This is just to rahash some of the stuff I have posted in the past. Some of which has been lost in the shuffle throughout time..

Most of this is intended for the Newcomer to Amateur astronomy, however, some may need it as a refresher or just to keep around as a reference.. 

There is a lot of information in this post. you would probably be better off if you print it so that you can read it at your leasure off-line and keep it for future references.

Getting Started:
To start with, I welcome you to the wonderful world of Astronomy.

You should do a few things first. While you are doing them start saving your money because you might want a nice telescope in the near future. However, hold off on that for a little while. Learn the night sky and do some research on the subject first.

One of the first things you should do is learn the night sky. Learn the constellations; start with the 12 constellations of the Zodiac and the brighter stars all around the night sky.  They are the easiest to learn. You can be learning to identify the many other constellations at the same time as they become visible to you... This will help you find and locate the many interesting objects in the universe.

Start out by just getting a blanket or a lawn chair. Get comfortable and watch the night sky. See which stars and constellations you are able to find and identify. At the same time there are two things that can aid you in learning the night sky. They will also be important for future use as well. These items would be a nice pair of binoculars and a book about astronomy. Go to any large bookstore and find one that includes star charts. These charts will help identify stars, constellations and some of the Deep Sky Objects.  There are many objects you will have the ability to locate with a pair of binoculars. Something around the size of about 10x50 is a good range to start with which is easy to handle without a tripod of anykind. You would be surprised what you can see through a good pair of binoculars.

A good book is a must to get started in the hobby of amateur astronomy. These books will include charts that you will need to help find your way around the night sky. Many other important aspects of astronomy will also be included.  A good book should explain what astronomy is all about, explain the Universe, Solar System, Planets, and Deep Sky Objects. You would also want it to explain the telescopes. To learn a little about the different telescopes and their mounts. In addition, some of the advantages and disadvantages of each telescope and mount. This will help when the time comes to make a decision as to what telescope you will want. There are many excellent book available on the market today. A couple of the more popular books for beginners are, Night Watch and Turn Left at Orion.

If you can, find an astronomy club close to your home. They usually welcome visitors and new members.  Check them out, learn from them, attend a star party where you can see and possible get some hands on experience with a variety of telescopes.  This will help you realize your expectations may not be what you actually expected to see through a telescope, or might exceed your expectations. They only way you will know is if you get some time under darks skies with a telescope. Visitng an astronomy clubs star party is the best way of doing that.

After you have done the above mentioned for a while, then if you are really interested and ready to go. You will be ready for the purchase of a nice telescope. Be prepared to spend some cash towards the purchase of a good quality telescope. These are not toys. They are an astronomical instrument that will give you a lifetime of enjoyment. Choose wisely and take care of it. The hobby of amateur astronomy can be very expensive to get started. This is a hobby that you can easily spend a small fortune on, depending on how involved and how far you want to take it. Particularly if you decide you want to do astrophotography…

It is offen recomended if you can, find a local astronomy club. Go to some of there meetings and go to one of the star parties. There you will gain more knowledge from the experiences of other amateur astronomers. You will get a chance to view a variety of telescopes personally. Most people are more than happy to allow you to look through them. This in-turn will give you a greater understanding and idea of what you will want giving you a little experience with the telescopes.  


The following are some of the more common acronyms and terms you will frequently hear and see used by amateur astronomers. Some of them will be explained in greater detail throughout this lengthy read..
This is not all the Acronyms often used however it is most that wil get you a good head start on things.

EPOCH:  An instant in time that is arbitrarily selected as a point of reference.
EP:………..Eyepiece
FOV:…….. field of view
AFOV:……apparent field of view
TFOV:…….true field of view
DSO:………Deep Sky Object / Deep Space Objects
EP:…………Eye Piece
OTA:………Optical Tube Assembly
APO:………Apochromatic refractor
ACHRO:…. Achromatic refreactor
EQ:…………..Equatorial
GEM:………..German Equatorial Mount
DOB:…………Dobsonian Mount
Newt:…………Newtonian Reflector Telescope. Named after the famed Astronomer English mathematician and scientist Sir Isaac Newton (1642- 1727). Credited for the development of the popular Reflector telescope.
SCT:…………Schmidt Cassegrain Telescope
MCT:……….. Maksutov Cassegrain Telescope
Catadioptric:…Optical Systems which involve both lenses and mirrors.

Reticle:……..An Eyepiece with a grid pattern or crosshairs used as a scale or aiming reference to maintain the position of a star. Or to establish the positions star, separations and or angles of separation of stars. (Measured in degrees of arc). 

M:………….Messier (pronounced Messy Ay) After the 18th century French Astronomer
Charles Messier ( June 1730 - April 1812).
in 1774 he published a catalogue of 45 DSO’s, such as nebulae, star clusters and galaxies. The purpose of his catalogue was to help comet hunters like himself and other astronomical observers to distinguish between permanent and transient objects in the sky.
By 1781 the catalogue became what it is today with 110 Messier Objects. The designation of these objects are from M1 to M110. This is the most popular and common catalogue for amateur astronomers to start with.. 

SAO:………..Smithsonian Astrophysical Observatory, SAO catalogue of 258996 stars,
NGC:……….New General Catalogue, A catalogue of objects ranging from 1 to 7840
IC:…………..Index Catalogue of Nebulae and Clusters of Stars and Galaxies.  This catalog is an index extension of the NGC catalogue..
Abell:………..A catalog of rich clusters of galaxies.  This catalog is an all sky catalog covering both northern and southern hemispheres containing  4,073 rich galaxy clusters. The original catalog of 2,712 rich clusters of galaxies was published in 1958 by George Ogden Abell (1927-83),  The catalog has been revised to include another 1,361

RA:…………..Right Ascension
DEC:…………Declination
ALT:…………Altitude
AZ :…………..Azimuth
CA:..………….Chromatic aberration
FL:……………Focal length
F/r, f/r,  f/n:…..Focal Ratio Or F/Stop
Lat:……………Latitude
Long:………….Longitude
GMT:…………Greenwich mean time
UT:…………….Universal Time

Collimation:….To make parallel; line up. To adjust the line of sight of (an optical device).

Averted Vision: Averted vision is a technique used by many experienced observers. This is a way for the your eyes receptors to absorb more light detail while observing an object through your Eye Piece (EP).
At night, the periphery of the eye's retina is more sensitive to faint light than the center. This area of the eye is more sensitive to light, color, and detail in objects. Looking slightly to one side of a faint object (averted vision), so that the faint light falls on the more sensitive outer part of the retina, usually reveals the object more clearly than looking directly at it. Even with brighter object this will reveal more detail..
If the object is in the center of the FOV avert your vision and give it a few moments. Allow your eyes to absorb the light. The longer you look, the more the area comes into focus..

Asterism : A group of stars that has a shape or appearance of something Not necessarily a constellation. Although they are often usually part of a constellations. A couple good examples for this would be the two frying pans in the sky known as the Big dipper and Little Dipper. There are many other asterism formed with the stars that are good for visual navigation around the night sky. Similar to land marks on the surface of the earth.. 

Sidereal:  A Measurement used to determine the apparent daily motion of the stars: (Sidereal Time). The time required for a complete rotation of the earth in reference to any star or to the vernal equinox at the meridian, equal to 23 hours, 56 minutes, 4.09 seconds in units of mean solar time. An apparent sidereal day is the time it takes for the Earth to rotate 360 degrees. To be more precise, it is the time it takes a typical star to make two successive upper meridian transits. (the time it take a star to return to the same location (meridian) in the sky). This is slightly shorter than a solar day. There are 366.2422 sidereal days in a tropical year, but 365.2422 solar days, resulting in a sidereal day of 86,164.09 seconds ( 23 Hrs, 56 Min, 4.09 seconds). Therefore giving us one more sidereal day than normal solar days.  The offset of one sidereal day gives us 365 ¼ days for observation. even though the planet itself rotates 366 ¼  times. (The Earth rotates in the same direction around its axis as it does around the Sun: Counter clockwise as seen from the northern sky).
A sidereal day: The stars rotate 360 degrees, one complete rotation around the earth is one Sidereal Day, For example: 360 x 60 x 60 arc-seconds in 86,164.09 seconds.
The Sidereal Rate is 15.04 arc-seconds / second.
Therefore, 100% of Sidereal is roughly equal to 15 ArcSecs / Sec

Minutes of arc (MOA), arcminute,  Seconds Of Arc (SOA) arc seconds: Arcminute is a unit of angular measurement that is equal to one sixtieth (1/60) of one degree.  Since one degree is defined as one three hundred and sixtieth (1/360) of a circle, 1 MOA is 1/21600 of the amount of arc in a closed circle, or (p/10800) radians. (such as the sphere of space surrounding earth). The symbol for marking the arcminute is the prime (')(U+2032, ′).One arcminute would be 1'. Arcseconds are  one sixtieth  (1/60) of  1’ arc minute. The symbol for arc second is (“) one arc second would be 1”.  Celestial coordinates / positions are typically indicated by use of degrees, minutes, and seconds of angles in two measurements: one for latitude, the angle north or south of the equator; and one for longitude, the angle east or west of the Prime Meridian  This Method is used to precisely calculate and locate any position on or above the surface of the Earth. Although, often because of the seemingly complicated or awkward nature of the base-60 nature of MOA and SOA, More and more people prefer to just give positions using degrees only, expressed in decimal form to an equal amount of precision. Degrees, given to three decimal places, give almost as much accuracy as degrees-minutes-seconds.
To convert from minutes (angles) to:
degrees, multiply by .01667.
quadrants, multiply by 1.852E-04.
radians, multiply by 2.909E-04.
seconds, multiply by 60.

Radian is a unit of angular measure equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle, approximately 57°17′44.6″.

Quadrant is a circular arc of 90°; one fourth of the circumference of a circle; Any of the four areas into which a plane is divided by the reference axes in a Cartesian coordinate system, designated first, second, third, and fourth, counting counterclockwise from the area in which both coordinates are positive.

Parsec: A unit of astronomical length based on the distance from Earth at which stellar parallax is one second of arc and equal to 3.258 light-years, 3.086 × 10¹³ kilometers, or 1.918 × 10¹³ miles.

Angular Diameter:  The diameter of an object as seen from a given position is the diameter measured as an angle.

Angular Size: is a measurement of how large or small something is using rotational measurement. It is useful for measuring things that are so far away that they appear two-dimensional.

Prime Meridian:

The Which is also the location for the starting of all time zones. This is know as Greenwich mean time (GMT) Or most commonly Universal Time (UT).

Meridian: The meridian is an imaginary line for your specific location. The line will extend from the celestial north pole directly over your head to the south.. This is the line that divides the east from the west.. As you face the north, anything on the right is east of the Meridian. Anything on the left is west of the meridian..

Ecliptic: The ecliptic is the path through the sky the sun, moon and most of the planets follow. It is the plane of the solar system..

Celestial pole: The celestial poles are the axis points the night sky revolves around the Earth..

True North: The direction from an observer's position to the geographic North Pole. The north direction of any geographic meridian.
The direction of true north is marked in the skies by the celestial north pole. For most practical purposes, this is the position of Polaris. However, due to the precession of the Earth's axis, true north rotates in an arc that takes approximately 25,000 years to complete. Currently, in 2002, Polaris is at its closest approach to the celestial north pole. 2,000 years ago, the closest star to the celestial north pole was Thuban.
On maps issued by the U.S. Geological Survey and the U.S. military, true north is marked with a line terminating in a five-pointed star. Maps issued by the Ordnance Survey contain a diagram showing the difference between true north, grid north and magnetic north at a point on the sheet.

Magnetic North: The direction of the earth's magnetic pole, to which the north-seeking pole of a magnetic needle points when free from local magnetic influence. Such as an abundance of Orr deposits in the ground.
The direction indicated by the north seeking pole of a freely suspended magnetic needle, influenced only by the Earth's magnetic field.
The magnetic pole is also on the constant but gradual drift. 

Grid North: The northerly or zero direction indicated by the grid datum of directional reference. Grid north is a navigational term referring to the direction northwards along the grid lines of a map projection. It is contrasted with true north (the direction of the North Pole) and Magnetic north (the direction of the Magnetic North Pole). Many topographic maps, including those of the U.S. Geological Survey and the Ordnance Survey, indicate the difference between grid north, true north, and magnetic north.

Aperture: Aperture is the diameter of the main / primary mirror or objective lens of the telescope. The larger the aperture, the better the resolution and the fainter the objects you can see. The more aperture a telescope has, the more light grasping ability it will have.. Thus allowing you to see farther and fainter objects.. Also allowing closer brighter objects to be seen in even greater clarity and detail

Most measurements used when referring to telescopes are in millimeters. Although, you will still find many that are in Inches..

1”in.  = 25.4 millimeters, 10mm = 1centimeter, 2.54cm = 1”in
39”in.  = 1meter

Some common telescope apertures used by many amateur astronomers,
note: Some measurements or rounded off to the nearest denominator.

Small telescopes
50mm = 1-15/16”
60mm = 2-3/8”
70mm = 2-¾”
80mm = 3-1/8”
90mm = 3-9/16”
100mm = 3-15/16”
114mm = 4-½”
120mm = 4-¾”
150mm = 6”
3”in. = 76mm
4”in.= 101mm
5”in.= 127mm
6”in.= 152mm
Medium Telescope
8”in.= 203
9-¼”in = 235mm
10”in.= 254mm
11”in.= 279mm
12”in.= 304mm
14”in. = 355mm
Large telescopes
16”in.= 406mm
18”in.= 457mm
20”.= 508mm
24”.= 609mm


Ready to purchase a telescope:

Some things you should consider prior to purchasing a telescope. In addition, a couple things to beware of.

First and probably most important thing to think about is your “Budget“. How much are you willing to invest in a nice telescope? Be prepared to invest some money for a good quality telescope.
You must also include in your budget, added accessories. Such as, more Eye Pieces, Collimating Eye Piece, Filters, , power supplies if needed, Etc… 

Second, what is your primary interest in astronomy? What type of observation (s) do you want to do? (Planetary, Lunar, Solar, Deep Space Objects or any combination of all), and are you considering Astrophotography. 

Third, Your Expectations! What exactly do you expect in performance characteristics from the telescope? Each kind of telescope has it primary design purpose along with its own characteristic advantages and disadvantages.
I hope that by the time you have gotten this far towards your decision, you have a decent book on astronomy and have learned a little about the different telescope types.

Fourth, probably one of the more important subjects to think about, “Storage and Portability“, Do you have the space?  Are you able to store a telescope with any size to it? Are you going to trouble moving the telescope around? If you need to move it to a dark sky location, do you have the vehicle capabilities to transport it.

Things that you will look for when getting ready to purchase from a dealer or distributor:  What is their reputation? What kind of reviews do they get? Read the reviews so you are aware of everything about them. Most importantly, how is their customer support? Once again, do the research and read the reviews.
Just because they are a well-established company that makes an excellent quality telescope and accessories, does not make them a good company. Some companies let their own success go to their heads while forgetting how they got there, by the customers purchases! Also, shop around at many of the online dealerships. Many of them sell the same brands of equipment. Often you can find a dealer / distributor that is having a sale and offering a better deal than others offer.

Last but not least, and certainly a very important thing, Be patient! Avoid jumping on what sounds like the first good deal you come across. If it is too good to be true, it probably is. Avoid department store telescope. These are more likely to be cheap junk telescopes. That will be more headache and frustration than enjoyment. They will do nothing but disappoint you. Be careful, many entry-level telescopes for beginners can also fall into this category. Although there are many, excellent entry-level telescopes available. You just need to do the research and shop around.   Even if they are a popular brand name, does not make them a good telescope.  Department Store telescopes are the bottom line cheap junk scopes.  Avoid the online auctions for cheap telescope, Buyer Beware, Remember You get what you pay for. Also remember, “If it sounds too good to be true it probably is“. Many of those great sounding deals are full of misleading hype. More often than not, you will be greatly disappointed in the purchase of any of those. If you want a good telescope, you are going to spend some money…

Many times people will start out with a small inexpensive telescope and high expectations... Often just to realize they cannot see what they were expecting to see. One reason is the aperture is just too small with a limited light grasping ability.  Astronomy photos and images do not help in this matter either. People often see these photos and think that is what they will see through any telescope. Very few objects will actually appear as they do in those photos / images. Other than a few objects, such as only a few nebulae and the planets, it is pretty much a colorless (black and white) universe to the casual observer. Those images are often long exposures allowing for color absorption then processed with image processing software. Even realizing that you cannot see objects, as those images show so greatly, People often want to see more than they can with a small telescope any ways. This is what many amateur astronomers refer to as Aperture fever. Because the larger that a telescope is, the more you can see. Larger aperture means greater light gathering ability. Which means you can find and see more objects. Then you will want to spend more money on a larger telescope. So that being said, more often than not you will hear recommendations to purchase a telescope with a 6” or 8” aperture to start with, Usually a Dobsonian, (More bang for the buck). Of course, this is keeping your budget and all other considerations in mind. 
 
The above are common recommendations to consider which you will frequently see and hear most any place you go. These are based on my own experiences and learning from others of more experience. These are pretty much the same recommendation to me by others members when I first got started in this very interesting Hobby.

Telescopes:

A brief description the three more common types of telescopes for beginners.

The three main types of telescopes, and the advantages and disadvantages of each:

The purpose of a telescope is to gather light and to bring that light to a point where it can be to focus. The larger the aperture of a telescope, the more light gathering ability / resolving power the telescope will have. The greater the light gathering ability means you will be able to see fainter objects. You will also have a greater ability to see more detail in many of the objects. Such as the Moon, planets, nebulae star clusters and galaxies.

REFLECTOR: Probably one of the more common types of telescopes:
A reflector telescope has a large primary mirror in the bottom end of the telescope for gathering light. The light is then reflected back up to the top/front end where a secondary mirror is placed at an angle to reflect the light out to the side of the Optical Tube Assembly (OTA) at a right angle to the focuser. This is where the light is focused to the eyepiece (EP) the main advantages to this are: More aperture for the dollar. Larger aperture means greater light gathering ability. These telescopes are great for general-purpose viewing. They are especially great in the area of Deep Space object (DSO's) observation.
You cannot use these for terrestrial observation. The views are inverted and reversed.
A reflecting telescope (reflector) is an optical telescope which uses mirrors, rather than lenses, to reflect light. The British scientist Sir Isaac Newton designed the first reflector circa 1670. He designed the reflector in order to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses. The traditional two-mirrored reflector is known as a Newtonian reflector.
While the Newtonian focus design still used in amateur astronomy, professionals now tend to use prime focus, Cassegrain focus, and coudé focus designs. By 2001, there were at least 49 reflectors with primary mirrors having diameters of 2m+.

Technical considerations
The primary mirror is the reflector telescope's basic optical element and creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or an eyepiece for visual observation.
Reflector mirrors eliminate chromatic aberration that can be found in achromatic refractors, but still contains other types of aberrations,  such as Coma near the outer fringes of the FOV, especially at lower magnifications. However a good quality eyepiece will correct much of that to the point it isn't noticeable.

Nearly all large research-grade astronomical telescopes are reflectors or a variation of. There are several reasons for this:
In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
Light of different wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration (CA) in poorly corrected or uncorrected lenses and creating an aberration-free large lens is a costly process. A mirror can eliminate this problem entirely.
There are structural problems involved in manufacturing and manipulating large-aperture lenses. A lens can only be held by its edge, which means that the sag due to gravity can be sufficient to distort the image. In contrast, a mirror can be supported by the whole side opposite its reflecting face.

REFRACTOR: These are what most people think a typical telescope should be. The ones you would see that old ships captains used. The type of telescope Galileo built and used to explore the universe.
A refracting or refractor telescope is a type of optical telescope which refracts light at each end using lenses. This refraction causes parallel light rays to converge at a focal point; while those which were not parallel converge upon a focal plane. This can enable a user to view a distant object as if it were brighter, clearer, and/or larger. These are similar to microscopes. The monocular is a type of refractor. A typical achromatic refractor uses two lenses, an objective lens element and an eyepiece lens(EP). The objective lens has two pieces of glass (with different densities), "crown" and "flint glass". Each side of each piece is ground and polished, and then the two pieces are glued together. The curvatures are designed to cancel (limit?) chromatic and spherical aberration.

The more common achromatic refractor has a lens assembly that is located in the front end of the OTA. The light enters through the lens to the bottom end of the OTA where the light is brought to its prime focus point.  The more expensive apochromatic(APO) refractor uses a more complex multiple lenses with higher grade glass to correct the refraction of the light and reducing or eliminating any chromatic aberration.
One of the greatest advantages of a refractor is that there is no light loss due to a central obstruction for a secondary mirror as in a Reflector or cassegrain.. This will allow for a much clearer and sharper image, especially on the planets and moon. Usually they are smaller, lighter weight, easier storage and portability. They can also be for both astronomical and terrestrial observation.

Technical difficulties
While initially the most common type of telescope, these are today used primarily by amateur astronomers and solar astronomers, and have been supplanted in professional night-time astronomy by reflecting telescopes. However, some relatively small instruments with 100-150mm objective lenses regularly produce astrophotography that rivals images created by professionals as recently as 20 years ago using what were then the largest telescopes on Earth. Vacuum solar telescopes such as the Swedish Solar Telescope often use the vacuum entrance window as a lens, and are thus refractor telescopes. The vacuum solar telescope design has produced the highest resolution images of the Sun currently available.
Refractors are criticized for their relatively high-degree of chromatic and spherical aberration. There is also the problem of lens sagging, a result of gravity affecting glass. There is a further problem of mis-refraction; caused by air bubbles trapped within the lenses. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed when it passes through glass. Many of these problems are avoided by using reflecting telescopes.

Understanding Refractor Terminology
Refractors seem to have a lot of terminology associated with them.  Here are some definitions of common terms.
Achromatic -- An achromatic objective lens is made two (Lens elements) made of  Crown and flint glass,  and is one in which red and blue light is focused to the same point, but there is residual secondary color.
Apochromatic -- An apochromatic (apo) objective uses 2 or 3 elements of special higher grade higher quality glasses. These glasses are much more efficient at focusing red, green, and blue light to a single point of focus and minimize or completely remove any secondary color.
Doublet -- A refractor objective (lens) consisting of two glass elements.
Triplet -- A refractor objective (lens) consisting of three glass elements.
Pyrex Glass
Borosilicate, A heat-resistant and chemical-resistant glass.  Although borosilicates had been produced before. Pyrex is a brand name of borosilicate glass first introduced as Pyrex by Corning Glass Works in 1924. Pyrex is widly used for the production of mirrors which are used in reflecting telescopes.
Crown and Flint Glass:
Two most common types of glass used in optics, in particular Achromatic refractors.
Crown Glass -- Soda-lime glass used to make lenses and prisms. It has a lower refractive index and less dispersion than flint glass, but is more durable.
Flint Glass -- Any highly-refractive lead-containing glass used to make lenses and prisms. Because it absorbs most ultraviolet light but comparatively little visible light, it is also used for telescope lenses. Flint glass typically has a much higher  refractive index than crown glass, and exhibits much higher dispersion, however it is less resistant to damage. Flint glass is used in conjunction with crown glass to make achromatic lenses.
ED -- Extra-low dispersion glass, which has special properties that reduce secondary color.  Similar to fluorite but less expensive.
Fluorite -- Fluorite is a special type of glass (actually a synthetic crystal) which has unusual properties yielding low secondary color and exceptional image quality.  Because flourite is expensive to manufacture and because it is brittle and hard to grind and polish for a high quality apochromatic refracting telescope they are very expensive.
Secondary Color -- The difference in focus between the primary wavelengths (red/blue in an achromat, red/green/blue in an apo) and the remaining wavelengths.
Chromatic Aberration -- The aberration that results from different wavelengths of light not focusing to the same point.


CASSEGRAIN /catadioptric : The best or comprimise of both refractor and reflector telescopes: Good long focal length general purpose telescopes. These are a very popular choice of telescope. These use both an objective or corrector lens in the front of the OTA and a primary Mirror cell in the rear of the OTA. The difference is that the primary mirror has a whole in the center and is spherical rather than parabolic like a newtonian reflector. The light first enters the primary objective lens to the primary mirror. Then it reflects back to a secondary mirror which is in or near the center of the corrector lens. (Similar to that of a reflector telescope) Then the light reflects back down through the whole in the center of the primary mirror to the prime focal point. These telescopes may be used with both astronomical and terrestrial observation too.
One great advantage of this design is that it allows for longer focal lengths with the use of a shorter OTA. Which for many this is a nice advantage, especially in the smaller sizes making them much more portable which is one reason they are popular with some people. 
a couple of the disadvantages associated with these telescopes are that they are harder to colimate and often take longer to equalize to the outdoor temperature.(cool down)

Variations
While there are countless variations, (both mirrors spherical, both mirrors aspherical, or one of each) they can be divided into two principal design forms: compact and non-compact. In the compact form, the corrector plate is located at or near the focus of the primary mirror. In the non-compact, the corrector plate remains at or near the center of curvature (twice the focal length) of the primary mirror. Typical examples of the compact design are Celestron and Meade commercial instruments, combining a fast primary mirror and a small, strongly curved secondary. This yields a very short tube length, at the expense of field curvature.
Non-compact designs keep the corrector at the center of curvature of the primary mirror. One very well-corrected design example would be the concentric (or monocentric) Schmidt-Cassegrain, where all the mirror surfaces and the focal surface are concentric to a single point: the center of curvature of the primary. Optically, non-compact designs often yield better aberration correction and a flatter field than a compact design, but at the expense of longer tube length.

Schmidt-Cassegrain Telescope, or SCT, invented by Bernhard Schmidt, is a catadioptric telescope. This type of telescope is advantageous because it combines the long focal length of the refractor telescope with the lower cost per aperture of the reflector telescope.
The optical design combines elements from both the Schmidt camera and the Cassegrain reflector. In this system the parabolic primary mirror is replaced by a spherical mirror, which introduces spherical aberration. This is corrected by the Schmidt corrector plate, found in the Schmidt camera. From the Cassegrain, it inherits the convex secondary mirror, perforated primary mirror, and a final focal plane located behind the primary. Some designs add additional optical elements (such as field flatteners) near the focal plane.

Maksutov Cassegrain,  or MCT, Invented by the Russian optician Dmitri Maksutov (1896-1964), the Maksutov telescope is a type of Cassegrain reflecting telescope that uses a spherical primary mirror in conjunction with a meniscus shaped correcting plate at the entrance pupil in order to correct for spherical aberration.
It differs from the similar Schmidt-Cassegrain telescope design in that the meniscus-shaped corrector plate allows for the use of an easily fabricated spherical secondary mirror rather than the hyperbolic mirror required for the Schmidt telescope. The Maksutov design also has excellent correction for off-axis aberrations such as coma, which is a significant problem in the simpler Newtonian reflecting telescope.
Light path inside a Maksutov Telescope
In some configurations, the Maksutov secondary mirror can be generated by simply silvering the central portion of the inner surface of the corrector plate. In this type of configuration, the Maksutov design has been mass-marketed as an inexpensive, very compact small-aperture telescope by several major manufacturers.
The chief disadvantage of the Maksutov design is that it does not scale up well to even moderate apertures (>250mm/10 inches), since the corrector plate rapidly becomes prohibitively large and heavy (and expensive) as the aperture increases. However specialist manufacturers do create models beyond 14 inches in aperture.

Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope with a hyperbolic primary and secondary mirror. It was invented in the early 1910s by American astronomer George Willis Ritchey (1864–1945) and French optician Henri Chrétien (1879–1956). Ritchey constructed the first successful RCT, which had a diameter of 0.5 metres, in 1927. The second RCT was a 1-metre instrument constructed by Ritchey for the United States Naval Observatory.
The Ritchey-Chrétien design is free of first-order coma and spherical aberration, although it does suffer from third-order coma, severe large-angle astigmatism, and comparatively severe field curvature (Rutten, 67). When focused midway between the sagittal and tangential focusing planes, stars are imaged as circles, making the RCT well suited for wide field and photographic observations. As with the other Cassegrain-configuration reflectors, the RCT has a very short optical tube assembly and compact design for a given focal length. The RCT offers good off-axis optical performance, but examples are relatively rare due to the high cost of hyperbolic primary mirror fabrication; Ritchey-Chrétien configurations are most commonly found on high-performance professional telescopes.
The curvature of the two mirrors in the Ritchey-Chrétien design are described by the following relationships:

Mounts:
In astronomy, the telescope mount is a very important part of the overall design of an operational telescope. Many sorts of mounts have been developed over the years, with the majority of effort being put into systems that can track the motion of the stars as the Earth rotates with a single motion.
The earliest types of mounts are today known as altitude-azimuth, or alt-az systems. The name refers to the way the system allows the telescope to be moved in altitude, up and down, or azimuth, side to side, as separate motions. They typically consisted of a two-prong fork with the telescope sitting between the prongs. Alternate alt-az systems used a single large pipe as one axis, typically the az, with the telescope mounted on a second pipe perpendicular to the first.
The introduction of the equatorial mount displaced most alt-az systems for serious users. By tilting the horizontal base of an alt-az system up until it is parallel to Earth's equatorial plane, the azimuth rotation then swings the telescope in an arc that follows the stars as they move across the sky due to Earth's rotation. By attaching a simple clockwork mechanism to this axis, the equatorial system makes long observation easy.

The following mounts are usually mounted on top of a tripod or pier. The sturdiness of the tripod is a very important consideration. Most larger telescopes use a large solid pier mount for better stability. Most often permanently set and mounted.
One of the more common is the German Equatorial Mount (GEM)or EQ). These are more complicated and more expensive. They not only have the altitude and azimuth axis's but declination (DEC) and right ascension(RA). Important for properly tracking celestial objects along with the rotation of the Earth. Great for locating objects with the use of celestial coordinates. And tracking for easy observation and astrophotography. These also come in computerized GOTO models or just manual hand controls.
A German Equatorial mount uses a counterweight on a long shaft opposite the telescope to counterbalance the weight of the telescope.  The telescope is able to track the sky about a polar axis to compensate for Earth's rotation.

The German  equatorial is the most popular form, consisting of two rods mounted in a T, with the telescope on the end of one of the T arms and the body tilted to match Earth's axis.
Today, the alt-az system is making a comeback in the form of the Dobsonian, a simplified and very rugged alt-az mount. The introduction of better electronics and inexpensive driver motors has allowed even alt-az systems to track the stars, making the complexity of the equatorial system less important.

Then you have the different mounts to consider also. They are as follows. One of the more common that you will see or hear about is Dobsonian mounts. This is a very simple alt az mount for a reflector telescope. usually made from formica veneer coated wood to mount the telescope on an easy swivel base / turn table. What makes these popular,  are the low cost to produce. Thus allowing for a larger reflector telescope for a lot less money.
There is also a simple Alt Az tripod mount. These are usually less expensive used for smaller less expensive telescope or spotting scopes or cameras.. Or smal grab and go telescopes used on trips away from home.

A German Equatorial mount with its Right Ascension (R.A.) axis which is aimed toward celestial pole, to polar align the mount.  Once aligned the telescope can track the sky using slow-motion controls or a clock drive to rotate the Right Ascension axis.  This axis allows motion from east to west.  The telescope rotates around the mount's declination (dec) axis in order to allow movement north and south.
PROS:
Allows automatic tracking with clock drive
Very stable
Easy to point to most areas of the sky
Good for photography or CCD imaging
CONS:
Heaviest type of mount
Longer setup time for large scopes

Fork Mount. This is the mount that a lot of cassegrain telescopes use. A more complicated and sturdier form of the Alt Az mount.  Most larger fork mounts are often computerized GOTO mounts. Meade Instruments has a popular line of telescopes using these mounts. Also does Celestron. To utilize this type of mount for astrophotography you will also need an additional accessory called a wedge..
Alt-azimuth or alt-azimuth mount is a simple mount used for moving a telescope or camera along two perpendicular axes of motion. The vertical movement is known as the altitude, while the horizontal motion is called the azimuth.
The biggest advantage of alt-azimuth mounts is their simplicity in both manufacture and use. They are often used for beginner telescopes, or for spotting scopes, but are still widely in use for more advanced telescopes. In the latter case, advanced electronics and motors are sometimes attached to compensate for the restrictions of the mount's simplicity.
In astronomy, alt-azimuth mounts were, for a time, surpassed in popularity by the more complex equatorial mount. The latter is more naturally suited for tracking astronomical objects in the night sky as the Earth spins on its axis, since its polar alignment means that only one axis need be adjusted rather than the two of an alt-azimuth mount. Being able to track such objects reliably is particularly important for astrophotography, as well as more advanced amateur astronomy, both of which became more accessible when equatorial mounts became affordable.
In recent decades, alt-azimuth mounts have once again become very popular for astronomical telescopes:

The Dobsonian mount—a variant of the alt-azimuth mount—has become very popular since about the 1960's. This is due to its ease of construction. Although it is difficult to attach an automated drive system to a dobsonian mounted telescope, it has become very popular for its suitability for very large amateur telescopes that cannot be conveniently mounted on an equatorial mount.
Affordability of modern electronics has been a further motivation for a return to alt-azimuth mounted telescopes, with their increased simplicity for manufacture and practical use. In particular, it has often proved more convenient to build a simple alt-azimuth mount and use a computer to manipulate both axes to track an object, than to build a more mechanically complex equatorial mount that employs only a single motor. When astrophotography is involved, a further motor may be used to rotate the camera to match the field of view.

These are just some simple descriptions of telescopes and mounts. This should help you with your choice of telescope.

Next I will be going over some more of the common terminology and some of the formulas used by Amateur Astronomers.  Knowing these terms and formulas will give you a greater understanding of what many things are and how things work..

Celestial Coordinates:
Understanding the Celestial Coordinate System and Using Setting Circles..
This is often easier said than done. First imagine you are standing on a very large ball,  the sky is a sphere all around you. Almost as though the earth is in the middle of a spherical dome.

Following is some information that I hope will help give a better understanding of this.. I am going to try and keep it as simple and in plain understandable language.

Units of measurement used for celestial observing and navigation:

Latitude and longitude are two measurements of degrees, Angle of arc in minutes, and in seconds.

Latitude is the degrees of measurement from 0 to 90 for the northern hemisphere.  From 0 to -90 for the southern hemisphere.. 0 degrees is the equator while 90 is the polar axis of  earth..

Longitude is lines (line of Meridian) extending from pole to pole dividing the earth into segments much like an orange.. There are 360 of these lines around the earth They are units of measurement from 0 to 180 degrees west and 0 to 180 degrees east.. Greenwich England is the point of 0 (Zero) Degrees.  Much like our times zones begin there also (Universal Time (UT)) so do the lines of longitude..

Degrees of measurement as visualized in the night sky,   Degrees are angular units of measurement. 1 degree equaling twice the size of the Full Moon or Sun as seen in the sky. Then you can divide degrees into the smaller units of measurement call Arc Minutes and Arc Seconds. 1 degree breaks down into 60' Arc Minutes, then 1' arc Min breaks down into 60" arc Seconds.

Declination. (DEC)
The angular distance of a celestial object north or south of the celestial equator, measured in degrees. One of the two coordinates that guide you to a celestial object with the help of a star chart and the setting circles on the telescope mount. (RA being the other coordinate) . This coordinate is called declination because the positions in degrees "decline" or decrease from 90 degrees at the north and south celestial poles. The poles being the area of which everything in the sky rotates or revolves around.  Then the Celestial equator would be zero degrees. Degrees of Declination are  positive if north of the equator (0 to +90) and negative if south of the equator (0 to -90).

Living in the northern latitudes Polaris (The North Star) is real close to the actual celestial pole. This is a nice advantage giving us in the northern hemisphere a very close reference point to the celestial pole. Whatever the latitude is for your location will be the angle from the northern horizon that the north celestial pole will be.  An example would be, if you live at 40 degrees north latitude then the north celestial pole would be at 40 degree angle above the northern horizon.  To clarify the location of the celestial equator, It is not at the southern horizon. The celestial equator would be 90 degrees from the celestial pole to the south. So if you are at 40 degrees north  the  Celestial Equator would be 50 degrees above the southern horizon…  


Right Ascension (RA)
Is the angular distance of a celestial object east of the vernal equinox, measured in hours, minutes and seconds. This is commonly one of the two coordinates (declination is the other) that help you locate celestial objects by using a the setting circles on the telescope mount.. When facing the north celestial pole, the stars will rise (ascend) from your right (East in the northern hemisphere). Thus we have the term "(RA) right ascension." The night sky makes a complete revolution around the earth in 24 hours., making each hour of right ascension equal to 1/24th of a circle, or 15 degrees. For example. If you are looking at an object in the night sky, one hour later it has moved 15 degrees. 3 hours it will have moved 45 degrees and so on. 1 degree of sky moves past a your position in four minutes. This rotation is especially noticeable at higher magnifications through a telescope.. 
Comparing right ascension (RA) to longitude. Longitude is for land (terrestrial) navigation purposes. Imagine projecting those line of longitude into the sky. Only instead of 360 lines you would have only 24. 
Now for the minutes and seconds. There are smaller more precise divisions / fractions of DEC and RA.  24 hours in a day, 60 minutes of arc in and hour, and 60 seconds of arc in a minute.  Lets say something is half way between RA 20 hrs and 21hrs. Then it would be RA 20hrs 30mins. For more precise then you would break it down to the seconds. RA coordinate would be as follows: hrs-min-sec.  Declination coordinate would be Degrees-min-sec For example the current location of Jupiter is RA 12h45m02sec, DEC -03d 29’49”

This should give you a better understanding of those numbered dials on the equatorial mount which are known as Setting Circles. Although they are a nice thing to learn. Setting circles are not used very often by your typical backyard amateur astronomer.. It is often easier to learn the night sky visually making it easier to navigate and locate objects. Many people are also using the ever growing in popularity Computerized GOTO telescopes.  They take away a great majority of the work involved in navigation and locating objects. Although, you still need a knowledge of the brighter stars used to set up and align the telescope.

For the purpose of celestial navigation there are four different angles / scales of measurement to be concerned with. They are Altitude (Alt), Angle or tilt above the horizon (0 to 90 degrees from the equator) .. Azimuth (Az), Circular angle around your point on the surface in 360 degrees. North as 0 and or 360 degrees, East as 90, South as 180 west as 270 degrees.  Then  RA and DEC as described above. There are two basic styles of mounts used by backyard amateur astronomers.  Those mounts that are just Alt Az. From a simple camera mount to a sturdier fork mount.  Then there is the German Equatorial Mount (GEM)..  The GEM uses all 4 angles of measurement and is better for locating and accurately tracking celestial objects in the same arcing motion they progress through the sky..

We are going to discuss the GEM and the setting circles. First you have probably noticed there are two set of numbers on the RA setting circles., The reason there are two sets of numbers on the RA dial,  one set is for use in the Northern Hemisphere and the other is for the southern hemisphere. Most likely the ones that are upside down or counter clockwise, are for the southern hemisphere.

To get the setting circles set right you first need to polar align your telescope mount on the celestial pole. This is where you will use the Alt and Az adjustments. Once you get the Alt Az set properly you will leave it alone.. You will want the mount facing 0 degrees Azimuth (north)  and the Altitude or elevation to whatever your latitude is.. For example I live at 34 degrees north. I would have my mount facing true north (not magnetic north) with the altitude / elevation set to 34 degrees. This is the angle above the northern horizon where I will find the celestial pole, and see the north star Polaris..

Guide Star Method of adjusting the Setting Circles:  

Once you have the telescope aligned on the celestial pole then you would set your Declination setting circle to 90 degrees. To set your RA setting circle you will need a star chart. Locate a star or object with a listed RA on the chart that is currently visible to you. Slew the telescope around to that star. Get it centered in the Field Of View (FOV). A reticle EP would be more accurate for this purpose. The star chart will indicate what the RA coordinates for that star. Usually only Hours and minutes. But that is accurate enough for the mount.  Now  you will turn the Setting Circle to correlate with the coordinate listed on the chart for that particular star.

This should help you get ready to locate objects in the night sky with the aid of setting circles..   

 

A much more complicated method is the Sidreal method

This method requires that you know your local sidereal time at the time of observation. You must either keep a clock set to sidereal time or you must calculate the sidereal time for the time of the intended observation.

First, calibrate your declination circle according to the methods above. Next, determine the target's hour angle by subtracting its right ascension from your local sidereal time. A positive hour angle indicates the target is west of your meridian; a negative hour angle indicates the target is east of your meridian. Now, turn your telescope to read the target's declination and then use the hour circle to point the calculated hours (and minutes or fractional hours) east or west of you meridian.

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More terms and formulas you should know, with more detailed and understandable explanations.

Aperture gain:
Although not a commonly heard term, this one is still important and helpful to know. It is the gain in aperture size as compared to the diameter of your pupil. On average, the human eye when dilated and adapted to darkness, is approximately 7 to 8mm in diameter.. 
Aperture gain will give you an idea of the faintest stars visible to you through a telescope of a specific aperture.. For example, If your pupil is 8mm in diameter, a 80mm aperture telescope is 100 times the size of your pupil. 100 x gain in aperture. That is the same as a five-magnitude difference, So what does that mean to you?  It means that if you can see a star of the 6th-magnitude with the naked eye, then you should be able to see a star of the 11th-magnitude in a 70-mm telescope. We will discuss magnitude later in this article.
This reasoning ignores loss of light in the optics and or due to the quality of optics. Also ignores atmospheric conditions effect on seeing...

To calculate what the aperture gain is for your telescope You would divide the aperture of your telescope by the diameter of your pupil squared. Or multiplied by itself, which ever you prefer. Unless you know the exact diameter you can only guess that you are an average person.. So we can say your pupil size when dark adapt is 7mm.. Let’s say you have a telescope with an aperture of 6”

Aperture gain = Objective Diameter / Eye Pupil Diameter 2

Since we know 6” is equal to 150mm you would take 150 / 7 = 21.42  or 462
So a 6” aperture telescope would have 462x aperture gain over the 7mm pupil of your eye. 

EP = Eye Piece:
The purpose and function of a telescope is to collect light and form a small fixed-size image at a point that is determined by the focal length of the (OTA) Optical tube assembly.  That point is called the Prime focus. With out an EP you can see this image by pointing the telescope at a bright object, such as the Moon. By holding a piece of paper behind the focuser. At some point or distance form there, you will see a small image of the Moon projected onto the paper. This is the prime focus image formed by the telescope.
Although, the human eyes cannot focus sharply on an image unless it's more than eight inches from the eye. More if you have eyes like I do. J  It is difficult to see to see any kind of detail in prime focus images formed by the telescope, Unless of course if you use an EP. 
Basically all an (EP) Eye Piece is,  a small microscope or series of magnifying glasses (Lenses). Most Eye Pieces are configured of multiple lens elements. Usually a minimum of two, more common 3 or 4. These lens elements are arranged and incased close together inside a plastic or metal mount. The lens element closes to the eye that you look into is called the eye lens. While the lens closest to the telescope end or bottom of the EP is called the Field lens. Multiple elements are used to correct and carry all light waves to the same point of focus.. The EP is used to allow you to focus on an image closer than eight inches from that fixed-focus image.  You will need an Eye Piece (EP) that will allow you to bring the prime focus image into focus to your eye..

You will find that EP’s come in various focal lengths (size). Focal lengths will by specified in millimeters. Often the focal length will be engraved or somehow labeled onto the outer casing  or barrel of the EP.. Such as a 25mm 15mm 12mm 10mm 5mm and so on etc. etc.. 
So as an example, With a 25mm EP, you can get focused on an the image with your eye at a distance of 25mm away from the prime focus image. (25mm is equal to 1” (one Inch)) A 12mm would be little less than half an inch and so on.. The closer you can get to focus on an image the larger the image will appear. So the longer the focal length the less magnification you can get. The shorter the focal length of the EP the more powerful the magnification will be.  Magnification will be discussed more later in this article..

Many people mistakenly distinguish the focal length of the EP as the size of the EP. When actually the size of the EP is the outer diameter or barrel size that inserts into the telescope focuser. . Most standard Eye Piece and focuser formats are 1.25” inch, or 2” inch. The less expensive cheaper telescopes, such as cheap department store junk telescopes will use an eye piece of  0.965 OD. Usually made of plastic rather than glass. 

Exit Pupil
Often overlooked, nevertheless still a very important consideration, especially when choosing eyepieces to match a yourself and the telescope.

This is the point at where all the light rays of the image come to focus.. A cone of light concentrating the light to a smaller point for a clear focused image.  A circular image or beam of light formed by the eyepiece of a telescope. Imagine looking through a straw or a cone with one end larger that the other. You are looking through the smaller end. Typically you want that small end of that cone of light to be as close to the size of your pupil when adapted to the dark.. In order to get the most advantage out of a telescopes aperture and Light gather ability. It is preferable that the exit pupil of the EP is no large than Your dark adapt eye’s pupil. This way all the light of the image is gathered and focused to your eye.

 The exit pupil of the EP used in a telescope is figured by dividing the EP focal length by the telescope focal ratio. Or by dividing the Aperture of the telescope by the magnification given by the EP  To calculate this you must first know how to calculate the magnification that an EP will give you with your telescope.. In this example I will use a 6” aperture telescope with a 1200mm focal length, a focal ratio of f/8

To calculate focal ratio you divide the focal length by the aperture. In this case 6” = 150mm

1200 / 150 = 8   so the focal ratio is f/8

The formula to calculate the magnification that an EP will give you for this telescope, you will divide the telescopes focal length by the EP focal length..

1200mm / 25mm = 48x magnification
 
Now we can calculate the exit pupil of the EP..
The EP is a 25mm and the focal ratio is f/8

EP Focal length divided by Telescope focal ratio = exit pupil

25 / 8 = 3.125

Or

Telescope aperture divided by Magnification

150 / 48 = 3.125

So in this example the exit pupil through the EP with this telescope is 3.125mm this is a smaller more concentrated cone of light on your pupil.. So your pupil is gathering all the light of the image..  
 
Unfortunately as we get older, the eye's ability to dilate declines. By the time the average person reaches the age of 50 the dark adapt eye only dilates to about 5mm instead of 7mm.  That is only one of the eye problems encountered with age. Other effects of age on the eyes will also effect your telescope observing.. Such as the ability for the eyes to focus on or slow to focus on objects at a close distance to the eyes.  At first for telescope observation this is not a real problem. As you get older and this problem increase and persist, then you will eventually need to wear glasses even when observing with a telescope.. Generally if you only need glasses for reading you will not need them for telescope observation.. Though you will still want them to read your star charts..    

To select eyepieces by the exit pupil, first keep in mind If you use eyepieces with an exit pupil larger than your dark adapt pupil size,  The edge of filed rays of the light cone won't be making it into your eyes, it would be the same as stopping down your telescope aperture.You want the cone of light through the telescope to be equal to preferably less than your pupil size. You want all the light gathered by the telescope (Light Cone) to make it into your eye to hit your eyes light receptors.  With an eyepiece exit pupil that is too large in a Newtonian reflector, the secondary shadowing might become too noticeable and even obtrusive at low magnifications.

To choose eyepiece to buy that would be best for you and your telescopes by the exit pupil you want from the eyepiece.
In a person with healthy eyes the upper limit for the exit pupil is about 7mm, Larger  exit pupils are what you get through longer focal length(low) magnification eyepieces. For the best visual acuity 2 to 3mm is preferred., Too small and your eyes won't be able to field the exit pupil. Generally you won't want an eyepiece with an exit pupil smaller than 0.5mm.
The lowest magnification you would need should have an exit pupil no larger than 7mm, and, the highest magnification having no smaller than a 0.5mm exit pupil.
The formula used to convert the exit pupil to eyepiece focal length would to multiply your telescope’s focal ratio by the exit pupils to equal the focal lengths of eyepieces. For example in my 10" f/4.7 Newtonian
7 * 4.7 = 32.9mm focal length,  0.5mm exit pupil would be 0.5 * 4.7 =  2.35mm focal length. 

Eye Relief
This is important especially if you need to wear eye glasses.. This is the distance from the lens of an eyepiece that you want to keep your eye at.. This is also the point behind the eyepiece where the light of the exit pupil comes to a focus and form the image. This is where you should have your eye positioned to see the full field of view (FOV) of the image through the EP. Also the best position to get the most detailed clarity of focus on the image. If you are someone who has to wear glasses due to an astigmatism, you'll want at least 15mm of eye relief. This will allow  you to see the full FOV with your glasses on.

A longer eye relief is an advantage to those who must wear their eye glasses. Although, there are a couple disadvantages that accompany long eye relief  EP’s.. One problem is you must have your eye centered over the EP exit pupil just right in order to see the image correctly.. If not what usually happens is, you will see a portion of the image darkened as though something is partially blocking the FOV.  This is more noticeable in lower magnification, longer focal length EP’s   Another problem can be easily stopped with the use of eye cups or cupping your hand around the EP and your eye.. If there is any stray light from house lights or street lights, that light can also find it’s way to your eye while trying to observe through the telescope. This is something we do not want.. Blocking stray light will significantly improve observations.. 

Focal Length
This is the length of the effective optical path through the telescopes (OTA) Optical tube assembly,  or  the eyepiece. In other words, it is the distance from the Primary Objective, (main mirror or lens) where the light is gathered to a point where the prime focus image is formed. The point at which you would focus the EP on the image.. 
Internationally, this is typically designated in millimeters. Focal length determines the magnification you can get from an EP of a specific focal length..

Focal Ratio
The only thing you really only need to know about this is that it is the ratio of the focal length of a lens or lens system to the effective diameter of its aperture. The focal length divided by the aperture in millimeters equals the focal ratio. Not to be confused with f-stop among photography enthusiasts.
Don't become overwhelmed by this number. For visual astronomy this number is not an issue of importance. Other than to calculate other numbers that might be more important. These will be covered in this article.  This number does have some importance for film photography of deep sky objects when calculating exposure times.
Telescopes with a shorter focal length to aperture will have a shorter / lower (faster) focal ratio.  Longer focal length to apertures will have longer(slower) focal ratio..
 A shorter focal length short focal ratio telescope will yield a wider (FOV) Field Of View. Generally a shorter focal ratio will result in brighter objects. However, if you have two telescopes of equal aperture and different focal lengths and focal ratios, You probably won't notice any difference in brightness. The human eye is not sensitive enough to notice such subtle changes. although for film photography purposes, film will pick up the difference. The focal ratio will play an important role in determining the exposure time.. Although these day more an more people are using Digital SLR cameras or CCD imagers. The CCD and CMOS sensor chips used in these imagers and DSLR cameras are more sensitive. In other words focal ratio does not have the same effect as it does with film photography. With Digital photography the only considerations other than the camera is focal length effect on magnification and aperture effects on resolution.. In this case Aperture definateluy rules.

Magnification
The act of magnifying or the state of being magnified.
The process of enlarging the size of something, as an optical image.
Something that has been magnified; an enlarged representation, image, or model.
The ratio of the size of an image to the size of an object.

With telescope we use Eyepieces to magnify and bring the image to focus. It is not the aperture of the telescope the magnifies the image. Rather it is the aperture that collects the light and allows the image to be magnified. Let me clarify this for you. The aperture of the telescope gathers the light to be focused. The larger the aperture, the brighter the image can appear. Not the larger it will appear.  Although with larger apertures you will have the ability to use higher magnifications and see fainter objects. Objects or images will also have greater detail with larger aperture telescopes. 
The focal length of the telescope and the focal length of the EP determine the magnification of an image.
The aperture of  the telescope will determine the maximum useful magnification you can use at a given aperture. Typically you can get 50 to 60 x per inch of aperture. Or about 2 to 2.4x per millimeter of aperture..

A 6”(150mm) telescope can get a maximum useful magnification of 360x. That is figuring the maximum at 60x per inch or 2.4x millimeter of aperture.. Typically you would only want to calculate this with the smaller figure. 50x per inch or 2x per mm.
More often you will not be able to achieve these higher magnification.. A condition known as seeing will determine how high you can achieve at any given time.. We will cover seeing conditions later in this article. 

If you have never purchased a telescope before, this information and these numbers are something you will want to remember . Unfortunately there are misleading advertisements and or sale persons that try to get the unaware shopper or patron to believe a telescope can reach unreasonable or unreachable magnifications. Just to get you to buy that telescope.  Remember a maximum of 60x per inch of aperture or 2.4x per mm. And that would be under ideal atmospheric conditions which is not as common as we would like.

Once again, to cover the formula used to calculate the magnification you can achieve with an Eye Piece..

Telescope focal length divided by the EP focal length.

1200mm / 25mm = 48x magnification..

Optimal Maximum Eyepiece Focal Length.

This is something which is more important with reflecting telescopes. Too long a focal length will yield too low of a magnification. Then you will start to introduce optical aberrations and other errors into your view. Such as severe Coma near and at the edge of the FOV, possible Astigmatism from the EP, and secondary mirror shadowing over the object and the FOV. 

To calculate the maximum eyepice focal length for a telescope you multiply the exit pupil of your eye by the focal ratio of the telescope. For a younger person with healthy eyes, the dark adapt pupil is an average of 7mm.  However, as you get older that my be a bit smaller, about 5mm or 6mm.
For my 10" f/4.7 reflector, (7mm * f/4.7 = 32.9). so the maximum focal length of the EP I would want for optimal low magnification performance would be about 33mm.

To calculate the minimum optimal (low)magnification for a telescope divide the aperture in millimeters by 7.
250 / 7 = 35.7x magnification

keeping the above two figures in mind you will see that if you divide the telescope focal length by the EP focal length you come up with a figure that is real close to what you want.
1200 / 33 = 36x magnification..

Some times you will see a telescope advertised with specifications such as 600x100mm. Do not allow yourself to be mislead by this specification. This does not mean the telescope has 600x magnification. Often this is confused due to the fact this is how the magnification and aperture of binoculars are identified. Such as 16x50. For binoculars that would mean they have an aperture of 50mm and a magnification of 16x.  An object at 1000 feet will appear 16 times closer than with the naked eye.. For telescopes this is not the same. These numbers are specifying that the telescope is a 100mm aperture telescope with a focal length of 600mm..

To choose eyepieces the magnifications that you want from them, starting with the optimal minimum magnification for your telescope, use 1.4X as a multiplier to produce the next higher magnification. For example let say the optimal minimum magnification for your telescope is 35x, multiply 35 * 1.4 = 49x,  then 49 * 1.4 = 68x,  68 * 1.4 = 95x and so on
Then convert that magnification into eyepiece focal length by dividing the telescopes focal length by the magnification.  For example an eyepiece yielding a magnification of 48x in a 1200mm focal length telescope would be 1200mm / 48x = 25mm

FOV: Field Of View
The Area, range or width of View as we see it measured in degrees

AFOV: Apparent Field f View
apparent field of view is the angular diameter, expressed in degrees, of the circle of light that the eye sees.

TFOV: True Field Of View
True Field Of view is the Field Of View Seen through the EP when used with a specific telescope. measured in degeees.

There are a few formulas that can be used to determine the True Field Of View

First we have determined to calculate the magnification an Ep will yield when use in a telescope of a specific focal length..

Telescope Focal Length divided by the EP Focal Length

example: 1200 / 25 = 48x magnification
Provided you already know the manufactures specifications for the Apparent Field Of View we can use the magnification to calculate the True FOV.

EP apparent FOV / Magnification of View = True FOV

example if the EP has an apparent FOV of 50 degrees and the EP yields a magnification of 48x

50 / 48 = 1.04 degrees True FOV

Or You can aim the telescope at a star near the celestial equator. Time the star as it moves from on side of the FOV to the other.
Mulltiply that time in minutes by 4
If it only take 15 second for the star to cross the FOV (.25 minutes)

4 x .25 = 1 degree True FOV

One of the more accurate is to messure the Field Stop located inside the Bottom end of the EP barral that secures the lens elements with in the EP. The bottom Lens element we have Already discussed is the Field lens. The Field Stop is the ring you see holding those in place. Measure across the diameter in millimeters.

Example: a 25 mm EP will have a field stop at or near 22mm

The formula to calculate the True FOV with this measurement is as follows.

Field Stop Diameter divided by Telescope focal length multiplied by 57.3

22 / 1200 x 57.3 = 1.05 degrees True FOV.

Light grasp
The primary and most important piece of equipment or part of a telescope is it’s heart.  This heart is the objective lens in a refractor telescope or the primary / main mirror in a reflector telescope. The function of the objective lens or primary mirror is to gather as much light as the aperture size will allow.. Light grasp is a term used for that amount of light the primary mirror or lens of the telescope can collect. The light gathering or collecting power of the telescope. Limiting magnitude of the objective or primary mirror.
The size of the lens or mirror,  such as 6” (or 150mm) refers to the size of the telescope.

Generally speaking our naked eye when adapted to the dark skies has an pupil / aperture of about 7mm. People with excellent eye sight can even dilate to 8mm for better night vision..

At any given time if conditions exist to allow,  you can see about 2000 stars with the naked eye. This is if your eyes are dark adapt, No light pollution from any artificial lighting, if there is no Moon illumination, and the atmospheric conditions are clear and stable. 

Light gathering ability is directly proportional to the collecting surface, (square of the aperture) , clear diameter of the lens or mirror. For example: If your eyes are dilated to 8mm and you are looking through a 16mm objective. The light gathering ability is increased to 4 times greater than with the naked eye. Theoretically this means you can see stars 4 times brighter and theoretically can see 4 times as many stars that are 4 times fainter than with the naked eye. Theoretically speaking only though. We will explain more on this when cover magnitudes and object brightness. 

The larger the aperture of a telescope the greater the light gathering / collecting ability of the telescope.. The greater the light collecting ability the brighter stars and objects will appear,  the greater the distance of and the fainter the stars will be that you will have the ability to see.. Larger aperture means the dimmer the magnitude of stars that can be seen. And the greater the limiting magnitude of the telescope.

Resolving Power:

A measure of the ability of a lens or optical system to form separate and distinct images of two objects with small angular separation.

An optical system cannot form a perfect image of a point (i.e.,point source). Instead, it performs what is essentially a Fourier transform, and the resolving power of an optical system may be expressed in terms of an optical transform (transfer function) called the modulation transfer function (MTF).

The resolving power of an optical system is ultimately limited by (a) the wavelenght involved, and (b) diffraction by the aperture, a larger aperture having greater resolving power than a smaller one.

Dawes Limit = 4.56 arc seconds divided by the aperture in inches. 

Magnitude

A system first developed about 150 BC by the Greek Astronomer, Mathematician, and Geographer named Hipparachus to determine the brightness of stars as seen by the naked eye. Original scale ranging from 1 to 6. With 1 being the brightest and 6 being the dimmest.. Just as a side note, Hipparachus also developed a system of grids similar to what we use today as our lines of latitude and longitude.
Modern magnitude scales developed with modern technology has revised the star magnitude system from negative numbers. The magnitude scale system has also been revised and broken down to decimal fractions. Stars can be an absolute magnitude of -8. This being the brightest stars in the sky.  Then as faint as an absolute magnitude of 16 or fainter. A one digit magnitude change indicates a 256% difference in brightness. A 4th magnitude stars is often the faintest visible to the naked eye from a light-polluted suburb. The closer you get to the city the less stars that can be seen. By comparison a 14th magnitude stars is approximately  1/10,000th as bright!  Typically a 6th magnitude star is about the faintest that can be seen by the average persons naked from a reasonably dark sky observing site. The Sun has the brightest apparent magnitude of -26.5.

Note: Never look directly into the sun. Never use the telescope for observing the sun without the proper solar filter.. 

The human eye senses brightness of light logarithmically, So for every 5 magnitudes of increase, means a decrease by a factor of 100 in brightness of a star. The Stars absolute magnitude is the magnitude it would have if viewed from a distance of  approximately 32.6 light years /10 parsecs.

parsec (pär'sec) [parallax+second], in astronomy, basic unit of length for measuring interstellar and intergalactic distances, equal to 206,265 times the distance from the earth to the sun, 3.26 light-years, or 3.08×1013 km (about 19 million million mi). The distance in parsecs of an object from the earth is the reciprocal of the parallax of the object. The nearest star, Proxima Centauri, has a parallax of 0.763? of arc and a distance of about 1.31 parsecs.

When viewing objects and stars we talk about the “absolute” magnitude of stars and the “apparent” magnitude of an object. Absolute magnitudes are how bright a star appears at a specified distance, As referred to in the above paragraph. This is often confusing for some when referring to the listed magnitude of an object.. These are not the same thing.. Apparent magnitude is the brightness that an object appears to us in the night sky, Or commonly referred to as integrated magnitude or integrated brightness.

On extended objects such as galaxies and nebulae, the magnitude is the brightness of the object if it would have all its light gathered (integrated) into a single point, like a star. A galaxy specified as having a magnitude of 6 will therefore appear dimmer than a 6th magnitude star because its light is not concentrated to one point like a star. Rather it is spread out over a larger area than a star. This is often refered to as visual magnitude A couple good example of this are the galaxies M33.  A 6th magnitude face on galaxy located in the Triangulum Constellation between Andromeda and Aries. This object is still hard to find even in a 8” or 10” telescope.. Because its magnitude 6 brightness is spread over nearly one square degree of sky. This is what you call an object that has a low surface brightness. (Surface Brightness is the brightness of the object as seen spread out over an area of space / the amount of light an object emits per area of the sky.). Even galaxies considered to have high luminosity can be hard to see beause of low surface brightness.  M33 is especially difficult if not impossible to find in Moon illuminated or light polluted skies. The Galaxy M101 is another example. There are also many Nebulae that fall into this category. Often to find these nebula a special filter is needed.. 

Here's a list of the the Messier Galaxies and their integrated magnitudes and  actual surface brightness.                         
Object  Mag.  Sur. Br.     Object  Mag.  Sur. Br.
  M31     3.4     13.6         M87     8.6     12.7
  M32     8.1     12.7         M88     9.6     12.6
  M33     5.7     14.2         M89     9.8     12.3
  M49     8.4     12.7         M90     9.5     13.6
  M51     8.4     12.6         M91   10.2     13.3
  M58     9.7     13.0         M94     8.2     13.5
  M59     9.6     12.5         M95     9.7     13.5
  M60     8.8     12.8         M96     9.2     12.9
  M61     9.7     13.4         M98   10.1     13.2
  M63     8.6     13.6         M99     9.9     13.0
  M64     8.5     12.4         M100   9.3     13.0
  M65     9.3     12.4         M101   7.9     14.8
  M66     8.9     12.5         M104   8.0     11.6
  M74     9.4     14.4         M105   9.3     12.1
  M77     8.9     13.2         M106   8.4     13.8
  M81     6.9     13.0         M108 10.0     13.0
  M82     8.4     12.8         M109   9.8     13.5
  M83     7.6     13.2         M110   8.1     13.9
  M84     9.1     12.3
  M85     9.1     13.0
  M86     8.9     13.9

As you can see there are a few galaxies that have a surface brightness greater than 14 magnitudes per square. In almost any size telescope these can be difficult to see under less than ideal conditions.

__________

Limiting Magnitude
This is the top limit of the magnitude or brightness of the faintest star that can be seen with a telescope. Magnitude is limited by the aperture of the objective lens or primary mirror of the telescope.. The larger the number, the fainter the star that can be seen.
This number does not take into consideration light loss within the telescope, the seeing conditions, observers vision characteristics (age), light pollution, etc..
The limiting magnitudes specified by telescope manufacturers, assume Ideal observing conditions and an experience observer.. (very dark skies, experience observers, excellent atmospheric transparency) . Under most average observing situations this limiting magnitude is rarely obtainable.
For the purpose of photography. Since film, CCD, or CMOS chips are more sensitive and more efficient at gathering light photons, The limiting magnitude will be more than the visual magnitude. Usually by a factor of two magnitudes difference...Although once again Light pollution is a big factor in astrophotography. The key to good images or photos is clear dark skies..

THE ATMOSPHERE
When a  nice breezy day turns into a nice clear dark night, with stars shinning brightly. We have a night that we can safely say is a night with high transparency. Clear dark sky and high contrast are ideal conditions for viewing the many objects visible in theNight sky. Objects such as distant galaxies, nebulae, and cluster of stars, the planets and more.. Unfortunately  we don’t always have these perfect conditions.. The atmosphere is often full of air turbulence. These nights we say have poor seeing. A small telescope or even the naked eye shows stars twinkling in the night,  but larger apertures telescopes will average out this twinkling giving us a more steady point of light. Twinkling stars are often the sign of a turbulent atmosphere.  

Another factor we must all consider and deal with is light pollution combined with haze from industrial air pollution throughout the atmosphere.. The best thing for an observer to do is to pack the car and travel to a remote area of country where the skies are darkest. For this reason portable telescopes of a variety of sizes, are becoming more common place among amateur astronomers.

Transparency:

The measurement of how dark the sky is on any given night. Transparency can be affected by humidity and dust and pollution in the atmosphere, as well as the amount of light pollution.
After allowing your eyes to become dark adapt. If you can see the faint outline of the milky way, and the four brighter stars in the bowl of the little dipper. In order of magnitude they are 2.2, 3.1, 4.3, and 5.0 being the faintest., You can safely say the transparency is rated as a 5. You are probably at a good dark sky location.. You should be able to use a 10”  telescope with no significant effects of light pollution..
If you could only see the Magnitude 4.3 star then your skies would only be down rated to a transparency of 4..Then you will probably be more limited observing with as small telescopes such as a 5” or 6”.  When a sky can only be rated as a 4 Transparency, then the condition can only be considered fair for observing DSO’s. At a transparency of 4 or lower, finding and observing objects will be more difficult..

A common way you can determine the transparency of your skies is to get a star chart.. With the use of the chart, Identify the faintest star you can easily see without using averted vision.. After allowing your eyes time to become dark adapt..  What ever that faintest stars magnitude is, will be the transparency of your skies.. If you can see a star of a magnitude of 6.4 then your transparency is a 6. This would be considered good to excellent transparency for telescope observation..

Seeing:
Is a term used in astronomy that describes the clarity that celestial objects can be observed. Seeing is one of the biggest problems faced by earth based telescopes. The atmospheric conditions are the primary determining factor of the seeing.  Most common and obvious phenomenon is when there is star twinkling or fluctuating.  This is also called by most astronomers as scintillation. This condition is caused by thermal motion of the air, which swirls in the different layers of air. This motion or turbulence in the motion of air is caused by differences in temperature and density. This turbulence creates very small alterations or deflections in the path the light from a star takes to your eyes.. Because different air densities will cause light rays to bend by different amounts. The closer a star is to the horizon the more noticeable this Twinkling will be. This is because the light has to travel through more of the Earths atmosphere. When you spot the planets such as Saturn and Jupiter, Since they are a disk rather than a point of light like a star. They usually do not twinkle, but undulations / wave like motions across its surface can be seen. Especially when closer to the horizon.  Another reason is due to the fact that the atmosphere is much denser at the bottom (Surface Level) than at the top.  Allowing for a more continual bending of light rays from stars. As you move closer to Zenith (Straight overhead)  The less bending of light rays so then less twinkling of stars is obvious.
The effects of the atmosphere can causes the sun or moon to appear elliptical when it is rising or setting because its bottom edge is raised more by the refraction of the atmosphere than its top.

Some of the best place to observe from are on mountain tops,  above the denser more turbulent layers of atmosphere. Also above and way from light pollution..

Below is a commonly used scale to judge the seeing conditions the sky is at your current time and location.

This scale is called the Pickering Scale, Named by Harvard Observatory's William H. Pickering (1858-1938). A 5-inch refractor was used to develop this scale. He noted the diffraction patterns will have to be modified for larger or smaller instruments. Although, this is still a good starting point for most amateur astronomers 
P1 worst to p10 the best.
p1.) The image of the star is usually twice the diameter of the third diffraction ring if the ring could be seen
p2.) Image occasionally twice the diameter of the third ring
p3.) Image about the same diameter as the third ring, and brighter at the center.
p4.) The central Airy diffraction disk often visible; arcs of diffraction rings sometimes seen on brighter stars.
p5.) Airy disk always visible; arcs frequently seen on brighter stars.
p6.) Airy disk always visible; short arcs constantly seen.
p7.) Disk sometimes sharply defined; diffraction rings seen as long arcs or complete circles.
p8.) Disk always sharply defined rings seen as long arcs or complete circles, but always in motion.
p9.) The inner diffraction ring is stationary. Outer rings momentarily stationary.
p10.) The complete diffraction pattern is stationary.


Atmospheric Prismatic Dispersion:
Most people who have observed the night sky has seen bright stars low to the horizon that appear to be glittering with blue fringe on top and a red fringe on the bottom. The bright star Sirius is a good example of this effect. When we see the light from stars in the night sky, that light is passing through the Earth's atmosphere where it is refracted, and the different wavelengths are dispersed. This is called atmospheric prismatic dispersion

Whenever light travels through one medium, such as air, water, or different types of glass, it slows down. For each different medium the speed of light is different. When light travels from one medium to another, it bends and changes direction as its speed changes. This is called refraction.

Light refraction causes celestial objects to appear as though they change in their apparent positions. Such as when you observe the Sun breaking over the horizon at Sunrise, it is actually about one solar diameter below the horizon, However, because the light is refracted, by the atmosphere, it appears above the horizon before it actually is. In other words when you look at a star in the night sky it is not actaully where it appears to be.

Different (colors) wavelengths of light bend differently, this is the dispersion effect. Longer red wavelengths don't slow down as much and bend less than the others. On the other hand the blue (shorter) wavelengths slow the greatest, and bend the most. Therefore you will see the color fringing at different points on an object as it is flickering in the night sky.. Usually blue on top an red on the bottom.. The lower to the horizon the more noticeable this effect is. The bright star Sirius and the planet Venus are always good for observing this effect/phenomenon. Coupled with unstable atmospheric seeing, bright stars such as Sirius will have the appearance of flickering and fluctuating colors between blue and red.

Optical Effects:

Airy Disc
Due to the wave nature of light, light passing through apertures it is diffracted, and the diffraction increases with decreasing aperture size.
The resulting diffraction pattern of a uniformly illuminated circular aperture has a bright region in the center, known as the Airy Disc, which is surrounded by concentric rings. The diameter of this disc is related to the wavelength of the illuminating light and the f-number of the circular aperture. The angle from the center at which the first minima occurs is

(CA) Chromatic aberration
This optical aberration is common in Achromatic refractor telescopes.
CA, common in telescopes is color distortion around the image produced by a lens, caused by the inability of the lens to bring the various colors of light to focus at a single point.  More pronounced on brighter objects and at higher magnification. Also more pronounce in  refractors with short focal lengths.. Also known as chromatism.

Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index with the different wavelengths of light. Since the focal length of a lens is dependent on the refractive index,  different wavelengths of light will be focused on different positions.  Chromatic aberration is noticeable as "fringes or halo" of color around the image, (Often the color violet), because each color in the light spectrum cannot be focused at a single common point on the optical axis. Although, there is a point called the circle of least confusion, where this effect can be minimized.

There are a few other ways to reduce the effects of Chromatic aberration. One is through the use of an achromatic doublet or achromat in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. By combining more than two lenses of different chemical composition, the amount of correction can be further increased. These are the more expensive (APO) Apochromatic refractor telescopes..

Many types of glass have been developed and used to reduce chromatic aberration, most notably, glasses containing fluorite. These hybrid glasses have a very low level of optical dispersion; only two compiled lenses made of these substances can yield a high level of correction.

Other ways to reduce the effect of chromatic aberration are, A minus violet or anti fringing filter. Or by the use of an aperture mask.. This is a cap that fit in place of the dust cover over the objective lens.. Thus in effect covering or shadowing the halo around the object. Especially useful on the moon Jupiter and Saturn.

Astigmatism:
A condition in which unequal curvatures along the different meridians in one or more of the refractive surfaces of the telescope an or Eyepiece cause the rays from a light source not to be focused at a single point on th axis of the focal plane. 

Coma:
An optical defect in a mirror or len ( more common in reflector telescopes) in which in-focus star images appear progressively more triangular or have a comet-like tail the closer they get to the edge of the field of view. The faster the focal ratio, the more prominent the coma.
The visually coma-free field of a telescope in millimeters is roughly equal to the square of the scope's focal ratio - For example, an f/5 focal ratio telescope has a 25mm field (5 squared = 25).
Since the internal diameter of a 1.25" eyepiece barrel is only about 29mm, and a 35mm film negative or slide measures 44mm across its diagonal, It will be much more noticeable that a 25mm coma-free field is more apparent in photos than it would be in visual observing.
Coma can superficially appear similar to a star's image in a poorly collimated telescope. With coma, however, the brightest portion of the comatic wedge (actually the Airy disk) always points toward the center of the field. This differs from an out-of-collimation telescope, where the Airy disks are all offset to the same side of the diffraction rings, no matter where in the field the star image is located.

Curvature of Field or Field Curvature:
An optical defect in which objects at the edge of the field of view can't be brought into sharp focus at the same time as objects in the center, and vice versa. More common with mirrored telescopes. Especially noticeable when the alignment of the mirrors (collimation) is incorrect.

Spherical aberration:(dioptric aberration)
The (astigmatic) out of fucused distorting of an image from the light around the margin of a lens or mirror with a spherical surface comes to a shorter focus than light from the central portion. The changing focal length is caused by deviations in the lens or mirror surface from a true sphere.

Ghosting
The appearance of one or more false images, This can be cause by something as simple as the reflection of your eye back onto the Eye Piece. Can be caused by hazy and unstable / turbulant conditions of the atmosphere.. Or something such as poor optics.. Poor coatings on the lens elements of the EP,  Or misaligned optics. Often when using binoculars, if not adjusted to the width of your eyes you can see the ghosting of, or double images. This can also be caused in the binoculars if the optics are faulty and misaligned.


A skill to Observing the Night Sky and the Universe:

Many people are easily frustrated and discouraged when unable to find an object in the night sky.. There are reasons these objects are hard to find.. Although these objects are still within the grasp of the telescope you are using.. There are some tricks or techniques you can use that will help.

Note: There are many Books, Atlases, Charts, Maps and Computer software that can help make locating objects much faster and easier. There are also additional accessories you can purchase for your telescope such as higher quality eyepieces with better image quality and usually a wider apparent field of view. Then there are different finder scopes that will help significantly.
Some of the more popular finder scopes are Zero / 1x magnification reflex sights.  When you look through one these it is the same as looking at the night sky with the naked eye. The more common ones paint an illuminated red dot or  gaged circular target rings with a bullseye. This makes it much easier to recognize an area of the sky when using in cojunction with a star atlas/chart or software
However, since not every one has all that immediately available to them, there are the more common age old techniques to locate and find the objects of interest with the finder scope and accessories that are usually supplied as stock items with a telescope.

First and most important is good dark skies with excellent atmospheric seeing conditions. Atmospheric conditions can often make observing hard. Conditions such as air turbulence at different levels of the atmosphere due to changes in wind direction and temperature differences. Some other factors are the amount of moisture in the air.. More often than not we can’t always have those perfect conditions. More so in the summer months when the air is more hot and humid. When you look at the brighter stars and they remind you of a nursery rhyme, (Twinkle Twinkle Little Star) odds are good the conditions are turbulent. Areas to avoid would be those around houses and buildings. The roof tops can cause air turbulence due to temperature differences between the roof and surrounding air temperature. Other areas would be those surrounded by trees heavy in foliage. Especially any kind of pine tree. And areas with lots of black top pavement. These areas will store then radiate heat creating heat wave turbulence above and around them…Much like you see radiating off the pavement of a road on a hot summer day.

Observing objects low in the sky close to the horizon is not a good idea either. When you are looking through your telescope at objects closer to the horizon, you are looking through more of the Earths moist atmosphere. Like viewing an object at the bottom of a rippling pool of water.. You will also be looking through more of the air turbulence caused by differences in the air temperature of surrounding objects as mentioned above. The higher the object is in the sky, the better it will be for observing. You will be able to achieve better focus and resolve much more detail when high in the sky..

There is also a better time of night to be observing objects. Observing early in the evening shortly after sunset the atmosphere is still unsettled and somewhat more turbulent in many cases.. The latter at night, closer to the early AM hours after midnight and prior to sunrise is better. This allows time for the atmosphere to stabilize allowing for better seeing conditions.. And of course we want to avoid as much light pollution and stray light as possible.. This leads me to my next tip..

Even during daylight hours, when ever out doors take care of your eyes. Avoid bright lights, and bright sunlight. Wear protective sun glasses when ever exposed to the sunlight.. Avoid alcohol, this will restrict the blood flow to your eyes reducing your ability to see good.. Avoid too much caffeine , too much can raise your blood pressure also effecting your vision. Not only that, it reduces your patience which is greatly needed for a good night of observing.. When ever indoors, also take care of your vision. To much time behind a television screen or computer monitor can be harmful to your eyes. It will also cause dry eyes making it hard on your vision. And again, avoid bright lighting. Other things to consider would be proper nutrition and vitamin supplements that can also help improve or at least maintain good vision.

When you are ready to go out and observe with your telescopes. Wait about 15 to 30 minutes allowing your eyes to become adapted to the dark skies.. Avoid any white light. If you need light, use red light..

Locating objects:
some are hard to find , some are easy and visible even to the unaided eye. Much easier when you have first taken the time to learn your way around the night sky. References are often a great assistance in learning the night sky and locating the many object in the universe. Books, Charts, Planisphere are good to have along with you at all times. You would want something you can take along that can handle the elements of the weather. In my area, it is often humid, items just laying around tend to gather dew.. A good pair of binoculars might be of help to assist in finding your way around. To help in pre-locating objects prior to using your telescope to find them.. A good detailed easy to read star chart or atlas will identify many objects of interest. These charts and atlases are of great assistance to help you identify and locate objects real easy. As long as you can identify the stars in the night sky.. The more detailed the chart or atlas the easier it will be to locate and identify the objects of interest..

Now it is time to decide what object you want to observe. Look it up and locate it on your chart or atlas. Look up at the sky and identify the area of where it should be.. Look at that area with your binoculars. You may be able to locate the object with them. This will also give you some star reference points to look at through your finder scope.. Aim your telescope using the finder scope at that area.. Start with a low power / longer focal length EP. If you have good charts and you are close to that point in space, and it is within the grasp of your telescope, all conditions allowing, you will find it easily.

Some objects are bright and easy to identify. Others are not, and are harder to locate. Objects with lower surface brightness are much less obvious and take time to find and focus your eyes on.. For those hard to find objects there is a couple things to do. First, keep observing that area and let your eyes adjust to looking through the EP, and the background FOV. Some times it takes a moment or two for the object to come into your eyes focus.. You will also use what is called averted vision.

If after a few moments the object does not come into focus then you probably have not properly located the area of the object.  First recheck your charts and verify the formation of stars in the area you want to look.. Then try to aim your telescope in the proper area once again.  If you still can not see the object, and you are sure you are close, then try a slow search pattern. I have used what I call a spiral or box type pattern.. Keep track of your point of origin (Starting point, A star or stars of reference.) Move just a little less than one FOV. Pause and observe that area for a few moments allowing your eyes to adjust while using the averted vision technique.. Repeat in a box or circular pattern as necessary until you locate the object. You should not have to go to far from your point of origin..

Some times it will take a little time to find and focus on some objects.. Patience will be required, do not get discouraged. It is not a race to see how many object you can find. But rather a rewarding challenge to find the object and observe it the best you can.. Even once you do find the object, take the time to study it for a while. See who much detail you can see in it.. Take notes or keep a record of your observing session, Some people even make drawings of what they observe.. Once you have found the object then change eye pieces to the next higher magnification. If you can and the conditions allow go to the highest magnification you can on the object..

The key to a good night of observing is:
Take care of your eyes day and night.. Learn the night sky, Take your time, relax and have patience.. Averted vision, Move slow in your search, One FOV stop Observe, Most importantly again “have Patience“. The rest is up to mother nature.  Good Stable seeing and dark skies will make your viewing / observing much easier and better. Poor seeing conditions, humid hazy skies, or ground fog can make it hard on your observing..  Illumination from the moon or light pollution from artificial lighting can hinder your observations and make it hard to see many objects in the night sky..


Enjoy

Have A Nice __________