For those of you with single-lens reflex cameras sitting on your shelves, here's some breaking news — 35mm astrophotography is not dead yet.
|If you listen to many advanced amateurs (and vendors), it seems you can't take a good astrophoto without an expensive CCD camera. This is simply not true. For a century and a half, astronomers — amateur and professional alike — used photographic emulsions to produce some outstanding images. And it still happens. If you are looking to capture the beauty of the heavens, don't discount film.|
A 35mm camera, tripod, and cable release are basic equipment.
Photo by David Healy
If you are planning to do film astrophotography, selecting a camera is quite simple. Choose a camera with the capability to hold open its shutter for extended periods of time. The shutter control must be of the manual type rather than the electronic or fully automatic variety. Most new 35mm cameras have electronic shutter controls, which make them unsuitable for long-exposure photography because the open shutter imparts a constant drain on the camera's battery. The small batteries cannot withstand long periods of holding the shutter open and will go dead quickly. With a manual shutter control, a cable release (with locking thumbscrew), and the camera's shutter speed set to the "B" (bulb) position, the shutter can be opened and held open indefinitely.
A second important feature to look for in a camera is "mirror lockup." When the shutter button on a camera is depressed, two things happen quickly.
First, the mirror flips upward to allow the light entering the lens to get to the film, and second, the shutter opens. The motion of the mirror and its rapid, sudden stop sets up a vibration in the camera that often causes blurred images on the photograph. The best camera for astrophotography is one that has the capability to "lock up" its mirror before the shutter is opened. This allows the vibrations to settle out before the film is exposed.
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A third consideration is to obtain a camera with interchangeable focus screens and the availability of screens that are clear or bright. Many budding astrophotographers have tried to focus on a star with a normal focus screen (sometimes referred to as a diamond or split image screen) — a very difficult feat. For astrophotography, a clear focusing screen is something you must have.
|Field of view|
Knowing a lens's field of view is important in order to gauge how many degrees of sky an image will cover. Use this information along with a star chart to determine which constellations or objects will fit on the film.
The formula for determining field of view is x=d(57.3/fl) where x represents film coverage in degrees, fl is the focal length in millimeters, and d is film dimension in millimeters. Remembering that the frame dimensions for 35mm film are 24mm vertically and 36mm horizontally, we arrive at the table below. Total area is given in square degrees.
|Lens field of view in degrees|
Bob and Janice Fera created this image with an 11-inch Celestron Schmidt-Cassegrain telescope at f/10 using hypersensitized Kodak Pro 400 PPF film. Three exposures of 90, 120, and 120 minutes were made and later electronically combined. The pictures were taken at Mt. Pinos, California on September 18, 1998.
Photo by Bob and Janice Fera
The best films for astrophotography are professional films, which means they're available only in the largest cities and they're more expensive than common film. It's also difficult to find a camera store that is willing to break open a package and sell you a single roll.
Before we start considering individual films, let's take a look at a problem encountered in long-exposure astrophotography. Take two exposures through the same camera and with the same film, the first a 1-second exposure at f/1.4 and the second a 128-second exposure at f/16. In a perfect world, both exposures would look the same; the bright and dark areas on one negative would be repeated on the other. Unfortunately, we must deal with a property of film called "reciprocity failure," which affects film speed. Film speeds are given by numbers such as "64" and "400." Below 100, films are classified as "slow," and above 100, films are said to be "fast."
Because of reciprocity failure, a 400-speed film will, after several minutes, act like a 100-speed film, or an even slower film. The popular black-and-white film Tri-X is a good example. For short exposures, this film has a speed of 400. However, if you expose it for an hour (not an uncommon occurrence for astrophotography), its effective speed drops to 10! Because a great deal of astrophotography involves lengthy time exposures, this is a real problem for amateur astronomers. We either have to live with the effects of reciprocity failure or try to get around it. One way to minimize reciprocity failure is to employ a technique known as "hypersensitization."
Hypersensitization is a film-treatment process that reduces reciprocity failure. This process replaces moisture and impurities in the film with a gas by pressure-cooking it into the film, making the film more resistant to reciprocity failure. The gas, by the way, is a combination of 92 percent nitrogen and 8 percent hydrogen and is called "forming gas." When hypersensitization was discovered, the gas used was pure hydrogen, which is quite dangerous. Forming gas, however, is inflammable, although some care still must be used in handling the pressurized containers.
The technique of gas hypering involves putting the film in an airtight canister, pumping the chamber interior to a very deep vacuum, purging the interior with forming gas, and revacuuming for two or three cycles. The chamber is then pressurized with forming gas at about 3 to 6 pounds per square inch and left to "bake" at about 100° Fahrenheit (38° Celsius) for a period from 4 hours up to 100 hours, depending on the baking pressure, temperature, and film type used.
Hypered film has a very short period during which it's usable. After the film comes out of the hypering tank, it's best to use it and develop it immediately. Some amateurs have had success sealing hypered film in airtight containers and freezing it until they're ready to use it, but even then, its shelf life is less than a week. The main enemies of hypersensitized film are heat and moisture. Developing hypered film can be done commercially at photo labs based on the film type. No special processing is required.
To be useful for astrophotography, black-and-white film must have low reciprocity failure and a good degree of red sensitivity. As a film undergoes a long exposure, it often loses sensitivity in the red region of the spectrum. Red is best for recording nebulosity, and if a film is not responsive in this area, your exposure won't be as good as one taken with a film with high red response. Because of this requirement, we immediately run into problems with most black-and-white emulsions.
The best film for black-and-white astrophotography is Kodak Technical Pan (TP-2415), preferably hypersensitized. This film is available in various formats, including 35mm. It has good response to red light, has high resolution, and is inexpensive. Regarding its resolution, Technical Pan film can resolve an amazing 320 lines/millimeter. Compare this to the resolution of Tri-X at 80 lines/mm. Another feature of Technical Pan film is that you can vary the image contrast by using different developers.
This 15-minute, unguided exposure was captured onto Kodak Elitechrome 200 slide film by a Minolta SRT101 camera with a 16mm lens at f/2.8 mounted on battery-driven, motorized camera mount. The image was digitized using a CanoscanFS2710 and auto-contrast adjusted using Photoshop6 to bring the image back as close as possible to the original.
Photo by Russell Cockman
Slides or prints? Really, it's your choice. I've seen terrific images made on both slide and print film. I prefer slides because they lend themselves easily to illustrated talks. For displaying your images on a computer, only a scanner that accommodates negatives or slides is required.
Unlike the one great black-and-white film choice astrophotographers have, many good color films exist. Most color films are well balanced throughout the entire spectrum and have good red sensitivity. If you hear other astrophotographers talking about an emulsion you haven't tried, my suggestion is to purchase a roll and shoot.
Film processing. Process black-and-white film at home. Send color film to a lab. If you are going to shoot black-and-white film after reading this, it probably will be Kodak Technical Pan 2415. This is the best ever for black-and-white astrophotography, but few photo labs are equipped to handle it to the specifications you are going to want. If you cannot — or do not wish to — set up a home darkroom, you can load film into a developing tank using a photographic changing bag. Such a device is small and, if used properly in a darkened room, will keep out ambient light that would fog your film.
Regarding color film (negatives or slides), I can make one important suggestion: Have all your rolls of film processed and returned to you uncut. Many amateur astrophotographers learn this lesson the hard way. Automatic slide mounting machines are poor judges as to where an astrophoto (especially of a faint object) starts and ends, and many good shots have been ruined by the slide being cut in the wrong spot. Once you have the uncut film in hand, you can cut and mount the slides in either plastic or glass slide mounts. Another suggestion I will make is to choose a processing lab that has some flexibility to allow push processing (developing the film for a longer time to increase its speed) your film, if you desire it. Some "one-hour" labs are locked into processing at a film's standard speed.
Image processing. Whether you shoot slide or negative film, you probably will want to transfer those images to your computer and maybe even onto the Internet. This transfer process offers an opportunity to enhance — often dramatically — your images. The processing end of astrophotography can make the difference between an okay shot and an excellent one.
The first step is to scan your images into your computer. It is preferable to use a negative scanner for print and slide film. Once you've scanned your images, software exists to enhance them. One popular image enhancement program is Adobe Photoshop. Another choice is JASC's popular software Paint Shop Pro. Check these and other software for the features you want at a price that makes you comfortable.
Tripod-mounted photographs. The equipment necessary for a tripod-mounted astrophotograph is minimal: A camera, lens, tripod, cable release (preferably with a lock), and a watch with a second hand (or a stopwatch). Focus the camera on infinity and lock open the shutter. Sometimes, astrophotographers include the horizon or an object in the foreground like a tree or mountain for a dramatic effect. It's really up to you. If star trails are your goal, try numerous exposures. And remember, long-exposure astrophotography requires a dark sky because light pollution fogs film.
Be sure to purchase a good tripod. This is an important photographic accessory. It's amazing how many people try to save money and end up with a shaky, inadequate tripod they eventually have to replace. A good tripod will work with any camera, so buy the best and strongest tripod you can afford.
Make sure the tripod you select has easy-to-operate controls that can be worked with gloves on in the winter. Ensure the tilt and pan movement locks are strong and will hold a front-heavy telephoto lens. A sturdy tripod also will allow you to mount two (or even more) cameras onto it at the same time using a bar or a plate. Finally, make sure the tripod you buy will point to the zenith.
The next necessity for tripod-mounted shots is a quality cable release. This item ensures that vibrations caused by your hand triggering the camera shutter don't ruin exposures. A cable release should be at least 18 inches in length to isolate the camera from the operator's motions. Several types of cable release locking mechanisms exist — pick one and use it.
Okay, you've mounted your camera to a tripod and pointed the lens at the sky. But you don't want to see "trailed" stars in your finished picture. Is there a way to figure out how long an exposure you can take without trailing? Yes, there is, and it can be determined with this formula x = 1000/fl*(cos d) where x is the exposure time in seconds, fl is the focal length in millimeters, and d is the declination of object, in degrees. Declination (d) is a factor because the closer you get to either Celestial Pole, the smaller the star trails are per unit time. Star trails are greatest when your camera is pointed at the equator.
Using this formula, the values in the table below were obtained.
|Maximum unguided exposure in seconds|
|declination||focal length of camera lens|
|(north or south)||28mm||35mm||50mm||135mm|
When setting the f-stop of your camera lens (how open the lens diaphragm is), remember these two words: wide open. Many astrophotographers stop down the lens (increase the f-stop) one or two clicks because the star images at the edges of the photograph are not pinpoints (due to lens irregularities). Rather than lose that light, which you will need if there are extended objects in the field (stars are pinpoint objects and are not affected by f-stop), shoot with your lens wide open and then either electronically crop the image or mask the slide with black photographic masking tape, which is available at photo-supply shops.
Piggyback astrophotography This photographic arrangement allows you to shoot with your camera while it is attached, or "piggybacked," to your telescope. Sometimes this is referred to as "guided wide-field astrophotography." By the way, you don't need a telescope to shoot piggybacked images. I often remove the optical tube assembly and simply use the mount and drive to guide the camera. Piggybacking allows you to take longer exposures without having to worry about the stars making "trails" on the film.
An 8-inch SCT rigged for piggyback photography is silhouetted against an evening sky.
Photo by David Healy
To do basic piggyback astrophotography, you'll need everything listed for tripod-mounted astrophotography except the tripod. In its place is an equatorially driven telescope or, at least, the drive. One more item you'll need is a way to attach the camera to the telescope. Use a camera-mounting bracket for this, either one commercially made or a mount you've built. Just be sure it can hold the camera securely.
Make certain you balance your telescope (or mount) after you've attached the camera and aimed it. A telescope's balance can be made optimum only for certain positions (because of all the extra weight hanging off the telescope at strange angles). Some astrophotographers slightly off-balance their scopes to the west because it ensures a good mesh of the telescope's right ascension axis drive gears.
Exposure times for piggyback astrophotography vary greatly. Sky conditions, film speed, accuracy of polar alignment, and drive accuracy all come into play. You also have to consider the focal length of the camera lens you're using. If it's a "short" lens (50mm or less), you probably can expose the film for 15-20 minutes. Longer lenses will magnify the errors in your alignment and drive. Shoot a test roll of film to gauge exposure times. Start with an exposure of 1 minute and work up to 30 minutes or so.
Astrophotography through the telescope. Mounting a camera with no lens to the focuser, essentially making the telescope the camera lens, produces "prime-focus photography." For this type of photography, you'll need (in addition to the equipment listed above) a T-adapter and T-ring for your camera, a guide scope or off-axis guider that contains an eyepiece with a crosshair pattern, and a corrector for the telescope's motor drive. The T-adapter is a machined tube, one end of which fits into the eyepiece holder. The other end is threaded to accept the T-ring. The other end of the T-ring has flanges exactly similar to a lens that couples to your 35mm camera.
This style of photography allows you to image deep-sky objects.
Photo by Astronomy.com
An advantage to using a separate guide scope rather than an assembly that uses the main telescope's optics to guide is that a suitable guide star can be found more easily because the guide scope can be aimed independently (within limits) of the imaging scope. And because both the main telescope and the guide scope are driven on the same mounting, any variance seen in the tracking of one also will be seen by the other.
The mounting of the guide scope to the main telescope should be of high mechanical quality to ensure perfect alignment with the imaging scope. Even the tiniest bit of flexing in either the guide scope optical tube or in the mounting to the imaging scope will cause a loss of alignment between the two. So, even if you believe your guiding is perfect, your final image may display trailed stars.
Into the eyepiece end of the guide scope goes a "reticle eyepiece." The two basic types of reticle eyepieces are illuminated and non-illuminated. Almost every astrophotographer I've met uses an illuminated reticle eyepiece. The illumination for the pattern comes from a light-emitting diode built into the eyepiece powered either by an internal battery or, more often, by external power.
Features to look for in a quality reticle eyepiece are variable brightness, blinking capability, and pattern. In my opinion, variable brightness is a necessity. An illuminated reticle eyepiece with no dimming capability will limit your guiding choices to only bright stars. The ability of an illuminated reticle to blink is important for some astrophotographers who find staring at an immobile crosshair pattern tiring to their eyes. Finally, choose a reticle pattern that works for you. Bulls-eye patterns or crosshairs are the most popular, especially the dual crosshair pattern which forms a small box in the center of the field of view. Movements of guide stars when framed in such a box that are generally easy to detect.
Unlike tripod-mounted and piggyback astrophotography, in through-the-telescope astrophotography, you cannot just focus a telescope on infinity, as you can with a camera. You must focus through the scope. The best way to do this is with the camera mounted on the telescope and a clear focusing screen in the camera. Pick a 2nd-magnitude star and make it as much a pinpoint as possible. The star may appear as a pinpoint throughout a range of the focusing knob's rotation. Target the middle of that range.
After you center an object in the camera's viewfinder, choose a nearby guide star. If you have an off-axis guider, rotate it while looking through the reticle until a suitable star comes into view. If no suitable guide star is near the object, you may have to move the object away from the center of the camera's field of view to help you locate one. Guiding is the key to producing a good image. All great prime-focus shots are guided either manually using a reticle eyepiece or automatically with a CCD autoguider.
|Eyepiece-projection photography. With eyepiece-projection photography, the image is projected on the film through the telescope's eyepiece. This type of astrophotography is used mainly for high-powered shots of the Moon and planets. The crucial piece of equipment is the eyepiece-projection holder.|
Because of the high magnifications used (which magnify any tracking error), eyepiece-projection photography is the most challenging of all types of astrophotography. The optics must be high quality, and the telescope must be vibration free and in perfect focus for this technique to work. Excellent seeing is the final requirement. Because the possibility of drive errors increase after about 30 seconds, exposures no longer than this should be taken.
|All-sky astrophotography. If your budget is large, an 8mm or 6mm camera lens will provide truly spectacular images of the sky. You can use such a lens either tripod-mounted or piggybacked.|
The inexpensive alternative is to suspend a camera above a spherical mirror. A spherical mirror with a diameter 1.4 times that of its radius of curvature will reflect the entire sky. If the sphere segment is any smaller, less than 180° of sky will be reflected toward the camera.
Many amateur astrophotographers have built all-sky camera setups. The most common method is to attach the camera to a mounting plate on an assembly that has three or four support struts. The assembly then is suspended over the mirror. A single strut holding the camera/mounting plate can used, but the strut must be made of thick metal to prevent flexure.
Considerations in all-sky setups are obtaining the correct distance between the camera and mirror, painting the mounting hardware black, and achieving focus. Most amateurs shooting all-sky images make the camera support plate round so the occulted spot in the center of the image matches the shape of the entire image.
Focusing an all-sky camera is much easier to do in the daytime than at night. A sky full of broken clouds (rather than a totally clear one) is best for focusing. Once you have the camera focused, secure the focusing ring (generally with tape) so it cannot move. You'll have to make a compromise in focus, as points on the spherical mirror's surface are at different distances from the lens. Settle on a focus where most of the image is reasonably sharp. And remember to use a fast film.
Finally, keep two things in mind about all-sky astrophotography: First, you'll be in the field of view because of the coverage of the lens/mirror combination. Second, the final image on film will be reversed. A reversed image is no problem if you're shooting slides, but if you're using print film, be sure to print (or ask your photo lab to print) the negative backward.
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