I 'm trying to understand the "light gain" of photography in astronomy.  If you can see down to 13th magnitude with your naked eye, what can a 10 minute exposure on film register?  I also want to try tripod photography:  a 35 mm camera with a cable release on a tripod.  I understand that exposures of 1 to 30 seconds should capture most constellations --- my problem is that this makes no sense!

Photographic film is properly exposed when the amount of light reaching it is 100/ISO times .008 to .8 lux-seconds. For example, a typical daylight scene bounces, say, 3000 lux to your camera.  If you're using ISO 100 film and set the camera at f/16 and 1/125 seconds (following the sunny 16 rule), then you get an exposure of .094 lux-seconds on your film --- a perfect exposure.

I gather that the light from a 0 magnitude star is about 2.7E-6 lux.  With my lens wide open that seems to mean exposure times of many hours, which is obviously wrong.  Is there something about point sources of light that throws off this kind of calculation?

FWIW, I've spent weeks on this question and looked at literally hundreds of Web  sites.  Maybe I just don't know how to ask the right questions.  Or maybe most informational Web sites assume that one is either totally ignorant of or totally at home with the material.  This is one of the few sites where amateurs can be advised by better-informed amateurs. 

Anyway, thanks in advance for any responses.

http://www.eskimo.com/~rachford/widefield/calc.html
http://www.answers.com/main/ntquery?s=reciprocity%20failure&gwp=16
http://www.company7.com/library/astforms.html

I don't know if they have what you are looking for or not, but that is all I could think of or find.


Have A Nice __________

tkerr wrote:

I don't know if they have what you are looking for or not, but that is all I could think of or find.

I'm glad you answered that Tim. The only thing I could think of was how many buttons the raincoat might have, and how easily they could be undone to flash someone.  

Tim,

Many thanks.  As always you’re a font of astronomical knowledge.

 I’ve saved those URLs and will study them later.  This will take some time.  I’ll let you know how it works out. 

 Kevin,

Okay, I’m naïve: so what raincoat are you talking about?  Are you under the impression that either my question or Tim’s response is a joke?  What am I missing?

Spacer wrote:

Kevin, Okay, I’m naïve: so what raincoat are you talking about?  Are you under the impression that either my question or Tim’s response is a joke?  What am I missing?

Hi Spacer, my comment was suppose to be the joke. "Exposure" and open raincoats, metaphorically speaking -  go hand in hand. I was just trying to be funny, sorry you took it the wrong way.   I would never joke about someones question, or answers. Especially Tim's, he's too knowledgable about this stuff, and I'm too new to this to make him mad at me. I still need help now and then too.  

I apologize for not clarifying myself, and for offending you. Hope you understand now.  Clear skies! 

Kevin Bozard wrote:

Spacer wrote:  Kevin, Okay, I’m naïve: so what raincoat are you talking about?  Are you under the impression that either my question or Tim’s response is a joke?  What am I missing? Hi Spacer, my comment was suppose to be the joke. "Exposure" and open raincoats, metaphorically speaking -  go hand in hand. I was just trying to be funny, sorry you took it the wrong way.   I would never joke about someones question, or answers. Especially Tim's, he's too knowledgable about this stuff, and I'm too new to this to make him mad at me. I still need help now and then too.

Knowledgeable? No not really, I only have more time than the average person to look around on the internet.  With Books, Magazines and the Internet there are answers for just about everthing. Many people have other things to do and just don't have the time to search. If I have the time I will search for those answers, especially when it is something I also have an interest in.
It just so happens I was also curious about the differences in daytime and nightsky photography when I got started. So luckily rather than trying to remember everthing I bookmark them for future references because many of them are hard to find. Fortunately I only had to look as far as my own bookmarks for those links or I don't know if I could have found them again.   I almost forgot all about them.
I hope they helped


Get Mad, No.. I don't get mad..


Have A Nice _________

Kevin,

Relax, I wasn't offended, just puzzled.  I think my comment was poorly worded. Funny thing is I got it just now on re-reading the thread.   



 

No problem Spacer, I just didn't want you to think I was a snob, or something. I hate getting off on the wrong foot with people I don't know. All's cool. Have a great weekend, and clear skies!

hi,

ive been shooting for years with my 35mm, and have read a great deal many of books and sites but when it comes down to it just set the the camera on infinity and at the widest app. and test your times for exposure.  the variable that makes the difference seems to be the types of film used, b&w, slide, print, and they do come close to what all those books say but always a little off too

ive even had good success with my 300mm zoom lens, just watch the app. time again and a barn door tracker helps but know your latitude.

good luck.

 

If you want to estimate the required exposure time for stars (point sources), try:

t = ( 1.0 / I * A^2) * 100 ^ ( (M + 8)/5 );

were t is the required exposure time (sec)

I is the nominal film ISO, take reciprosity failure into consideration

A is the aperture in cm

M is the star's magnitude

example: mag 0 star using 100mm f/4 lens w/ ISO100 film (which isn't the best speed to use) would require:

t = ( 1.0 / 100 * 2.5^2) * 100 ^ (8/5) = 2.5 sec

If you are working with extended (resolved) objects, of any kind, using film OR CCD/CMOS, the focal ratio (aperture AND focal length) should be used for exposure time calculations (especially if you are attempting to achieve specific S/N, as with a linear electronic sensor).

Robert

Hmmm.

Problem is, you are trying to think of this in too scientific of terms.   Dump that lux stuff...has no place in astrophotography. 

Film is difficult to judge, simply because any computations you make fail after the shutter is open for a couple of seconds.   Reciprocity failure KILLS you ability to collect light over time.  Some film is better than others, but still, it's not worth trying to compute.

With film, your task is to find a film that has the least reciprocity failure.   Back when I did film, Kodak E200, Fuji Provia 400F, and hypered Techpan were the films of choice for that reason.   Personally, I felt E200 provided the best sensitivity in h-alpha, whereas the Provia had the best balance.   Techpan, while very sensitive, was too much trouble to shoot for me, since it required great care to prevent oxidation...plus, it was black and white.

But it goes to show something important....these are "slow" films by ISO standards.   You might think that ISO1000 or higher is preferred, but they weren't, simply because such film just didn't work after 30 seconds or so.   The ones I mentioned did quite fine, and one-hour long exposures were common.  Even so, 90% of the data was recorded in the first 30 minutes of the image.  

Thus, if you are trying to use formulae with ISO as a factor, you are wishing on the wrong star.

Whatever setup you have, you will find that 10 to 15 minute exposures are optimum.  Any more and you get diminishing returns.  Any less and you won't record the information you want.   

I had great success with piggyback film images using lenses between 300 and 420mm in the f/2.8 to f/4 range.   My exposures were using E200 for 30 to 60 minutes.    The only reason I could manage those exposure lengths were because my sky was very dark.   In most sky conditions, you'll likely just be wasting time above 10 minutes.   So, dark skies are critical for film work...which is yet another thing that you can't compute with a formula.

With CCDs, you can literally count photons...so the science is helpful.  But with film, the best advice I can give is just "let 'er rip."   

ill just add the frustration levels with film are gong to be extreme, along with the fact expensive- i was a diehard film buff- one of the last bunch hanging on the end of that rope until i got a DSLR, i always thought film would give a better resolution being no pixels involved- i was dead wrong, also the fact film lacks and kind of depth over digital- id use too buy and shoot 1 or 2 rolls night, pay for developing- then 95% chance they hit the trash can- as far as your exposure question goes since thats main question- i found 5 minutes max for me om film depending on conditions- anything over that was wasted, but 5 minutes of film for something deep- is like wasted time

Absolutely right, john.  I HATED the notion that I could have a night of productive work, go home, spend money to develop my shots, wait a day, and get back photos that would immediately get tossed away. 

Feedback is the key factor to success in this hobby...and immediate feedback helps VOLUMES.

If you want to get a little closer to a good expopsure, the first time, and certainly get it within a reasonable bracket, look up Barry Gordon's book on exposure. His suggestions will put you in the correct ballpark much faster than the WAG based approach, that some film shooters may suggest. That won't help the focus problems though. If you happen to have a lot of film laying around, and you have nothing better to do for several nights,....

Robert

if you aint figuire it out yet and if someone already mention this book, i just bought it today and i luck out that half-price books had a copy "Astrophotography for the Amateur " by micheal covington and from i read so far i know this book would be a great help to me since i just got a older meade DSI-C plus i have a old olympus 35mm with a remote cable and i am in the process of getting a t-ring and adapter for my telescope, so i am planning on using both for astrophotography.