Star trails and long exposures (60min+)

[qwibbled]

hi, i have a 30d and have taken quite a few long exposures with it but never anything over 10 minutes. i've always gotten great results but the other day i tried a two hour exposure and it turned out horriblly. i had tons of noise and a blue glow in the bottom right hand side of my shot. before taking the two hour shot i took an 8min one at iso200, f8. it was properly exposed so i changed the exposure settings to iso100 and f22 to obtain the same exposure with a two hour shutter. i assumed everything would remain the same except the star trails would be longer. is this not a valid assumption? i also took a dark frame and tried a subtraction but that actually made it look worse, although its possible my dark frame exposure was considerably longer than two hours. i just let it go until the battery died..

i originally posted this question in the general forum and was pointed here, someone mentionned that stacking a dozen 10 minute exposures would probably work better. typically how are these long star trail shots taken? by stacking or really long exposure? and if its the latter, how come the results are so much nicer than mine?

[bowtie`]

Well it would help a bit if we could see said offending image. Failing that, I have quite a number of questions for you. What lens were you using? If using a zoom, what was youe focal length? Was the image taken inside city limits or out in a dark rural site? Close to a house or out in the middle of a field? Were there people running around with flashlights or a gon-fire possibly? Was the camera set up close to a white car, building or other structure?

You say you have good luck with 10 minute exposures, maybe you should slowly work your way up to something like 2 hours. Maybe taking it 10 to 15 minutes more each increment. That way you can tell what you need or where the problems are occurring. There is something called sky-fog that eventually will saturate an exposure. Maybe you hit the sky-fog limit.

The longest I have done so far is only 41 minutes. I posted the shot here, http://photography-on-the.net/forum/...03&postcount=7

So I have no experience at over an hour, yet.

[qwibbled]

heres a zip with my original and dark frame raw files, 15mb

http://www.sendspace.com/file/cjcbrq

[Celestron]

First off you should never let the exposer go until the battery is dead . When this happens you loose some info needed and it may loose any or all data . Second if your f/stop is too high you can't collect the correct amount of data needed for an image . Noise is a factor with all digitals at some point or another . Collecting noise in night shots is a comon factor . Thirdly you may have had the ISO set too low . I know low ISO is for very bright objects like the moon and planets but this also means longer exposers . I would suggest backing off to short exposers and stacking them . Stacking eliminates alot of the noise and helps keep down LP . If your shooting anywhere close to a city your going to have LP no matter what you do . Like bowtie mentions , post a image and show your example .

[qwibbled]

so when you stack dozens or hundreds of 30 second exposures, does the brightness of the final image increase as you stack more and more exposures or does it always remain the same brightness/exposure of a single 30 second shot?

[Celestron]

Originally Posted by qwibbled

so when you stack dozens or hundreds of 30 second exposures, does the brightness of the final image increase as you stack more and more exposures or does it always remain the same brightness/exposure of a single 30 second shot?

If you collected enough signal detail then the stacking will improve your image overall and make it easier to PP . Also it helps to illiminate noise but not all of it .

[le_R]

Yes stacking 1 or 2 minutes frames is much better than 60+min single frames.

First of all, you'll get a better signal to noise ratio.
then your shots are very less polluted by electronical noise (the strange glow in the bottom right) and thermal noise.

But of course with seconds or minutes shot for a star trail, you can't reach the same magnitude than a several hours shot.

[Adrena1in]

Originally Posted by le_R

But of course with seconds or minutes shot for a star trail, you can't reach the same magnitude than a several hours shot.

Is that true though? I admit that a longer exposure when tracking will reveal those lower magnitude stars, but whether you expose for 1-minute or 60-minutes, if the camera is static, because the stars are moving you'll get the same magnitude trails appearing on either picture, won't you? The only difference will be the increased noise on the 60-minute shot and the increased haze due to light-pollution and what not. (I do admit that going too short on the exposures will obviously limit the light gathered.)

For trails I will always shoot shorted exposures and stack. The noise even on a couple of 12-minute exposures I took the other night was appalling.