star trail

hi guys





recently i went to a trip and i had the chance to try my first Star trail picture.i had my 40D on the tripod,set the aperture to f 4 on my tokina 11-16mm lens and kept the shutter open for 40 minutes.i am happy with the result, being just a test shoot but my concern is that it took my 40D almost as long as 40-50minutes to process the shot.is this normal?it was set on jpeg+raw but i don't think this is the problem.my battery almost died during this.am i doing something wrong?can anyone tell me please?





thank you





qatarezu

As far as I know if you had the noise reduction feature turned on then it takes the same amount of time as the exposure to process the image in camera.


thanks


joel

Post an example of the picture...we'd love to see it

thank you joel, you are of course right.





thank you

i don;t have it with me now, but i will post it soon

Adrian,


a second thing you can try, and it gives a less noisy image, is image stacking.


1: Determine your single exp time, say f16 @ 30 min.


2: Break the exposuer into smaller segments, f16@ 7 min or so


3: merge usingphotoshop/picture window pro/gimp or 3rd party stacking tools. This is NOT HDR merging, but stacking the images ontop of another, lightening the image and showing the star trails.


http://www.tawbaware.com/maxlyons/cgi-bin/image.pl?gallery=8is an example of stacking images.

thank you very much, i'll try this soon, seems like a better way to do it

Ehcalum, do you use image stacking? If so how do you get one photo to start right after the last one ended, do you make it some kind of atime-lapseor something?

Does it harm the CMOS sensor doing such long time exposure? Excessive heat generated?

I don't believe stacking in the way you describe will give less noise. Comparing stacking to single exposure, photon noise is exactly the same and read noise is worse. I believe thermal noise is almost the same (a little worse with stacking because of hot pixels which saturate in the long exposure).


Stacking, eg 20 10 min exposures will give less noise than a single 10 min exposure, but is actually worse than a single 200 min exposure.


Unless, of course, I'm wrong. [:)]


Stacking has other advantages, though.

Tim,


I have a homemade involameter. You can just use a watch or a stopwatch. Generally you want to start the next exposuer within a few seconds of the previous one.

i have one more question regarding this subject...


what can i do if i want to take a 5-6 hours shot? i just bought one grip but i don't think my batteries will last that long.any ac-adapter to work with my 40D? any ideeas?





thank you

I know this is a digital forum but from what I have read of film, it seems to be a better choice for a 5-6 hour shot based on no sensor to heat up. No I dont have experience with this, so I could be completely wrong.


If film would be better it would give me a reason to get the EOS 1V and try out some 5-6 hour exposures [:)].


But for an adapter for the 40D, I found a website with some info on that http://www.astronomiser.co.uk/canonpower.htm


Thank


Joel