Quote:
Originally Posted by RickS
That's very topical, Simon. There's another discussion about whether there's any benefit in taking more than 30 subs.
The answer is that you'll collect the same number of photons with 600 x 6 seconds or 6 x 600 seconds but you have to consider the effect of read noise. If you have a very low read noise camera and 6 second subs are sky limited (i.e. the effect of read noise is negligible) then you'll get the same final SNR either way. If you have higher read noise and the short subs are read noise limited, then you'll get lower SNR from the many short subs.
One of the best things you can do when imaging is figure out what exposure time is needed to get sky limited. There's a thread by Shiraz which describes a clever method for doing this:
http://www.iceinspace.com.au/forum/s...d.php?t=117010
The attached graph shows the effect of read noise on overall SNR.
Cheers,
Rick. |
Thanks Rick for that answer. So if I've got a very quite camera I can get away with short exposures and conversely a noisy camera needs longer exposures as the pixels that are noisy get filled and stay filled, with a longer exposure they can only be filled once, but if you use a short exposure on a noisy camera when you combine the frames there will be noise on noise on noise at that pixel and it will blow out? Now this comes to mind if the bad pixel gets filled will it bleed over to a good pixel? Also what is considered a long exposure? I can only reasonably get 300 sec before some trailing appears is that a 'long' exposure time? To me 1200 sec and above start to be 'long'.
So the options is to get an expensive quite camera and use it on a mediocre mount or a dslr on and expensive mount of course the ultimate is a quite camera on an expensive mount!