So I did a quick test last night while trying to collect more data on Andromeda Galaxy. I continued most of the night with my standard 5 minutes (300seconds) at ISO800 for most of the imaging session then jumped up 20sec subs at ISO8000. The purpose of this was to see how much extra noise is introduced by the higher ISO setting as the SNR (signal to noise ratio) is basically what you want to maximize in any image. However, mount tracking and guiding is also a factor, tracking 20 seconds is pretty easy, whereas 5 minutes presents a bit more challenge – now for my mount/guider combination I’m confident I can guide for as long as I would like so that isn’t a huge worry but shorter subs does give a higher degree of confidence.
So why do it? Planes/Satellites etc that pass through a sub and hit the “target”, basically when you have twelve 5 minute subs and an airplane flys through 1 or 2 of them, that’s a lot of data lost. That’s the main reason I don’t do 1.5 hour sub s at L1.0 setting on the Nikon (which I tested and is the limit before the sky background really starts to rise).
Now this is a “worse case” scenario because with the images shown there are 4 things to note:
1. This is just stacked LIGHT frames (no darks/flats/bias etc)
2. This is fifteen 20sec subs vs. a single 5 minute
3. Its summer, so the camera is operating as warm as it would in any imaging session. I tried one night at +25C but that was a mess as I was sweating and dew was awful.
4. I picked ISO8000 on a whim
Post Processing steps were also pretty simple, I mainly just stacked the images as required and applied a background correction and stretch. Its not perfect and its not suppose to be, it just enough processing to show what side by side comparable images would be for 5 minutes of imaging at iso800 vs iso8000.
At the End of the Day I’m going to use all the subs (minus the direct plane streaked ones) in my new and improved Andromeda Galaxy image with proper flats bias etc. which you can see here: Andromeda 2.0