No problem - looking forward to what you come up with! I'm here to help.Misange wrote:Thank you Ivo for your comments and your time. It is exactly what I needed to improve ! I did not knew if the problem was from my data set or from its development. Now I know Yes my flats were not correct and I will try longer exposures. I understand that the signal/noise ratio is greatly improved with longer exposures. What would you recommend: 2 minutes ? I am still confused about the ISO to use, even after a careful reading of this article http://dslr-astrophotography.com/iso-ds ... otography/ With my camera (nikon D5600) if I understand well it is of no use to increase the ISO above 200 which seems quite low to get a signal with a galaxy.
It might be that the sky will be clear here tonight. If so I will give a new try.
There is a lot of misinformation on how ISO settings work on digital cameras. Your sensor only has one sensitivity. It is fixed and cannot change. All the ISO (usually) does, is multiply (or divide) the signal in the digital domain (once it has already been captured by the sensor in the analog domain). Ideally you don't want the signal to be multiplied (because that just overexposes the image quicker), nor do you want the signal to be divided (because then you lose out on photon counts).
E.g. if I captured this 2x2 pixel image (numbers indicate brightness, where 100% is max brightness);
And then use an ISO that multiplies these numbers in the digital domain, say by 2.0x, I get
Three pixels are now fully exposed "white" (100%). The first pixel (25%->50%) is now brighter, but any noise will also have been multiplied by 2. Also, because I multiplied everything by 2, there will never be odd numbers like 51% or 63%. The dynamic range that these odd numbers could use is now going to waste. It's just a trick to make it seem like your camera is more sensitive, but it's just that; a trick!
You can see that picking a multiplier other than 1 does nothing and actually makes things worse. If ISO 200 corresponds to a multiplier of 1 (also known as "native ISO") for your camera, then keep using that ISO setting.
Hope this helps!