The Iris Nebula, I picked this target to test combining data from two separate nights. I haven’t really done this in the past, having kept target imaging to a few hours in one night and then integrating for a final result. Mainly because my post-processing wasn’t up to snuff an combining images often resulted in lots of undesirable artifact matching.
Multiple nights are a must for faint nebulousity as the single to noise ratio is so low compared to background that several hours are required, or a very fast scope (I was using a Celestron EdgeHD 8″ which is an F10 – so quite slow) to gather enough light.
So you end up looking that the three stages of the image as more data is added you can see more and more detail appear and noise is reduced. Seeing on both nights was quite good, but if I had to guess, the first night was a tad better.
Being in a dark site helps tremendously, as does the target being closer to the Zenith; both cases causing a reduction in atmospheric light pollution. Of course for most people, dark skies are often a dream (or a very long drive) away, so filters really help in those situations.
Of course, to really reduce the noise you start needing a lot of integration time, tens of hours if not longer, to start to really notice an image becoming noise free.
And at some point I need to consider a Go-To mount, it took almost an hour the second night to find the nebula, despite knowing where in the sky it was located (its in reality quite a faint object and without significant exposure looks like a normal star with only a hit of nebulosity).