Vignetting and saturation - is this caused by high cloud?

[BrendanC]

Hi all,

Last night I took 16x120s of the part of the North America Nebula just to the right of the Cygnus Wall (I'm doing a mosaic), and 28x240s. The reason for doing this is that I've found taking shorter exposures while the object is lower in the sky, helps with light pollution where I live. Then, when it's higher, I can move onto the longer exposures.

This worked just fine for the Cygnus Wall bit, which I did a month ago:

However, last night's images have come out pretty badly. I've done a VERY quick stack in DSS and process in StarTools (no noise reduction or owt), and it's not looking good:

It's using essentially exactly the same calibration approach as the other image - 25 flats taken on the night, 50 darks from the library, a master dark flat, no bias frames.

What I'm finding is that the 120s exposures show bad vignetting, which I'd have thought the flats would have fixed...

... while the 240s exposures are oversaturated (and noisy):

I've tried EVERYTHING to find out what's causing this. I've used different flats from another night, retrieved old darks from an older library, tried without any calibration at all, using different stacking algorithms, and it just seems to be there, in the subs. 

So, this is weird, and I don't quite understand it. The conditions seemed perfect last night. I've not had such bad vignetting or saturation, and I'm confused as to why this is a split between the 120s and 240s subs.

One theory is that perhaps there was very high cloud last night? Could this have caused it? My guiding was pretty good, which isn't usually the case with high cloud.

I've noticed that particularly with the 240s subs there's quite a variation between lighter subs and darker ones, which would bear this theory out.

Or, is it just simply not enough data for this to give good results? It's similar to what I captured for the Cygnus Wall image.

But frankly, if it isn't something like that, I'm stumped! Again!

Thanks, Brendan

Edited July 15, 2021 by BrendanC

[BrendanC]

So I took the exact same shots last night and they came out much better. It must have been high cloud. That's my working hypothesis anyway, until/unless anyone else can offer another one!