Please help with noise/gradient on M31

[Sterrenland]

Managed to get the scope (Skywatcher ED80) out last night for the first time in weeks and try out the Skywatcher 0.85x reducer/flattener for the first time.

I chose M31 and after throwing away sub-standard lights I was left with about 80mins worth of data. There was moonlight to contend with so am pleased I got what I managed, but am annoyed at the dark line of noise/light gradient near the top of the photo.

Please ignore the dust motes...what I'm not sure of is the long dark linear feature that stretches from about M110 to M31. Any ideas what has caused this? I don't think it's the new reducer/flattener as it appears only here in the image as a linear feature. I've marginally lightened the 2nd lower image to make the dark line show even more clearly.

 

[vlaiv]

Calibration files are your friends

If a dust speck causes a circular shadow, what kind of shadow could a thin piece of hair or similar produce?

Indeed - it will be like very elongated circle - or rather line with circular endings - pretty much like feature you are asking about.

It might be that it is due to that or it might be feature that is produced by bias signal - and removed with bias or darks. I have heard that some DSLR cameras have issues with AF pixels - it creates a line. Stacking can make this line appear in different places and result in thicker feature than single row of pixels.

What ever the actual cause is - using calibration frames will rule many possible sources of this and either fix it or narrow down possible suspects.