Uneven field

[69boss302]

Whenever I do background subtraction in PixInsight, I get a very uneven field, but it seems to be consistent among my photo sessions. (see extracted background below).  PI seems to take care of it just fine but it seems to me it shouldnt be like this.  I have a Z73 with 73a field flattener and ZWO EAF. Camera is ZWO 2600 MC Pro OSC, cooler on, and AA+ taking the pictures.  I use dynamic background extraction, not automatic. Seems to work better but still get same basic results. Is this normal or should I be concerned?

[69boss302]

Also, on the main picture I always have this 'clear' dot in the same spot all the time. This is the artificially stretched shot, but you do still see it in the final product:

[69boss302]

Here is that dot in the finished product

[69boss302]

That dot does not appear to be in any of my original .fit files, nor in the autosave.tif file from DSS. Only in PI

[wimvb]

Do you use flats? If not, you really should start using them. The background pattern is ordinary vignetting. The circular dark area is the shadow of a speck of dust on probably a filter. Both are taken care of by flat calibration.

The vignetting is best corrected in PI by using division as the correction method. But since gradients are a combined effect of vignetting (multiplicative) and sky glow (additive), removing them with DBE is less likely to succeed than using flats for vignetting and DBE for sky glow.

Edited November 25, 2021 by wimvb

[69boss302]

@wimvb Thank you for your very helpful reply. Yes, generally I do use flats, but did not this time. I do have flats that I shot during the session and will reprocess and see if it helps. Thank you.  Im glad its a normal occurrence and nothing to worry about.  As for the dot, I will clean my optical train and see what happens! Once again, thank you for your reply.

Bob