Regarding this image, I set out with the goal of finding variable stars near M31 to study and in the process also end up with a decent image of the galaxy to boot.  As it turned out, I didn’t find any variables of note, but I did find a noxious noise problem in the stacked image of the galaxy.  Some research led me to the so-called hair effect.  I’m sure many Astrobinners know about this, but I had never seen any posts discussing it.  In the event, then, that it is of use to explain it, here goes. If the RA guiding slips enough during the acquisition of many subs, the result is that the image drifts over the sensor.  When these images are stacked, the stars get aligned and cropping is necessary because, in essence, the frame moved relative to the image, so stacking by the image (the stars) throws the frame alignment off.  Not only that, but pixel response variations, including hot pixels, move with the frame because the sensor is the frame. This causes the pixel variations to turn into streaks that resemble brush strokes or even hair, a rather nasty form of noise.  Flat frames cannot correct for this because it would be essentially impossible to prepare a streaked flat field image that emulates the streaks in the image.  Prevention of the hairbrush effect could include dithering, syncing to the sky and recentering between image batches, or best of all, getting a better mount (which obviously is something I’d like Santa to bring me)!  But once the damage is done, what can be done?  To get the posted image of M31 at least decent, I used PS dust and scratches, PS noise removal, the StarTools wipe module, custom masks and StarXterminator to get starless views to avoid messing up the stars while smoothing out the noise.  Because of all of this nutso stuff, the image does not tolerate enlargement very well, but I thought I’d post it anyway. 

The image was taken on 10/08/2023; 150 30-s frames were acquired for a total integration time of 1.25-h.  A synthetic luminance frame was prepared by restacking 35 selected frames over the duration of the imaging run with 3x3 drizzle and then conversion to gray scale and this was blended with the original stacked color image in PS.