Strange image artifacts/blemishes

[adge87]

Hello all,    

In an attempt to master the post-processing aspect of AP, I was playing in photoshop with a stacked image of Bode's Galaxy. As I progressed through, attempting and tweaking various aspects, some strange artefacts began to appear on the image, as seen here.    

 

   

 

For information, the subs are as follows:    

41 lights @ 300 seconds, 120 gain, -15 celsius  

15 Darks @ 300 seconds, 120 gain, -15 celsius  

15 Flats @ exposure set by APT via the flats aid    

 

Relevant Equipment used:    

ASI291MC

 Explore Scientific ED80 FCD100 APO    

 

An indication of the "marks" or "blemishes" is seemingly visible in the raw sub files, one also attached for reference (https://i.imgur.com/A8ISMaN.png). Although this could be a separate issue and not linked at all!    

 

Any clues as to the culprit of this? Is it a smudge/blemish on the lens of the telescope, for example?    

 

Many thanks

[michael8554]

Appears to multiple pasted Clones of the same small circular Selection of part of the image.

Michael

[vlaiv]

Shifted flats.

If you stack your subs without flats - you'll see dark patches in the places where you now see this artifact.

When flats shift - flat correction creates "bevel / emboss" effect in places of the dark patches:

Same effect - only produced on dust shadow circles.

Question is - why did your setup move between lights and flats? Do you have something loose in optical train - like loose focuser? If you put your scope horizontally to take flats - gravity might be causing shift of loose component.

Even if you think you have everything tight - if you use 2" nose piece with compression ring - that could tilt (this is why threaded connection is preferred for astrophotography).

Other than that - it could be processing issue - for example applying flats after stacking instead of before stacking (subs get shifted in process of registration when stars are aligned).

Those blemishes that you linked on a separate single sub - could be again related to dust - but closer to the sensor (maybe on sensor cover window). Do they show on flats as well?