very strange shape of stars

Try to get rid of the filter first and report back. And no processing, just STF.

Hi Taras_M,

when I look at your star shapes, it feels like there is some movement in one of your mount axis. I might be wrong, but this would explan the elongated stars (that appear only in one direction) and the light falloff. The reason for that is hard to guess, but maybe one of your axis is out of balance. Another possible reason might be a problem with the selected guide star. In my case, I sometimes realize, that PHD2 is selecting a star but as time goes by, the light might change and that star is getting brighter until it saturates the pixels of the guide cam. This may lead to similar problems, too. Maybe you had some wind during the exposure. Because the wind is blowing mainly from one diretion, it causes such problems, too. Its a bit hard to tell without any further details of your exposure time, etc. The size of the effect is quite small so maybe wind or bad balance (or even both) might be the best guess at least for me so far.

Maybe this helps somehow.

CS

Christian

Hi Taras_M,

on the top right corner the shape of stars looks different, these lines go in opposite direction. Maybe this is an optical problem with the lens? I know such strange shapes from my DSLR lenses when I try to do astrophotography with cheap ones. (Coma or Astigmatism?)

CS,
Konrad

Although your problem looks a little different than the one I had you might check fan vibration.  ASI cameras are known to have issues with this.  To check it just compare images taken with the cooler on and cooler off.  I had a problem with elongated stars in one direction.  After trying many expert suggestions to no avail I found the Cloudynights discussion linked below.  It solved the problem.  In the future I will change the fan mounting on any ASI camera as soon as I get it.  This may not solve your problem but it is a cheap fix to avoid future problems.  

https://www.cloudynights.com/topic/837559-fan-vibration-asi2600mc-fixed/

THe OP might not use a flattener (Sharpstar doesn't make them for this unit) or the unit reducer. Given the size of sensor they might not need one at all.

While at it take some images without the reducer, too. That should remove issues with tilt and spacing out of the equation. Or put them firmly in.

Don't need to takes loads of them, just few 60s/120s shots at a crowded field will do. Just make sure you are well focused.

See image below for reference. The star shapes correspond to "Camera Sensor Too Far Away" for the top half of your image. Since you don't have the same issue at the bottom of the image, you're likely looking at tilt/sagging.

Taras_MThanks, I used this pics as a reference already. I removed all the small spacers I had before. My back focus is 54,25 already

You backfocus distance should come out to exactly 55mm using only included spacers (16.5 and 21mm). How did you arrive at 54.25? Also, where did you put your filter? Do you have a drawer or did you attach it to the front of the reducer? If you put it in front, it won't impact your backfocus. Filters shorten the optical path by about 1/3 of their thickness - so if it's behind the reducer, you will have to compensate by adding fine adjustment rings. Antlia filter thickness is about 2mm. That would come out to 0.6-0.7mm of additional backfocus.

That said, I am still pretty certain you have tilt in your setup. You can use ASTAP to analyze your tilt (and other aberrations) given a sub. That will tell you for sure.