Martin,

That is a pretty decent image of B33 itself. Alnitak is obviously causing glare.

As to the star shape problem. What processing software are you using?

Martin,

That is a pretty decent image of B33 itself. Alnitak is obviously causing glare.

As to the star shape problem. What processing software are you using?

 

I did stacking, integration and image processing all in Affinity Photo.

Only used plenty of graduation curves and high pass frequency filter for details and denoise in the end. Partially with masks, but nothing weird / black box macros or similar...

But taking a look at the lightframes and the single channel stacked images I already do see a tendenca for square stars there...

Hello Martin, 

That's a very nice colour rendition. Having only just started going through the recent images again, maybe I've come across your question a bit late. Anyway... 

I have also encountered this problem but as yet have not come up with a definitive explanation as to what causes it. My only thought is that it might have been clipping of the white point, and as a result there is insufficient differentiation between the outer star pixels to give the appearance of roundness. Probably not the right answer. 

Providing that there were only a reasonable number, I did find a partial remedy using Photoshop. This may also be available in other processing packages. After carefully selecting the star, I chose the 'distort' filter option and within that 'spherize'. By carefully adjusting the slider to a negative value the star begins to appear a bit less square. This may not work in all circumstances as doing this may introduce other undesirable effects

If you haven't already tried to fix it, I hope this will help. 

Cheers and CS, 

Ray