Spikes problem

Oh boy, that is a tough one!  Your stars appear to be reasonably uniform and round so the alignment of your scope looks ok.  If you look at the brightest star near the bottom of the field you can see that the flare appears to change with field position.  That makes me think that the flared light might be vignetted.  Without being able to examine the design details of this scope it’s hard for me to say exactly what might be causing this problem.  It could be due to a surface finish problem in one of the components.  I personally don’t see how Andrea’s idea of a tilted component could be causing this kind of problem.  However I do completely agree that changing one thing at a time is the best way to track down this kind of problem.  

You’ve run into a simple fact about evaluating any imaging system.  The acid test of performance doesn’t come from imaging something faint.  One of the most difficult tests is to see how well the system handles a bright star.  That’s where baffling problems, stray reflections, and surface finish (or material) problems quickly show up.

John

Konstantin Firsov:
Most likely your main mirror has a so-called turned down edge. Try covering the outer part of the mirror with the opaque (black) ring. Try a narrow one first. Say, 3 mm thick. Place it right above the mirror. E.g. on the mirror cell.

I had the same issue on my Sharpstar HNT15028 fast newtonian. A 3.5 mm ring above the mirror fixed it completely.

That's an interesting idea Konstantin.  A turned down edge creates high order spherical aberration and I suppose that you might see something like this if the surface finish was poor.  Even with high-order SA, the defocused light won't produce the spike structure in the OP's image.  It will simply produce a smooth halo of light.  However, stopping the primary down a little slows the optical speed, which will change the vignetting characteristics. The diffraction pattern may be coming from a rough edge on a vignetting aperture and by stopping the system down a little, it may cause the light bundle to miss the offending edge.  The fact that the pattern is field dependent is a strong clue that this isn't due solely to high order SA.

John

Hi there @Apprenti,

Fellow (former) Newtonian owner here! I used to have an almost identical issue on my Orion 8" f/3.9 Newt, so I can very confidently say that this almost certainly the primary mirror clips causing unwanted diffracting spikes on your image.

The way to easily fix this is by making an aperture mask. Basically, if you can cover up the primary mirror clips, you can prevent the flaring issue. I made my aperture mask out of some thin, matte dark grey flexible foam that I bought at Michael's (arts & crafts store). I measured and cut out a ring-shaped aperture mask. The outer diameter was wide enough to fit the width of the inside of the OTA, and the inner diameter was about 1" or so to make sure that the primary mirror clips were covered. Here are some more detailed instructions on how to do it: https://www.cloudynights.com/topic/656915-cleaner-stars-how-to-install-an-aperture-mask-on-your-newtonian/

Here was my Alnitak before the aperture mask:


And here was my Alnitak after installing the aperture mask:


Much better, in my opinion. Hope this helps!