I might be silly, but what telescope are you using?
The only thing that i see there is guide error, tilt or possibly flexure.
If you are using a Newt, your stars will have slight pins on the small stars because the stars aren't bright enough to throw fully formed diffraction spikes. The big stars should show off the spikes and look quite nice. Looking at these spikes if they are RED GREEN BLUE rainbow spikes and are inline with each other EG + then you don't have a problem.
Is your scope collimated? this is one of the main reasons for things going skew-if, Astrophotography requires a much much higher level of collimation than visual, If you don't have a set of high end collimation tools. get some if you are serious as they will change everything!
it doesn't matter if PHD is sitting on the money your shots can still wonder around. ive seen it too many times! easiest way is to keep your eye ball on it until you are truly confident with your equipment!
If you are using a VC200L, square/triangle stars are the norm unless you modify the vanes.
Your mask shot means nothing. If you are using a newt and your star isn't 100% in the dead set middle of the sensor then it will never be round no matter how much you pay the hethan gods! as the tube will cut a little bit of the light off! Plus it just means nothing !
Astigmatism i am very familiar with If you have it, you will get a shadow of your secondary coming in round until your just about to approach the focal plane when it will go to a oblong once you go though focus it will flip 90 Degrees and come out the way it went in. Very easy to pick.
If you have 4 mirror clips and your getting astigmatism from that. dyam you have perfectly tightened them all up together so the astigmatism is concentric... this is virtually impossible.
I hate mirror clips. they cause so much grief it isn't funny. Do yourself a favor and chuck them as far away from your telescope as you possibly can and use Silicone or high quality velcro to adhere your primary to the Cell it is the only way to achieve a truly pinch less system.
But before we can tailor our advice can you give us some insight to your equipment.!
Brendan
) i have a 8" newt produced by konusky it is about 8 years old (although little used till about six months ago. the primary is probably approaching the end of its coatings life so there is that to figure in aswell. I collimate atm with a laser collimater although i think i am hearing i might need better then that sigh more money. I will have to test the defraction spikes theroy i tried for the astigmatism and couldn't see any major problems although i might not have had the mag up high enough. As for clips on the primary i have rectified that earlier today although only temp till i shorten the tube (again) so i can focus with the coma corrector in. Not sure what else i should mention.
if the laser isn't centered within the holder game over.
