[Shiraz (Ray)]

Hi Robin. all looks depressingly familiar. I have an f4 newt with an MPCC and have spent countless hours trying to track down exactly what causes odd star shapes - with limited success.

Earlier posts are good advice (maybe JJJ's comment sums it up), but the following may also be useful - apologies if you have already been down these directions:
1. With fast Newts, if the secondary is not properly positioned and offset, the primary optical axis may not be aligned anywhere near the tube axis and you can get the end of the tube encroaching into the light column, causing flat spots on the out-of-focus images of stars and odd shaped in-focus star images. You have checked the secondary offset, but if you need to tilt the focuser to centre the secondary - and are still getting odd star shapes/vignetting - then either the secondary is actually offset away from the focuser by an incorrect amount or it is not in the right fore/aft position along the tube. try putting the focuser back to normal and move the secondary in/out to centre it. Also, it is always worth standing back and looking into the front of the OTA with nothing in the focuser - you can see if the secondary holder (or the edges of the secondary itself) looks tilted WRT the OTA tube and this will give some clues as to what way it might be misaligned.
2. A laser alignment tool can be very useful for initial alignment, since it is relatively easy to work out causes of gross misalignment - the passive tools are probably better for fine alignment, but interpreting what is going on is easier with a laser.
3. The MPCC is intolerant to misalignment. I would first try to get everything aligned without it - try to get a symmetrical coma pattern about the centre of the FofV. Then re-introduce the MPCC and see what it does. Even with the best alignment I could get, my f4 system cannot produce round stars out to the edge of the FofV. The MPCC probably meets the claim of "10 microns" when measured at half power width, but the PSF skirts are distorted and star shapes vary with brightness. The MPCC is definitely worth having, but not perfect at f4. And it is totally intolerant of misalignment - if you cannot get symmetrical star coma patterns without it, you will get bad results with it in place, so do your inital alignment without it.
4. I suspect that you may also have focal plane tilt in the camera, judging by the star patterns, but I would leave that as the last thing to check when you are convinced that everything else is properly aligned. A camera tilt that is fine at f6 may be disastrous at f3.8 and even a few microns of tilt will upset the star shapes. Make sure that the camera is perfectly seated in the focuser (without the MPCC) as a first step and then use any tilt adjustments that the camera may have to try to get a symmetrical pattern in the star coma patterns around the centre of the FofV. If the camera does not have tilt adjustment, check that tilt is the problem by rotating the camera in the focuser. The star pattern will rotate, but if the general nature of the star pattern distortions does not change (eg elongated in top left), then the chip is probably misaligned. Then you will need to adjust it - I spent many nervous hours with shims trying to get my QHY8 chip aligned with the camera body (at least to within about 10 microns) and it made a big difference. But only attempt it if you really need to.
Good luck - if you get it lined up, a fast optical system can be be great fun. Regards Ray