Cheers for that.
1 - Where is the indication that there it tilt in my image train? I have a tilt plate I can install to try and fix this but I'm not sure what I'm looking for to see the tilt in my image.
2 - Its probably a combination of both t.b.h. I'm right on the weight limit for my CEM25p. Max payload is 12.3kg and I'm rocking about 10.3kg. I know its a lot but the CEM mounts seem to be able to take a bit more than the GEM ones. (I'm planning on upgrading to a CEM40 which has a max payload of 18kg soon. Just need to sell the cem25p).
I'm guiding with a QHY5L-II and a 60mm guide scope. I'll see if I can dig up the guiding log from PHD2.
I'm pretty sure the collimation is a bit out as well. The scope took a big knock just after I got it, and although I have recollimated it with a Takahashi collimating scope I have always felt it was never quite right. But with RCs being rather difficult I have been hesitant to tweak it too much as I don't want to make things worse. I'll figure it out one day though.
3 - I am using a 0.75 focal reducer that has been known to cause reflections with bright stars as can be seen here (top left corner there is a reflection of the star in the bottom right. This is just a trash sub taken while testing).
In saying that, this image was taken without the focal reducer and if you really stretch it, it also has a 'dark ring' in the center.
The M78 image was stacked and processed in DSS, but I'll try using PI to calibrate and process it to see if its present in the subs (still learning PI so tend to gravitate to DSS for stacking for its ease of use).
And just out of curiosity, why should I not be using the bias frames?
1 - The star shapes change over the image from left top (worst) to right top (best). I think this is a very minor issue, and difficult to get better without special software (CCD inspector).
2 - Yes, as I said, I wouldn‘t fuss more with collimation. Check if stars are rounder in shorter subs, if yes, it is your tracking. Btw, I could not attach my guidescope rigidly enough to my CF RC, so switched to an OAG with much improved results.
3 - If this is reflection, it is from stray light bouncing off somewhere in the image train, not from bright stars.
Bias: The bias on many CMOS cameras is unstable. Also, you would need it only for dark scaling, which won‘t work well due to some amp glow of that sensor. If you use matching flat darks for your flats and matching darks for your lights you will not need any bias frames. Make sure to turn „dark scaling“/„dark optimization“ off when calibrating and do not use bias, maybe this already fixes the ring.