[alistairsam]

Hi,
I'm using an 8" F4 newt on a HEQ5Pro with a QHY8CCD, baader mpcc with the qhy8 spacer from peter tan, orion St80 guidescope with a qhy5 via a diagonal for guiding and st4 autoguiding.

I seem to have field rotation, curvature, coma and I don't know what else.

The attached images are single 3 minute subs, not stacked just colour corrected. As you can see, stars seem to have rotated at the corners in both images, and with an animation of both images that were taken a minute apart, the image seems to have rotated. I can't upload an animated gif here, but if you click the images consecutively you can see the shift. the OTA wasn't touched between exposures so framing is identical.
can someone pls tell me what's happening?

I initially had collimation issues with the F4, but I collimated with a cheshire just before this imaging run and it looked ok, managed to get the cross hairs aligned.

The ST80 guidscope is fitted to to the guide rings and not the OTA and is fitted very securely, but I didn't align the guidescope perfectly to the OTA, just roughly. will using a diagonal with the qhy5 cause issues?

I'm using a bintel 10:1 focuser and have checked there's hardly any slop. But I haven't checked the qhy8 ccd sensor itself to see if that's perfectly square.

The phd guiding graph was very smooth to well within 1 segment either side of the central line so just above and below. I used default phd settings and guiding in both ra and dec.
The figures were around 0.22 and 0.15 I believe, will attach that shortly.

my polar alignment was also quite good, I did drift align with the meridian star and the east star and didn't notice any n/s drift. I didn't spend too much time on it though.

I also ran the fits files through ccd inspector and the outputs attached which shows severe curvature or tilt or both, I'm not sure.

what normally causes field rotation in an eq mount? what other issues are apparent? should I increase the mpcc spacing?

I will be getting the cats eye kit for the 8" F4 and my 10" F4 but I don't think these are just collimation issues.
where do I start?

Thanks