Stars don't look right. Help

[ollypenrice]

My understanding is that the spacing on SCT focal reducers is not that critical. It becomes very fussy at fast F ratios.* If I were to apply Sara's link to your first image I would say, if anything, that it was in tune with 'distance too much' anyway. The fit with either diagram is not good. Like Steve I remain sceptical, but playing with the distance to refine it is inexpensive and worth a go.

I have no doubt that neither flexure nor guiding are the cause. You can't get a tracking error trail going in different directions on different parts of the same image...

Tilt and collimation are possibilities worth exploring.

Does each sub show the same distortions? I'm just wondering if we might be looking at an effect due in some part to the stack.

Olly

*And wide fields of view (large chips.)

 

[Filroden]

I don't know if this will help diagnosis, but here's a plot of FWHM and eccentricity across the image (using PixInsight).