astroct They are forming a radial pattern similar to if the sensor is too far away from the coma corrector
A spacing that is too short will produce a radial pattern (elongated stars at the edge of the image circle that looks like short lines which radiate from the optical axis).
A spacing that is too long will create elongated stars that look like they are short arcs which rotate around the optical axis.
See diagram (Back Focus Spacing Guide) about half way down this web page:
https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus
If some corners show that it is too short and some corners show that it is too long, you have a tilt problem, and/or your camera is not centered on the optical axis of the reducer.
Remember that the physical distance of the sensor from the reducer needs to be increased by 1/3 the thickness of the glass filter that you have inserted in between them (for glass with refractive index of about 1.5).
So, if the manufacturer specifies 56.2mm as the back focus (most Takahashi telescopes, as example), the mechanical back focus needs to be 57.2mm when you are using Radian and Chroma filters (3mm thick). Cheaper filters tend to be thinner, and you will need to add less compensation. With a filter that is 2.1mm thick, you add 0.7mm to the optical backfocus to get the mechanical backfocus.
Remember too that ZWO's specs are very loose. I remember seeing a number like +/- 0.5 mm for their sensor to flange distance. On top of that, I think ZWO's drawings are for mechanical flange distance that do not include the additional optical path posed by their protection windows.
In general, you need to fine tune everything yourself by adding and subtracting shims.
If you see stars that radiate from the optical axis, increase the spacing. If you see stars that rotate around the optical axis (assuming it is not from field rotation), you need to reduce the spacing.
When you are happy, just replace all the spacers and shims by something you get custom made by PreciseParts.com (warning: their backlog is of the order of 3 to 4 weeks). There are a couple of services like that in Europe, too.
Chen
