Cassegrain telescopes have their focal plan at a fixed distance from the rear of the mirror cell / rear of the instrument. From a CCD/CMOS sensor point of view, what is an appropriate tolerance (+/- mm) in the metal back distance for getting images with no loss of definition? Can I work it out from the focal ratio and pixel size for example?
The reason I ask: the Tak reflector I am looking at requires a metal back distance of 56.2mm while ZWO state that their ASI imaging train has a length of 55.0mm. I can replace one of the Tak couplings with a custom-made alternative made 1.2mm longer to resolve this. This is not expensive and will be rigid, but of course is not adjustable. However I'm wondering whether I can really expect a machine-shop to deliver such an adapter within a tolerance in length better than +/- 0.1mm.
Many thanks for your thoughts.