extremely critical, probably not, but I would like to assume the optimal conditions.
Thank you, Stefan
Optimal conditions? Every SCT OTA has variant optics so that if you put 10 samples side by side and measured the actual distance between primary and secondary and the 'optimal conditions' they will vary in seperation distance and hence, backfocus as measured from the rear of the OTA.
Further, I'll cite the excellent optics documentation from https://www.telescop...cs.net/SCT2.htm
"The relation implies that, for nominal primary mirror shift ∆, error induced by extending back focus .... depends on secondary magnification and primary's focal ratio. For F1=2, m=5, and K2=0, every mm of reduction in mirror separation (∆=1), or nearly for every inch of focus extension, induces ~1/23 wave P-V wavefront error of over-correction, and as much of under-correction for widening the separation."
So, for every 25.4mm(1 inch) change between the primary and secondary(this is a huge value in seperation) the relative effect is 1/23 wave error. That is not significant error when talking P-V when high P-V values are 1/10, and average values for SCTs falls between at worst 1/4 and 1/10th.
In real world effect, be aware, if you were to measure a particular set of optics, you may find that the optimal correction therefore is NOT some magic number dictated by the manufacturer, but a number above or below that figure.
By the way, the actual distance a typical SCT mirror travels for focusing near infinity is well below that 1" change threshold so therefore the actual error induced is even smaller than 1/23.