It’s not a bad idea Martin,

Home Forums Imaging Metal back distance It’s not a bad idea Martin,

Andrew Read

It’s not a bad idea Martin, but I’m dealing with the metal back distance here (focus is on the secondary mirror), so the only way to adjust it is with spacers or custom made adapters between the rear port of the scope and the camera.  At each iteration I would have to refocus and measure the quality of the image (perhaps the FWHM of suitably bright stars). 

Ray-trace spot diagrams would replicate these tests exactly, and presumably manufacturers must run them in order to specify the optimal metal back distance of the scope.  What doesn’t seem to be specified is any kind of tolerance, or the response of the spot-diagrams to different metal back distances