Given that the inner diameter of the primary baffle on the currently-produced Celestron C90 is between 16 and 17 millimeters, what's the largest diameter eyepiece field-stop that one can expect the scope's light-cone to fill at the stock visual-back, with the shortest light-path that the scope allows?
Does lengthening that light-path allow the light-cone to fill wider field-stops?
If so, at what cost?
Also, are there any after-market dual-speed focus-knobs mechanically compatible with the currently-produced C90's moving-mirror focuser?
Failing that, is anything like this available in a version compatible with the currently-produced C90?
My testing notes show any eyepiece with a field stop larger than 22mm will begin to vignette.