Actually, I've measured the rear baffle with digital calipers. The opening is 16mm according to my measurement. That information is buried somewhere back in this very thread. If we limit the size of eyepiece we use in the C90 by that 16mm rear baffle, then we shouldn't use any eyepiece with a field stop larger than about a 17mm Plossl. We know that's not right.
But keep in mind that this is an f/13.9 instrument. By the time the light cone reaches the rear baffle, it has significantly decreased in diameter from the 90mm or so entrance pupil.
In any case, in my spreadsheet you can see that I've colored in red the field stop cell for eyepieces with the largest field stops, as an indication that there might be vignetting.
But the bottom line is: if you don't see it visually, does it really matter?
No, I don't know of any way to widen the rear baffle. If there were, I wouldn't try it. Just accept it and work with it.
Edited by Sarkikos, Yesterday, 05:13 PM.