I'll have to admit that it is difficult for me to believe that for a SCT slightly less than perfect collimation can affect image quality over the entire FOV. Yes, I do believe perfect collimation is beneficial. The counsel of experienced users of SCTs shows the benefits of perfect collimation. But I must force myself to wholeheartedly accept this counsel.
My "attitude problem" comes from how a slight miss-collimation affects a Newtonian image. One method to see what collimation adjustment is needed is to place a slightly out of focus star image in the FOV in a location where the shadow of the secondary is centered in the disc. This is where the axis of the primary mirror is pointed. One then just adjusts the collimation screws to bring that out-of-focus image (with shadow centered) to the center of the FOV. When the "sweet spot" is not centered in the FOV, the image on-axis (i.e center of high power FOV) suffers from off-axis coma and perhaps other aberrations. But other than off axis aberrations, the consequence of slight miss-collimation isn't an overall degradation of the image. It just isn't putting the best part of the image on axis.
But if my understanding is correct, this simplistic view is not applicable to the Schmidt-Cassegrain system. The primary has no adjustment screws. It is the adjustment of the secondary which directs the optical axis of the primary back down the center of the baffle tube in the primary. And if it is off a bit, it isn't just a matter of the "sweet spot" not being in the center of the FOV. If I understand things correctly a miss-collimated secondary negatively affects the image over the entire FOV.
It has taken me understanding (and believing) that negative consequence, to be willing to check collimation on my Celestron-11, especially when seeing is good enough to benefit from perfect collimation. It seems like my C-11 doesn't hold perfect collimation very long. Just normal handling in moving from garage to permanent pier causes it to drift off a bit. Maybe the secondary housing is loose. I intend to replace Bob's Knobs with Allen head screws.
Yeah, I do believe perfect collimation is very beneficial. This is especially so when I fight my "attitude problem". So for experienced SCT users, is my thinking on this correct? Are SCT and Newtonian collimation issues fundamentally different as I described? Please help me with my attitude problem.
Edited by Rustler46, 18 July 2019 - 01:37 AM.