Hi everyone -
I recently was introduced to the Casady collimation method.
I thought "how nifty - this seems very smart, I must try it immediately!"
To my horror, my trusty 1977 orange tube, when evaluated in this fashion, appeared to be significantly out of collimation. The rings in the reflection were not concentric. I felt as if I had failed as an astronomer.
Quickly regaining my composure, I thought back to my most recent collimation, done using Ed's Guide to SCT Collimation.
I recalled how satisfying it was to have all those little diffraction rings so nice and symmetric. How could my scope have become mis-collimated so quickly? How could I not have noticed this last night when I was actually *looking* at stuff?
Like an aging fluorescent bulb, an idea flickered to life in my tired brain: if the reflections in the mirror in the Casady method are the internals of the OTA, then maybe the OTA's physical axis is not parallel to the optical axis! I quickly retrieved a ruler from my kid's box of craft supplies, and verified - with both satisfaction and irritation - that, behold, the secondary is not precisely centered in the corrector cell! It's offset to one side by about 1/16", or 1.6mm. This, I believe, is why the Casady method was showing me as being out-of-collimation while actual star-viewing suggested I was collimated.
I can, should I so choose, readily disassemble the scope and cut a new set of cork shims to center the secondary. This will, logically, require a re-collimation. The question is, will doing this provide any optical improvement in the scope? Is having the secondary centered critical to optical performance? Or, alternatively, does proper collimation render the 1.6mm offset of the secondary irrelevant?
Thanks in advance for guidance on this!