SCT general optical alignment

If the just out of focus images of diffraction rings look similar and symmetric on both sides of focus, then this is probably nothing to worry about, and the corrector could have been set that way when the OTA was assembled by Celestron. If not, you can send the OTA to Celestron in Torrance, CA, for a collimation and cleaning; I have done that with my C11 and C14 OTAs and have been pleased with the results.

The corrector plate is typically never, never centered with respect to the physical corrector holder.. What is centered is the secondary mirror on the optical axis. Note, I said optical, not mechanical. In order for the corrector plate to be perfectfectly centered with the mechanical tube, it would need to be CNC manchined to a very high tolarance. Looking at the casting of that corrector ring and aluminum tube, do you think it is anywhere near that tolerance?

Due the nature of the spherical mirror system, the secondary can be off center and still colimate. This is why Celectron never took a lot of care with centering on the older non-Edge scopes. Not sure about the Edge. Collimation is beter if the scecondary is centered.

Now consider the fact that you have a floating primary mirror sliding on a greesed tube in order to acheive focus. What is the likelyhood that that mirrow is going to stay perfectly orthogonal with a fixed secondary/corrector? It will not. It all works because the primary and secondary is spherical. A sphere is a spheer, no matter how it rotates. Primary mirror flop, shift is a well know aspect of SCTs.

Why is there adjustment screws at all in the corrector holder, if it was be centered in the corrector holder? There wouldn't need to be any. But there is no way Celestron can bring a scope to markek at there prices, if the tube/corrector holder needs to manufactured at that level of tolerance. Consequently the adjustment screws.

LEAVE THEM ALONE !!. Yes, they were more likely adjusted at the factory.

A little history and understanding of Celestron SCT. The older or regular non-Edge model have shimes to adjust the corrector mirror. They were place at the factory. Due to the spherical nature of the mirror system, centering the secondary was not crutial. It works well enough for visual. I have owned an around 2000 yr 8” and two 6'‘ Celetron SCTs. I have a Edge 8, a carbon fiber non-edge C11 & C9.25. I have recenter all the scondary mirrors by ajusting the corrector plate with shimes, ACCEPT the Edge8. Celestron did a good job on the Edge 8. It was very necessary when doing Hyperstar with the C11. In the case of the C9.25, I had to shift the condary all the way to one side, and that still was not enough. I also had to shime the secodary to one side. The sondary is now centered on the OPTICAL axis and works well. This is all the consequence of a well made tube, but nowhere orthogonal enough to house a foating primary system. with cast non-CNC machined parts.

When Celestron put a field flattenter in the baffle tube for the Edge system, it became necessary to do a better job on centering the secondary, in oder to have the optical cone centered on the flattener. An off center light cone would have a negitive effect on the flat field. The baffle tube is not manufactured to the level of tolarance to assure it is perfectly orthogonality centered. They then put screws in the corrector holder, which made it much easier to adust, and do a better job of it. I'm not sure if they changed that on the regular non-Edge SCTs.

A NOTE: It is not crutial that that the corrector, itself, be rotated to a perticular position. What is crutial, is the scondary and Since it is attached to te corrector, it is required that the corrector be rotated to aparticular position. With non-Edge SCTs there is a black mark on the edge of the corrector plate. That mark needs to be placed at the 3 o'clock position when looking at the frount of the scope. (there may also be such amark on the Edge corrector) There is also a screw at that position.. At the factory Celestron (Meade did also) rotates a choosen best match secondary, to best rotation in order to acheive least spherical aberration. When removing the corrector, care needs to be taken to assure it is replaced back in the correct rotation.

Don't get me wrong. I think Celestron makes good SCTs. They are primarily responsible for the advance of amateur astronomy in the last two decades. IMO, they are the best for visual for 8 to 11 “. I love my Edge 8 on the Evoluton mount for visual. I haven't used it for imaging. A C9.25 is excellent for planetary. I'm getting my C9.25 set up now for galaxies. My preferred reftactors don't have enough focal length.

Lynn K.