Mirror flop

[BobSki]

having read recently of the subtle differences between Schmidt-Cassegrain and Maksutov-Casseegrain scopes, I was wondering if they both suffer from mirror flop, or is one system better than the other in this regard, any ideas?

[Davey-T]

My Meade SCT has a mirror lock which works OK, I believe Celestron have something similar, no idea about Mak's.

Dave

[BobSki]

Cheers Dave,

[Avocette]

My SkyMax 150 does show some slight image shift during focusing. This is acceptable during visual observation, but If I'm using the scope to image with a webcam, then the shift may be sufficient to move a small image (such as of a planet) off the image sensor. I have an SCT Crayford focuser which screws on the Mak replacing the visual back. I use the Mak focuser for first level adjustment, and then do all final adjustments with the Crayford.

[John]

I think mirror flop and image shift are different things and have different degrees of severity / annoyance. In some scope designs some mirror shift is very hard to avoid completely because the primary mirror is being moved to achieve focus. This is liveable with I think as long as it's relatively slight and can be contained and is consistent.

Mirror flop can be more severe to the point of putting a scope out of collimation when the tube points at certain angles and the weight of the mirror causes a larger movement. I had this on a 15 year old schmidt-cassegrain and it was rather annoying to say the least because I had to re-collimate the scope when it occurred. The slight irony was that the scope otherwise had superb optics and when in collimation showed me some of the best views of Saturn that I've ever seen though any scope

[Stu]

My OMC200 had a brilliant focussing system, with zero image shift even at stupidly high magnifications. It was not a complex design, just well thought out and implemented. I don't see why inherently there has to be either flop or shift on these scopes but they do seem to

[russ.will]

Some time back, Meade did a few SCTs where the mirror was moved on three rotating shafts equidistantly spaced around the mirror. Two were geared to the focusing shaft by a tensioned toothed belt, so it was pushed/pulled on all sides so the mirror didn't shift during focusing. Seems like a good idea that doesn't sound expensive, but I don't design scopes!

FWIW, I though mirror flop was basically caused by the same slack around the baffle tube, but was the name given to the mirror shifting after an EQ merdian flip.

Russell