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Mirror shift happens because the focus is achieved by the primary mirror being moved back and forth along the baffle tube. The shift happens because of the way that the mechanism is designed - being a poorly designed but cheap system - so the result is slop when the mirror is moved back and forth. Celestron and Meade SCT's were notorious for showing mirror shift (also called mirror flop), and Synta Maks can show it too. You will notice this mirror shift in your scope when focusing at high magnification, seen as a small shift in the image from one side to the other as you adjust focus. The thing is this shift will affect collimation. If you collimate in one position of the primary mirror, then it will be out of collimation when the primary mirror is shift in the other direction.
Because of this mirror shift, when focusing and hence also when collimating, you must get into the habit of moving the focus knob in a set pattern all the time so that it sets the mirror back into position for when its focused position is the same as the collimated one. So you defocus say by turning the knob anti-clockwise, and focus only be turning the knob slowly clockwise. If you go too far your repeat the afore mentioned pattern and do not try to focus by turning the knob anti-clockwise.
When collimating, you need to follow the following (following the how-to instructions of the screw manipulation)
1, Use high magnification - you cannot collimate accurately enough with low power
2, Do gross collimation with a large "doughnut". Wind the focus knob anti-clockwise to get a large doughnut and then turn it a little clockwise as if focusing just a touch but leaving that large doughnut. Tweak the colli screws are required.
3, Now do fine collimation with a very small doughnut. Wind in the focus knob to reduce the size of the doughnut so it is very small. This is because a large doughnut will eventually see whatever error remains in the collimation process be evened out. A small doughnut will allow you to check for whatever small tweak needs to be done.
This may all sound very dramatic, and it can lead some people to distraction, but while it really should not be there, it is easy to work around it. My little 127mm Mak does show some mirror shift, but it does not bother me as I am well accustomed to working with mirror shift.
Mirror shift does not affect all Cassegrains that focus with primary mirror movement. Russian made Maks show no mirror shift and they had this figured out over 30 years ago. But these Russian made Maks are very special beasties! Mass production SCT's are not in the same league as these scopes.
Alex.
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