SCT image shift

[Digger1895]

Could someone please explain what this is and what happens when it occurs,

Cheers muchly!

[CodnorPaul]

It is when the mirror slips against the baffle tube causing the image to move - if doing visual it is not too bad, but if doing AP it can move off the camera chip.  It occurs when focusing or when doing meridian flip, and affects some scopes more than others.  As it slips it generally also tilts which is why the image moves rather than goes out of focus.

I have only noticed it once in my C8 and to rectify I just racked the focus knob all the way through its range a few times to re-distribute the grease - not had a problem since.

The new Edge OTAs by Celestron have mirror locks so if doing AP it cannot move, but as I say it is not an issue on my OTA.  I think it is more likely to occur in the large SCTs - 11" upwards

[JamesF]

During focusing the shift is particularly obvious when the focuser changes direction, causing the mirror carriage to rock on the baffle tube.

The mirror carriage cannot be an interference fit on the baffle tube otherwise it wouldn't move back and forth, so there's always a (very small) gap between the outside of the baffle tube and the inside of the mirror carriage.  That effectively allows the carriage enough play to rock on the baffle as the load on the carriage it changes.

Sometimes the mirror moving under its own weight is called "mirror flop" and moving as the focuser changes direction "mirror shift", but they're largely interchangeable terms I think.

I've rarely noticed it when using a scope visually, but on my 127 Mak it was enough to move the image right off the camera sensor when I was using it for planetary imaging (albeit with a focal length of getting on for four metres).

James

[Digger1895]

Gotcha, cheers for the info guys.