Compound Telescope Moving Mirror

[saurabh dh]

Was wondering why  MAK & SCT use  focusing knobs that move the mirror, rather than have a  regular  focuser  like  Craford, Rack and Pinion....

 Is there a specific reason behind this, i read some thing about the Image shift, this method causes so why not upgrade the design  

[hobbit]

my (limited) understanding is those are for collimating, not focusing.

[nightfisher]

the mirror shift can be a a little annoying so i have added a crayford to my 150 mak some makes like the russian maks have a rack and pinion focuser

[Peter Drew]

Primary mirror movement for focussing gives a very increased range of focus position allowing the use of accessories like cameras, diagonals and binoviewers without further extension to the rear of the telescope. The downside to this is sensitivity and the magnification of any play in the system which causes unwanted image shift. 

[nicoscy]

You can always add a helical focuser or a regular focuser to a MAK/SCT later on, but in terms of the why, Peter nailed it.

[Stu]

I've never understood why mirror shift has to be present. My Orion Optics OMC200 had fairly basic mechanics internally, but the important stuff was right ie very accurately fitting sleeve over the central shaft so no mirror shift even at insane magnifications ie x500 or more.

The Edge 8" I've just got is a very nice scope, but does have an amount of image shift so I'll add a focuser to it and lock the mirror in place.

[nightfisher]

I've never understood why mirror shift has to be present. My Orion Optics OMC200 had fairly basic mechanics internally, but the important stuff was right ie very accurately fitting sleeve over the central shaft so no mirror shift even at insane magnifications ie x500 or more.

The Edge 8" I've just got is a very nice scope, but does have an amount of image shift so I'll add a focuser to it and lock the mirror in place.

Dont "lock" the mirror as after market focusers have very little travel, the Revelation i fitted to my 150 only has 25mm travel

[saurabh dh]

Primary mirror movement for focussing gives a very increased range of focus position allowing the use of accessories like cameras, diagonals and binoviewers without further extension to the rear of the telescope. The downside to this is sensitivity and the magnification of any play in the system which causes unwanted image shift. 

wouldn't the fixed mirrors give a steadier image,

the newtonian design OTS use all the accessories also so....