"Normal" focus shift in a Cassegrain

In Cassegrain systems, where focusing is made by moving the primary mirror, mirror shift is a common "problem". How much mirror shift - in arcseconds - is acceptable? Is it possible to decrease the shift?

I've got a NexStar 8 and want to know how it compares to other telescopes.

Hi:

Most current SCTs are pretty good in this regard, especially the 8s. What I see these days averages out to about 45" or less, which I consider quite acceptable. Shift of this magnitude won't usually put the image off the chip of even a fairly small CCD sensor.

Peace,
Rod Mollise