Re: Focus repeatability issue

Barry Megdal

The comments other have made about CFZ (Critical Focus Zone) are very relevant.  You need to find out what the step size of your focuser means in actual linear distance (e.g. microns), to see if the variation you are observing is even an issue.


Don Goldman and I wrote an article for Sky and Telescope ( ) a number of years ago which calculated a new, more restrictive definition of the CFZ, but once you know the step size and the optical parameters of your scope you can see if this variation is even an issue – Focusmax or other focusing software will always show some variation – the question is just whether it is significant.  The difference between what we published and the traditional definition of CFZ was that we showed that the traditional definition was roughly equivalent to a ¼ wave error, but 1/10 wave is more appropriate for critical imaging, so that is what I used in doing the math.


-        Barry


Dr. Barry Megdal



Shb Instruments, Inc.

19215 Parthenia St.  Suite A

Northridge, CA 91324

(818) 773-2000  (818)773-2005 fax



Faculty (retired)

Dept. of Electrical Engineering



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