Matteo and Bernd, Both of you lost me on your comments about pixel size. Camera pixel size doesn't enter the equation. Microns per motor step is the derived quantity we're trying to find, and that is found by running the focuser some predefined number of counts, and then measuring the drawtube travel. An alternative method would be to know the pitch of the focuser screw thread (a linear distance per revolution) and then factor the counts per revolution of the focus motor. Given that the average Joe observer is going to most easily move the focus motor by some number of counts (say 1000), and then measure the millimeters of focuser drawtube travel, the GUI could allow for those fields and then convert to microns per motor count. The rest is just the formula as given by the critical focus zone web link (which includes f-ratio, aperture size, a user preference for focus tolerance, and an estimate of seeing). See here:

www.goldastro.com/goldfocus/ncfz.php

CS Doug

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