Re: ATM 11.2" F/2 Primary testing questions :)

[Aplanatic@aol.com]

James,

> #2 How to Focus CCD?  (not impossible, 
> but more difficult)

Also consider moving the primary mirror for focusing.  Yes, with three independent screws one risks changing the collimation of the instrument when focusing by this method.  However, the Schmidt camera is not terribly sensitive to collimation errors to begin with, so with careful adjustment of all three screws, it should be quite possible to accurately focus without affecting the collimation.

Because of the very small useful field for the paraboloidal implementation, you will have to be very careful about collimation error and flexure during horizon to zenith (H2Z) slews if you decide to stick with this type of instrument.

For either optical system (Schmidt or paraboloid), focusing and focus shift with temperature will be critical.  At f/2 and 10 micron pixels, your allowable focus error is less than 1 mil (25 microns).  Be very careful about the design of the OTA.  Try to eliminate any mechanism that changes the distance between the mirror and the CCD camera, including:

1) Temperature.
2) OTA flexure during H2Z slews.
3) Mirror mount spring during H2Z slews.
4) Etc., etc.

Don't be daunted by the problems --and I can tell that you won't be.  Just be sure you understand them.  The worst thing in the world is to find out about some problem *after* you take your first images.  Yes, it's happened to me from time to time. <g>

Dave Rowe