Re[2]: [ATM] Back Focal Distance design considerations

[Michael Peck <mpeck1@ix.netcom.com>]

At 08:31 2/14/05, Richard wrote:
>MP> right that you're building a Cassegrain. I'd attach the focusser directly
>MP> to the rear mirror support, then use a standard 2" or 1.25" star diagonal
>MP> with eyepieces or remove it for imaging. Then the length of the racked in
>MP> drawtube would be limited by the distance to the back of the primary 
>or the
>MP> first mechanical obstacle it encountered.
>
>Again, the position of the corrector doesn't allow the focuser to
>attach behind the mirror cell, which is a great shame. Looks like
>the gain in image quality from the corrector is causing me other
>problems now.
>

I guess I still don't understand. Why doesn't the corrector allow the 
focuser to attach to the rear mirror support?

You're still at the design stage, correct? When I was seriously considering 
building something like this I found that the distance from the last 
corrector surface to the focal plane could be freely set as a design 
parameter (within limits). So why not design a system that puts the focal 
plane ~40 (focuser height) +55 (camera depth) mm behind the OTA back plate? 
That will allow you to attach the CCD camera directly to the focuser with 
no intervening diagonal.

That will also solve or at least mitigate the focus travel problem: a 2" 
star diagonal requires about 2" or more of additional back focus. A low 
power eyepiece that needs to be positioned ~50mm behind focus should only 
require ~40-60 mm. of focuser travel with the focus located ~55mm behind 
the top of the focuser.

Mike Peck


------
Michael Peck
mpeck1@ix.netcom.com

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