Re: ATM Compliant Focuser

[Donald Clement <clement@commquest.com>]

andrew barrett wrote:

> I had been thinking of this but more so of a moving primary for my new
> scope
> ( still heavily in design stage) or perhaps combining moving primary and
> crayford.

Hello Andy,

Is your new scope a cassigrain? The main reason for moving either the
mirror or secondary in a cassigrain is to take advantage of decrease of
the travel distance that the focuser has to move and that accessories
and EPs are fixed. But this shorter focuser travel comes at a cost.
Moving the mirror or secondary in a cassigrain greatly amplifies any
defect in the focuser. All commercial SCT users are familiar with the
dreaded "image shift" while focusing. My first attempt to build a moving
mirror focuser was to use three lead screws that move together. This
worked reasonably well. Then I thought of how to harness the advantages
of a compliant or spring movement and apply it to a moving mirror
focuser. I'am still thinking of how to use compliant bearings for the
scope axis!  The advantages are: almost zero friction, no lubrication
required,  precision fabrication would not be necessary, and it will
work in harsh environments-- such as the RTMC sand blast area :>) What I
have come up with is shown at
http://www.inetworld.net/clement/focuser.html  Although not totally
completed yet, I haven't mounted the mirror on the focuser for the real
test, but the movement appears silky smooth and the structure is rock
solid. If I don't wind up using this compliant focuser to move the
mirror, it should work as a low profile type, 1" height at minimum, 
with 2.25" of focuser travel. I don't think that a crayford or helical
focuser could match this combination of low profile and long travel. 

Donald Clement
San Diego/Running Springs