Re: Crayford style focuser for Vario-Finder 10x60 ; 61x250 f/4.1


Mike Dodd
 

On 10/18/2020 1:48 AM, CurtisC via groups.io wrote:
Rolando has definite opinions about this. As I understand it, the main
scope is (usually) mounted in rings lined with felt. As the scope
moves, it moves around a tiny bit inside the felt lining. But the guide
scope, in the setup you describe, is mounted rigidly on top of the
rings. The main scope is moving within the rings. The guide scope is
not moving. This introduces errors in the guiding.
When I was tracking down differential flexure, I considered EVERYTHING that might move, summarized here: <http://astronomy.mdodd.com/flexure.html#FlexureTable>

* Replaced the ring felt with solid styrene.

* Moved the guide scope/camera from atop the OTA rings, because a ring itself can flex with that extra weight.

* Bolted the guide camera to a dovetail clamp so it didn't hang on the guide scope focuser (in fact my guide scope no longer has a focuser).

* Machined a 1/2" aluminum "backstop" for the ST-402 guide camera because its case flexed when attached with a single 1/4"-20 bolt in its tripod socket.

Almost certainly a lightweight CMOS camera hanging on a robust focuser will have little, if any, flexure. But the only way to be sure is to test various things until the flexure disappears.

I even bought from Optec a tube to temporarily replace my Pyxis rotator, in case the rotator was flexing (it was not).

FWIW.

--- Mike

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