Re: Maximum SAFE shimming thickness under a DOVELM162 clamp set ?

Christopher Erickson

I have worked on a 20" RCOS scope where everything was seemingly rigid, however there was a "parallelogram" sag going on that would progressively push the pointing off as the OTA pointed to low altitudes.  The highly-convex secondary really exaggerated the sag.
Christopher Erickson
Consulting Engineer
Summit Kinetics
Waikoloa, HI 96738

From: ap-gto@... [mailto:ap-gto@...]
Sent: Friday, July 25, 2014 11:12 AM
To: ap-gto@...
Subject: Re: [ap-gto] Maximum SAFE shimming thickness under a DOVELM162 clamp set ?

Thanks Chris,
    Can’t see adding upto 10 mm under between saddle and D-plate. Guess you meant you had added a 10 mm “riser” on some other scopes.
    Done the RC-14 collimations several times, ad nauseam. Stars look OK, for my taste. Hand cranked EVERY screw on the entire truss – none moved.
Also, I reached in and gave the edge of the 14 inch mirror a very firm “rattle tug” – none felt, seemed rock solid, so don’t think its 20 lb weight sags any amount in its cell, on a flip.
    Finally, last night I bolted on another 14” Losmandy D-Plate,  UNDER the DOVELM162, as an extension plate and hung my telephoto mirror  500 mm f/8 lens with RGH attached, (giving an almost identical FOV to RC),    comparing CCD views before & after a meridian flip. This telephoto lens is the perfect guide scope. The RGH view seemed “symmetrically rotated” after the flip, while the RC lost its centered target. So, everything up to the saddle  (with the RGH & guide lens) seems squared. Only other thought is maybe the RC optics are collimated “as a paired set”, but their optical axis is slightly askew of the mount DEC  axis.
    Still pondering the test results, but RC truss orthogonality is still suspect. I had hoped that a simple test with a brass shim stuck in between the front of the saddle and the D-plate, would quickly confirm a case of non-orthogonality. But ~2.5 mm separation from the clamps has me concerned about attempting it.

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