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

Christopher Erickson

One of the things that RCOS did that I suspect contributed to the sag/flexure in the 20" scope is that they offset the truss mounting points on the secondary assembly from the primary assembly.  There are a number of other locations where they did similar things.  Even though the heavy aluminum rings seemed pretty-thick at those locations, they all obviously sum together into noticeable parallelogram-type flexure at low pointing altitudes.
When it was all said and done, it was easier to have a good T-point model and program a custom tracking rate than it was to re-engineer the RCOS OTA infrastructure.
FWIW, RCOS never acknowledged that there was any flexure, in spite of the accumulated evidence.
Christopher Erickson
Consulting Engineer
Summit Kinetics
Waikoloa, HI 96738

From: ap-gto@... [mailto:ap-gto@...]
Sent: Friday, July 25, 2014 5:46 PM
To: ap-gto@...
Subject: Re: [ap-gto] Maximum SAFE shimming thickness under a DOVELM162 clamp set ?

    I see your point – didn’t know that the RCOS truss flexures got that bad! I have eliminated that as a possibility on my smaller RC-14,  since it is always the precise same amount of arc-minutes when the AP-900 does a meridian flip, on a source at, or very near, zenith (rather than sagging at low skies), and reverses the offset exactly by doing more reverse meridian flips to about the same star location. I notice that the truss concept works in principle, but the RCOS truss poles have no “truss plates” at their ends, just pinned by end swing joint bolts. Some DOB trusses would be “somewhat” stiffened by their shaped angle truss pole “boots”. I believe major observatory truss tubes, do have truss joint plates.  Bridges constructed as trusses, always have truss joint plates – certainly wouldn’t risk driving over one without them.
    I wonder if a simple engineering fix, for existing truss OTA’s, would be a simple “truss plate CLAM Shell”,  formed to the angle and shape of two poles at their joints. That way, if there is any “sag”, it would be a quick, cheap, and easily user-added accessory to eliminate or minimize truss flexure, when heavy secondary cages, such as your 20-inch OTA,  point at lower sky angles.
    But we digress ... I was asking whether there was some “limiting thickness of brass shim”,  inserted between AP saddle and D-plate, that might preclude the possibility of loosening – (or dropping) -  the OTA. My need of a 2.5 mm shim doesn’t seem like much, but the notch height itself,  in each clamp is probably only about 5 mm high, thus not leaving much contact, and even that would be skewed  at the error angle. The clamp may not even fully close down, on the mating groove, unless there is a bit of give/wobble & rotation in the clamp halves, to actually mate with the now slightly raised D-Plate groove.
Perhaps my thinking here  is too convoluted.
    Then again ...  my estimate above,  was originally arrived at, based on my full ~22 arc-Minute offset angle. Perhaps I only need to shim for HALF that offset error. In which case, a thinner  1.25 mm brass shim then seems a bit safer, to test as a fix. Hope that the DOVELM162 clamps have a bit of “give” above the saddle, to at least accommodate a 1.5 mm vertical movement when closed.
Worth a try, with some concern, perhaps with a C-Clamp locked to the saddle & D-plate, for insurance?
Joe Z.
Sent: Friday, July 25, 2014 7:25 PM
Subject: RE: [ap-gto] Maximum SAFE shimming thickness under a DOVELM162 clamp set ?

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

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