Re: Maximum SAFE shimming thickness under a DOVELM162 clamp set ?
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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 www.summitkinetics.com 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 ? Chris,
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.
From: mailto:ap-gto@...
Sent: Friday, July 25, 2014 7:25 PM
To: ap-gto@...
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 www.summitkinetics.com
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