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

Christopher Erickson

Sorry for the delay in answering your message. I have been really-busy.

The VYSOS-20 RCOS telescope flexure is very predictable. Solution has been
to have a good T-Point model and use variable tracking rates at different
altitudes above the horizon. The scope is operating without an autoguider
because the scientists don't want to use a rotator to find guide stars
because they are building image mosaics and don't want to rotate the camera.
Yes, I know that camera rotation is not an unsolvable problem when building
mosaics. I just have not yet convinced the right person to experiment with
a different solution. The 3600GTO mount is so solid and versatile that
T-point and custom tracking have given them what they want.

I don't think that loosening and resettling all of the trusses on an RCOS
scope will have any positive effect on flexure.

I don't think that a drop in shipment would contribute to flexure unless
something was actually broken. Maybe check the bonding between the truss
tubes and truss tube connectors to make sure that there hasn't been a glue
bond failure someplace.

I don't think that polar aligning on both sides of the meridian would have
much effect. Sounds like chasing gophers down rabbit holes with a Jeep.
Lots of effort, cursing & dust and not much in results.

Personally I would polar-align, do a good T-point model and then autoguide.
After all, just about all scopes have some optical-axis flexure and you can
never eliminate all of it. Trusses, plates, mirror cells, secondary
assemblies, tubes, focusers, threadings, couplings, brackets, coupling
clearances, fasteners, rotators, bearings, bushings, cameras, etc.

I hope this helps.

Christopher Erickson
Consulting Engineer
Summit Kinetics
Waikoloa, HI 96738

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Friday, September 12, 2014 7:32 PM
To: ap-gto@...
Subject: Re: [ap-gto] Maximum SAFE shimming thickness under a DOVELM162
clamp set ?


In that situation that you had been consulting on - a 24-inch badly
flexing truss RC (in Hawaii ??).
Was that a case of a dynamic flexure - i.e. it shifted out of alignment,

and returned "exactly" to where the scope started from, every time the
scope dipped? That would be difficult to solve even by welding extra "truss
gusset plates".

I briefly considered pointing my truss OTA at the zenith in Park-1, and
loosening all the lower and upper truss pole boot screws, to let the truss
"center itself"before retightening, and hopefully remove any residual flex

or warp, in its mechanical axis. But then, it could also leave it adjusted,
far worse, after settling.

I think my situation is somewhat more stable, OTA doesn't flex in
opposite directions on a flip, since the shifted RA error does not seem
change markedly in size after a flip. The meridian flip just points the
scope too far downward instead of up. The truss might have been warped in
UPS shipment, and thus is a case of "static" flexure. i.e. It was
seriously flexed only once when possibly dropped in transit, and remained
stuck, with a residual a 3D vector mechanical misalignment angle, of the
collimated optical axis, from the mechanical OTA axis. The vector
X-component has been nulled out with a shim to make the optical axis once
again orthogonal with the DEC axle. However, the Y-component (RA) is still
stuck at a small residual angle, at any elevation of the truss OTA. Of
course, the Z-component has no effect, since it is just pure DEC rotation,
which an RCAL handles nicely.

My concern is whether (at least) a correct polar alignment "of the
mount" can be better approximated, by doing the PA on BOTH sides of the
pier, and then decreasing the Alt/Az adjustments by "half the difference"
on the side that gives poorer star drift results. I am thinking along
analogous lines of Roland's special "optical" method for alignment, and
doing the same, using PemPro graphing of CCD results.

I know I can get down to about 10-arc-seconds drift, or better, on one
side - doesn't matter which side, east or west of pier, I start with.
Flipping over, does result in a much larger Dec/RA star drift, because of
that truss warp. After flipping, I would then change the PemPro PA
adjustments for Alt/Az by exactly "half the difference" in the worst case
drift. Then flip back to the original starting pier side, redo the PA, and
"purposely worsen" the drift by that same amount. So, lowering the drift on
the worse side of the pier, and "raising" the drift back up on the original
best drift side, might get the PA closer to a midpoint optimum solution.
Otherwise, the PA is biased too far to the "initial set" of PA alignments,
which are in fact not quite correct.

I'm hoping that PemPro can be made to "optimize" a poor polar alignment
situation caused by flexure - rather than arbitrarily choosing one pier side

results as gospel. PemPro is meant for a perfect scope, and a single PA run
can be chosen for one side or the the other. But the user procedure I
describe might make it work for worse situations.

I'm guessing, but would this not make a bad and unfixable situation,
with any case of dynamic flex or warp, somewhat better or at least
tolerable, and perhaps even closer to a more accurate mount polar alignment?

One would still have to handle, (or model), the residual shift in RA after

every meridian flip, but at least the polar axle might be pointed closer to
the NCP, rather than at some larger error offset angle, on one pier side or
the other, and not letting PA further add to an existing mechanical

Ray, Rolando - Would this work with PemPro?

-----Original Message-----
From: 'Christopher Erickson' christopher.k.erickson@... [ap-gto]
Sent: Friday, September 12, 2014 4:22 PM
To: ap-gto@...
Subject: RE: [ap-gto] Maximum SAFE shimming thickness under a DOVELM162
clamp set ?

The offset between the RCOS inner-truss mounting points and the outer-truss
mounting points is one flex point but I suspect that there is another nasty
one in the RCOS primary mirror mounting system. They are both
likely-fixable but I am not sure it would be worth the effort and expense
when software can make up for it rather easily.

Christopher Erickson
Consulting Engineer
Summit Kinetics
Waikoloa, HI 96738

Posted by: "Joseph Zeglinski" <J.Zeglinski@...>

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