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

Christopher Erickson

Like I said afore, RCOS scopes have more optical axis shift from OTA flexure
than you would expect.

DEC error after meridian flip doesn't surprise me in the least.

If it is indeed OTA flexure, it will vary according to OTA orientation to

I hope this helps.

Christopher Erickson
Consulting Engineer
Summit Kinetics
Waikoloa, HI 96738

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Thursday, September 11, 2014 8:50 PM
To: ap-gto@...
Subject: Re: [ap-gto] Maximum SAFE shimming thickness under a DOVELM162
clamp set ?

Ray - Thanks for the additional leads on my problem. However,

(1) Doubt PA setting is actually "shifting", because I really lock down the
Alt knobs hard after getting the best possible Alt adjustment, , and the Az
is already firmly locked by the adjuster knobs on each side. The AP rotating

pier adapter's, rotation locking screw was NOT initially tightened (just
factory set), but locking it down firmly, had no effect on this problem.

(2) Tube currents aren't an issue in my RC "open truss". Images and star
trails are stable. Besides, tube currents shouldn't cause a shift on the
order of that many "arc-minutes" - perhaps just arc-seconds ???

I appreciate the clues. However, not sure we should be calling it
"DRIFT" since I am not talking about "tracking" and watching the star
"gradually" drift away. The DEC shift appears right after the flip. The
star's original position, very close to the meridian, at the equator, is
first precisely centered in the frame and is RCAL'd, before doing the
After the flip, the star is immediately off-target by 3 arc-min in RA (quite

understandable and easily fixed with another DEC shim),
and an inexplicable 8 arc-min in DEC - I don't know what causes it, or how

to decrease this jump.

During the day, I have used a special (one foot long) precision bubble
level, "carefully balanced" on the mount's AZ adjuster block, then doing
several flips, to see if the mount shook at all, or even leaned somewhat to

one side during a flip and back again, thus causing a potential PA change -
It was rock stable, bubble remained centered. So, no shift on the Losmandy
tripod or its sunken concrete block foot pads.

Countless PA runs, in fact carefully running them half the night,
patiently tweaking AZ and ALT knobs, then locking those down hard, with the
hex wrench, didn't change that DEC offset. I can easily take 15 minute
unguided photos with no star drift, so PA is pretty good.

This post meridian flip target centering is very frustrating.

It is interesting that there is a large difference in a PA run repeated
on the flip side, minutes after a near perfect PA adjustment on the original

side. It would seem there is some "common cause" that the meridian flip
changes the initially good PA done on one side, just like there is a DEC
image shift on ANY meridian flip. Would seem that some kind of shift is
responsible for both situations - different PA run results, and different
post meridian flip star centering.

The DEC shift remains the same size, no matter which side the centering

and RCAL is done from first. Doing it on the east side of the pier, and the
flip results in the same 8 arc-min image shift on the flip side. Repeating
the centering and RCAL there, and reverse flipping, results in the same DEC

shift - of course, now in the inverted frame direction (N or S).

Obviously, something is shifting during the flip, and it is consistent
and always precisely the same size.
I have already checked the footing, the pier, the mount, and the DEC worm
mesh. I have even grabbed hold of the edge of the RC-14 mirror and given
it a firm shake in the truss - and the mirror doesn't shift or rattle in its

cell, with that force. Did the same with the secondary baffle, no rattle
either. So, I don't know what else might shift so easily - just under

I have been trying to resolve this nuisance problem for several years now,
so I would appreciate further thoughts, at any time, if you think of any
other possibilities.

Many thanks for trying,

-----Original Message-----
From: 'Ray Gralak (Groups)' groups2@... [ap-gto]
Sent: Friday, September 12, 2014 1:00 AM
To: ap-gto@...
Subject: RE: [ap-gto] Maximum SAFE shimming thickness under a DOVELM162
clamp set ?

You have a polar misalignment in altitude, and that will produce the
offset that you experience. Do a thought
experiment - lets say that your altitude is off by 4 degrees. Then if you
start from a star on the east side, do a
meridian flip to the west side, how far off would the star be on the west
side? Give up?
In a perfect telescope 4 degrees in polar alignment altitude error would not

affect the polar alignment at the meridian. At or very near the meridian an
error in altitude polar misalignment causes virtually no effect on
declination drift. As the scope moves further from the meridian then the
scope would start to see some effects from altitude error.

If I had to guess the delta between sides is being caused by either:

1) The polar alignment actually shifting, or
2) By a difference in tube currents. Flipping the scope mixes the air up in
the tube... as the warmer air raises to the top over time it can cause a
change in the refractive position of the measurement star.

-Ray Gralak
Author of Astro-Physics Command Center (APCC)
Author of PEMPro:
Author of Astro-Physics V2 ASCOM Driver:
Author of PulseGuide:
Author of Sigma:

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

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