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

Christopher Erickson

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

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Friday, September 12, 2014 6:06 AM
To: ap-gto@...
Subject: Re: [ap-gto] Maximum SAFE shimming thickness under a DOVELM162
clamp set ?

Ray, and Chris Erickson,

I think you are bang on ! It must most likely be "truss flexure".

Thinking further last night, about how this DEC shift could come about,
I can see a "Non-orthogonality of the Optical and Polar axes" would most
likely be caused by tube (truss) flexure. The optical axis, (for whatever
reason), fails to intersect the mount's polar axis. at the centre of

Chris once mentioned in his post, a "parallelogram type shift" in the
OTA truss poles. But, it should disappear if the flip is done at zenith, yet

if anything, the DEC shift grew slightly larger. But, wouldn't a truss flex

also result in a star's corresponding shift in RA, mimicking a DEC
non-orthogonality? When I shimmed the OTA's DEC plate, the star's RA shift
after meridian flip, has been hugely reduced, and can easily be brought to
zero with a further shim. So, if the problem were purely truss flexure,
then this DEC axle shimming, should not have had any effect.

Or, just perhaps, a shifted positioning of the secondary spider ?
Looking through my TAK alignment scope, I see the secondary mirror
reflections of one pair of spider vanes "slightly shifted vertically" from
their corresponding reflections in the primary mirror. Thierry Legault's
document on "Collimating mirrors using the TAK Scope", points out this
spider shift anomaly. Unfortunately, all attempts to even up this spider
vane pair, showed no change no matter how far I decentered the spider. I
concluded that the spider, in this case, was not the cause of my problem.

Thank you gentlemen. I see I have a lot more investigation ahead of me.

The problem, whatever the cause, seems to involve polar axis
non-orthogonality. Perhaps, in the case of a truss, not much can be done,
other than coming close, and resorting to a pointing model.


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

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