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Old 02-02-2009, 09:48 AM
ausastronomer (John Bambury)
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ausastronomer is offline
Join Date: Mar 2005
Location: Shoalhaven Heads, NSW
Posts: 2,513
Originally Posted by erick View Post
I have the scope in this article:-

Look for the part:-

"Next problem: when I depressed the telescope tube from zenith to horizon, the tube shifts 8 mm to one side. I presume the altitude bearing is not perfectly circular. In any event, the end result is that my encoder assembly attached as it is to the altitude bearing also moves 8 mm to the side. The photo above illustrates this shift. My solution was, again, to avoid screwing the encoder arm to the rocker box. This is actually the recommended technique for many telescopes, including the Obsession: dont completely tighten either encoder arm, to account for such minor wiggling, although the 8mm shift noted here was more than most premium scopes would manifest. On the Obsession Yahoo group, Id heard about not even screwing the altitude encoder arm down at all, but rather letting gravity hold it motionless. This worked great in my case.
Hi Erick,

Allowing the encoder arm to float will certainly ease the mechanical load on the encoder and on the encoder tangent arm. Unfortunately with 8mm of lateral movement large pointing errors will eventuate. Whilst TPAS does a great job of compensating for mount errors, better pointing accuracy will result if the mechanical mount errors are minimised. That lateral movement of the scope on the altitude bearings is easily eliminated by "shimming" the altitude trunions with teflon pads. My 10" scope had about 7mm of sideways movement before I fitted argo to it. I shimmed it with 3mm teflon pads either side and it is now perfect. I will post picks later today when I get home from work. A further improvement can be obtained by "bracing" the side of the rocker box with a 90 degree tapered brace, to reduce flexure of the sides of the rocker box.

John B
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