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Subject: Re: [M]: LX200 2" diagonal
From: Ed Stewart
Reply To: mapug@shore.net
Date: Fri Aug 08 02:57:06 1997

>>Try an O-ring between the diagonal and the telescope.

>Won't that allow the diagonal/eyepiece to flex too much? After the recent
>comments here about not even moving the diagonal after alignment, an O-ring
>seems awfully risky. How much flex does actually allow? Does it really
>make it easier to rotate the diagonal? I would like to hear from anyone
>with experience on this point. Bill Arnett

I decided to do some testing with O-rings of several sizes and in different
locations. In examining the interface where the lips of the threaded
connection on the back of the scope meets the one on the diagonal, I
determined that the "grip" between these two lips is a result of the surface
friction between them and the lateral force produced by the threaded collar.
The interface surfaces are very smooth on both, being painted metal. So there
is little friction between them to resist the rotational forces of a large eps
in the diagonal. This causes us to have to vigorously tighten the threaded
collar--notice that there is very little rotation possible once the slack is
taken up in the threads, because there is no compression in the metal parts.

This lack of any compression holds the accuracy of the alignment of the
diagonal to the scope, but the trade-off is that the friction can suddenly
release, causing the ep to rotate towards the ground and possible fall out.
Also, there is no range of friction to adjust so that the diagonal can be
rotated without unlocking. Perhaps this was all evident to everyone else, but
I need to understand exactly what was happening in order to proceed with the
O-ring test.

I purchased a 1/8" (about 3 mm) and a 1/32" (about 1 mm) O-ring. First, I
placed the 1/8" inside the threaded collar so that it was between the
interface lips. When the threaded collar first seemed to tighten there was
about 5-10 degrees more rotation as the O-ring compressed based on how hard I
tighten the collar. It was possible to rotate the diagonal clockwise when the
collar was not too tight, but when attempting to rotate CCW, the diagonal
would release. Was unable to find a "magic" tension that would allow safe
rotation and not risk a breakaway with a heavy ep. As to maintaining
alignment, I could induce several degrees of deflection by pushing/pulling on
the ep, but whether the weight of just the ep caused deflection is
uncertain--would think a 20 mm Nagler would cause at least 1-2 degree
deflection when OTA is near hortizontal, but less when pointing higher up.

Second, the 1/32" was tried in this location. The amount of compression was
obviously less, but there was no adjustment that would work while rotating CW
and CCW.

Third, the 1/32" O-ring was slipped over the diagonal's lip onto its tube that
the threaded collar is captive on. Thus it was between the backside of the
diagonal's lip and the interior surface of the threaded collar. This is a
better placement since the metal-to-metal contact interface will hold the
alignment while the lateral force has the adjustable compression of the O-ring
to the make various tensions possible. But, again, I couldn't find a tension
that would work while rotating CW and CCW. Perhaps some grease would have been
helpful, if using lighter eps, to allow rotation without releasing the
necessary tension on the interface. Of all the combinations with O-rings, this
was the best.

Fourth, I decided to try some 1/32" or 1 mm abestost gray gasket material
(like that used for auto water thermostat housings). It has very little
compression, but has a matte surface to provide more friction between the
interface lips. When the threaded collar first tightens, I can get 1-1.5
degrees more rotation than without. I can use less brute force to get a secure
grip so that a large ep won't break loose, but there is still no joy for
making the diagonal rotateable without releasing the threaded collar slightly.
BTW, this gasket was glued with a very thin layer of "Goop" to the diagonal's
lip and tighten to the scope to dry so that it would setup in proper
alignment, i.e., flat to the interface lips. Superglue would be a good choice too.


BTW, the Mapug Topical Archive was updated earlier this week.
--
Ed Stewart, Austin, TX <stargzer@austin.cc.tx.us>
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<http://www.austin.cc.tx.us/astro-ES/AstroDesigns/index.htm>
Mapug (Meade Adv. Products Users Group) Topical Archive site.
Accessory designs and information for the Meade LX200 and Dobsonian telescopes.