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Subject: Re: [M]: LX 200 loading
From: Bill Arnett
Reply To: mapug@shore.net
Date: Sun Oct 05 01:25:06 1997
At 8:44 AM -0700 10/3/97, R. A. Greiner wrote:
>... I have just this morning measured the friction (stiction
>actually) of the 10" in A/A position. The stiction of the Dec bearing
>on this one is 2.5 foot pounds (torque). That is very much...
Hmmm. I tried to reproduce this result with my 12" LX200. I seem to have
only about a half a ft-pound of stiction. Lacking Doc's fancy basement
lab, it did the following:
With my LX200 in it usual home on my subwoofer in my living room, I pointed
it at the the ceiling. In this position it is pretty well balanced. I put
a long carpenter's level across the lens cap and a yard stick along side
it. And I filled a plastic bottle with exactly one pound of water
(counting the bottle). First I made sure that the level was level. I then
proceeded to set the bottle down on top of the level at various distances
from the centerline (== the optical axis in this case). I found that from
zero to five inches on either side the bottle would just sit. But starting
at about six inches out when I (very gently) set the bottle down then the
OTA would start to rotate (very easy to tell by watching the bubble on the
level). At seven inches it moved reasonably quickly. One pound at six
inches equals one-half foot-pound of torque.
Measuring it with the OTA level (pointed at the "horizon") was harder since
balancing it was not easy. So I just offset the heavy level until I got
the same stiction readings on both sides. Again, I got about six
inch-pounds.
I tried again with my Pronto sitting on top of the lens cap to see if
additional weight increased the stiction. But I got about the same 6
inch-pounds.
I estimate (wild ass guess, actually) that these numbers are correct to
about 10%.
I wonder why I got such very different results from Doc. Is my method
valid? Is there that much variation between LX200s?
My conclusion, if my method is valid, is that "good enough" means "no more
than 6 inch-pounds off true balance in either axis".
---
Bill Arnett billa@znet.com http://www.seds.org/billa/
"Science is a way of trying not to fool yourself." -- Richard Feynman