----- Original Message -----
Sent: Tuesday, May 22, 2001 1:17 PM
Subject: [M]: Pointing accuracy, permanent piers, and leveling


> A couple days ago I posted a message about a problem I was having with
pointing
> accuracy being 3-5 degrees. Someone emailed me suggesting that I read
part of
> the article on modifing the LX200 Dec bearings written by Michael Hart
that is
> on Doc G's website. Anyways, the article had a section describing how to
align
> the optical tube with the fork mount by centering the eyepiece light cone
on
> the RA bearing retaining screw (behide the plate at the base of the fork
> mount). Anyways, this seemed to help drastically and seems to have
reduced the
> error down to 30-60 arc-minutes (I haven't throughly tested it yet).
>
> That leads to my questions. When testing this fix tonight I did a fairly
quick
> level that wasn't perfect (like any are). How good does the leveling have
to
> be? Is this precision achievable with the LX200 bubble level or with a
simple
> plastic level or is a digital level required? According to the LX200
manual
> the leveling isn't critical in 2-star ALTAZ mode. Is this true? I set up
in
> both ALTAZ and POLAR mode and got about the same pointing accuracy.

The general consensus is the levelling is less important than alignment star
positioning. Use a reticle eyepiece (or high the highest magnification you
have)
to centre each of the alignment stars as accurately as possible. Pay careful
attention also, to which alignment stars you use. Choose stars far enough
apart, and avoid stars near the celestial pole.

My understanding is that levelling isn't necessarily that difficult, so it's
worth spending the 2 minutes or so required to get at least the bubble
roughly centred - but don't assume that your bubble level is perfectly level
with the LX200 in the first place.

> My second question is on the error contribution from the surface the
tripod is
> set up on. I set up tonight on soft cushy grass. It seems to me that
would
> cause quite a bit of error since as the scope rotates the weight is
distributed
> to various tripod legs which thus sink more than the others and throw off
the
> level. Do you think this would be enough to cause the 30-60 arc-min
error?
> Also I suspect some of this error was caused by mirror shift especially
since I
> was slewing thru the meridian.

"Soft cushy" grass is definitely not a solid enough surface to expect zero
errors, and a degree or so of error doesn't seem unlikely in my opinion.
Don't waste your time on the mirror shift theory until you set up on solid
surface.

> So I guess my main is question is whether 60 arc-min of pointing error (in
> either ALTAZ or POLAR) is typical when only a "quick" setup on a grassy
surface
> is done?
>
> Finally, assuming everything is perfectly level and the tripod is setup on
> concrete how much of an improvement in pointing accuracy do you get by
having a
> permenent pier?

In theory you can get just as accurate pointing on the tripod as you can on
a permanent pier. Tripods have other issues, such as vibration, accidentally
kicking the legs out of alignment, the hassel of setting them up etc.
against them more than anything.

> Thanks,
> John Teel
>
>
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