Geometry and Leveling of Equatorial Mounts?

Hi Davoud,

Will you be able to rotate the wedge about the pier? It doesn't need a lot,
just a little bit. In other words, will you have the ability to adjust the
wedge base so it points true north? If so, you don't need any other
adjustment outside of the wedge's ability to adjust for latitude.

It may sound wrong, but picture what is happening. All of the adjustments
have nothing to do with being level. The only key adjustment is making sure
the polar axis is aligned with the earth's axis of rotation. The polar axis
will be set at a right angle to the top of the wedge. If you can adjust the
wedge base around (az) and the wedge is adjustable (alt), you have a simple
alt-az mount for pointing the RA axis. If instead of your big scope, you
mounted a plate with a small polar pointing scope welded in place, at a 90
degree angle to the plate, you would be pointing that scope at Polaris, with
the offset dialed in.

It is the same as a GEM. Unless the goto requires it, a GEM does not need to
be level. In fact, you could bolt it to the side of your house so it stuck
out parallel to the ground (or at any angle you wanted). Then just use the
az adjustment and the latitude adjustment to bring the polar axis in line
with the earth's axis. Once that is done, the scope will track perfectly.

However the part about being a few degrees off not hurting anything is
wrong. That is not why you don't need the extra provisions for adjustment.
Being off a few degrees would hurt your images. This is why people take time
for drift alignment. The reason you don't need the extra adjustment screws
is because by rotating the wedge in az and adjusting it up and down, you can
set it so you aren't off by a couple of degrees. Your accuracy is only
limited by the time you put into it.

Hope this helps.

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
Lunar Picture of the Day http://www.lpod.org/
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"Davoud" wrote in message
...
I got into a little argument today with an expert on telescopes and
just about everything to do with them (not a self-styled expert,
either, but a recognized expert). I showed him my drawings for a pier
that I'm going to have built.

My base mounts in the same manner as an AstroPier
http://www.astropier.com/installation.html, which means that it will
be mounted slightly above the concrete foundation by the use of hex
nuts on the J-bolts above and below the pier's base plate to allow for
precise leveling of the top surface to which my Milburn wedge will
mount.

The expert said that it is unnecessary to mount the pier in this
manner; it should bolt directly to the concrete footing for better
stability. He said that it does not matter whether the pier is exactly
perpendicular to the base; a couple of degrees in any direction will
not affect telescope tracking.

I argued that the base of the wedge (and, if shims are to be avoided,
the surface on which it mounts) must be as level as possible;
perpendicular to a line dropped from the bottom of the wedge to the
center of the Earth, if you will. I've always assumed that that is why
they put bubble levels on wedges.

Otherwise, I reason, as the telescope follows a fixed star, adjustments
in declination will be required as well as movement in R.A. The result
of that, I believe, would be slight field rotation over time.

So, who's right?

Davoud

http://www.davidillig.com/observatory.shtml

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