Davoud:
...the pier needed to be approximately 15 cm (6 in.) due south of the
center of the dome.

Now, however, I am in the process of installing a GEM. A GEM ought to
be in the center of the dome, I am told...

I don't see why you think it would be an issue since you'll be there to
manually move the dome. If the dome was motorized, and under computer
control, it still wouldn't be an issue since all you would need to do is
define the dome/scope/mount geometry. This dome's controller could then
insure the slit would always be positioned in front of the scope's
aperture. But if the offset really bothers you, you could always install a
6" offset plate on top of the pier to center the mount.
-JATO
http://jatobservatory.org

I appreciate you taking time to reply. I asked not so much because I'm
worried but because I don't know why the offset is necessary for a
wedge/fork mount combination but not for a GEM. (Agreeing with you
entirely) I can't see why there would be a problem if I am there to
manually rotate the dome as I have always done.

I'm expect this will all explain itself once I use the mount in my tiny
zerbatory (a lot less fancy than JATO!) I only just put the mount in
place on Christmas eve, and haven't done my drift alignment yet
(though, if I may brag a bit, the polar alignment on my fork mount was
very accurate, and using the north index mark I had placed on the pier
to roughly position the GEM in its adapter, I found the alignment to be
good enough for visual use with no further adjustment!

Thanks again,

Davoud
http://www.davidillig.com/observatory14.shtml

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usenet *at* davidillig dawt com