Dome Geometry?

When I built my tiny zerbatory it was not in my mind to switch from a
fork mount to a German Equatorial Mount (GEM). Using my profound
knowledge of both plane and spherical geometry, which knowledge is
measured in truly irrational numbers on the Euclid scale, and relying
on information provided by Le Sueur, my pier manufacturer, I determined
that the pier needed to be approximately 15 cm (6 in.) due south of the
center of the dome. I hit the target it within a millimeter or so and
it has worked like a charm. Dome rotation is not automated, but that
did not present a problem; I could observe for quite some time or make
quite a number of DSLR images before I had to manually rotate the dome
a little bit to keep the target in sight of the finder, the Telrad, and
the 8" SCT.

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. Moving he pier -- and the
1,600 lb. concrete footer it rests on -- is not an option. Rebuilding
the zerbatory around the pier is an option, but not one to be desired
or taken lightly. What kinds of problems, if any, am I likely to
encounter in tracking and fine guiding with the GEM improperly located?

TIA!

Davoud

--
usenet *at* davidillig dawt com

It might well depend more on your own circumference ralative to the
internal diameter of the dome wall. Forget the maths and draw a simple
plan view. You may end up as a planetary gear driving the dome rotation
yourself if you're not careful. (and honest about the scale of the
problem) ;-)

On Mon, 26 Dec 2005 01:56:18 -0500, Davoud wrote:

When I built my tiny zerbatory it was not in my mind to switch from a
fork mount to a German Equatorial Mount (GEM). Using my profound
knowledge of both plane and spherical geometry, which knowledge is
measured in truly irrational numbers on the Euclid scale, and relying
on information provided by Le Sueur, my pier manufacturer, I determined
that the pier needed to be approximately 15 cm (6 in.) due south of the
center of the dome. I hit the target it within a millimeter or so and
it has worked like a charm. Dome rotation is not automated, but that
did not present a problem; I could observe for quite some time or make
quite a number of DSLR images before I had to manually rotate the dome
a little bit to keep the target in sight of the finder, the Telrad, and
the 8" SCT.

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. Moving he pier -- and the
1,600 lb. concrete footer it rests on -- is not an option. Rebuilding
the zerbatory around the pier is an option, but not one to be desired
or taken lightly. What kinds of problems, if any, am I likely to
encounter in tracking and fine guiding with the GEM improperly located?

TIA!

Davoud


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

On Mon, 26 Dec 2005 01:56:18 -0500, Davoud wrote:

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. Moving he pier -- and the
1,600 lb. concrete footer it rests on -- is not an option. Rebuilding
the zerbatory around the pier is an option, but not one to be desired
or taken lightly. What kinds of problems, if any, am I likely to
encounter in tracking and fine guiding with the GEM improperly located?

The only real trouble you would have is when the telescope or
counterweight bumps into the dome. I think you would minimize the
diameter of the dome when the intersection of the polar and
declination axes is near the center of the dome. This is not
necessarily at the center of the pier. The offsets for your two
mounts are likely to be similar.

Even research observatories can have trouble with this. The staff at
one facility was chagrined to discover their new pier was too short
for their new telescope when it was all assembled.

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

--
usenet *at* davidillig dawt com

Davoud:
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...

William Hamblen:
The only real trouble you would have is when the telescope or
counterweight bumps into the dome. I think you would minimize the
diameter of the dome when the intersection of the polar and
declination axes is near the center of the dome. This is not
necessarily at the center of the pier. The offsets for your two
mounts are likely to be similar.

Thanks. My zerbatory http://www.davidillig.com/observatory14.shtml is
rather tiny, but it's not so tiny (or the Losmandy G-11 counterweight
shaft is not so long -- take your choice) that the counterweight shaft
will bump the wall or dome. I may have to rethink my sitting position
and my laptop position, however.

Even research observatories can have trouble with this. The staff at
one facility was chagrined to discover their new pier was too short
for their new telescope when it was all assembled.

I've just installed the mount, but haven't used it yet. I think it will
be OK, but the pier could be a bit tall. If so, I'll live with it or
order a shorter pier from Le Sueur; I'm not going to cut that beautiful
pier!

Davoud

--
usenet *at* davidillig dawt com

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.

Davoud,

I have experience using a fork mounted C-14 on a central pier in a dome.
Years ago there were two different Newts on GEMs in the dome. While the dome
is big enough to accommodate the 'problem' the C-14 is still in a difficult
position when pointing at the southern sky. The eyepiece is close enough to
the North side that the observer must be on the side of the scope rather
then behind it. This is a problem because the observatory design requires
using a rolling ladder to reach the eyepiece and the entrance is thru a
hatch in the north side of the floor. The scope would be easier to use if
the pier were about 12 inches south of center, or if the C-14 were on a GEM.

George Normandin

I may have to rethink my sitting position and my laptop position, however.

If your sitting position was fixed up-to-now, you may have to transfer
this onto a trolley to account for very different positions when the
scope is used East or West of the pier or to have longer cabling (to
laptop and imagers).

Pierre MK-UK