Re: [APML] OT: Losmandy GM 100 users

[Jerry Lodriguss <jerry@astropix.com>]

Aike asked me off list about the polar alignment scope that I use with my 
Losmandy GM 100.

Since this may be of interest to others with that, or similar mounts, I 
thought I would post my reply to the entire list.

There is a picture of the home-made rifle-scope polar alignment scope on my 
web page at: http://www.astropix.com/HTML/I_ASTROP/I02/I02.HTM  Scroll down 
to the bottom of the page to see it.

There is not much to it. Just a piece of channel aluminum that the rifle 
scope happens to fit in. The channel aluminum fits flat against the flat 
back of the right ascension clutch lock knob. The round aluminum stock was 
machined to the size of the rifle scope barrel and is attached to the flat 
face of the channel aluminum. A nylon screw locks the rifle scope down.

Take the big clutch lock knob off the mount completely to drill and tap it. 
A large washer and 1/4 - 20 thumbscrew is used to secure the channel 
aluminum to the clutch lock knob.

The rifle scope is a very inexpensive $15 scope.

Calibrating the rifle scope to the pole.

The first time you use it, you have to drift align the mount before you 
calibrate the rifle scope. You just do this by trial and error. The first 
time it may take some time because you can't get close with the polar 
finder to start.

After you have drift aligned the mount, then you put the polar finder on to 
calibrate it.

Mark one of the cross hair adjustment knobs on the outside of the rifle 
scope so that you can identify it. Orient that cross hair that you marked 
by rotating the rifle scope so that the marked cross hair points exactly to 
5 Ursa Minor.

5 UMi is a 4th magnitude star right next to the 2nd magnitude Beta UMi. You 
kind of have to do this with one eye to the guidescope and the other eye 
picking up 5 UMi.  It helps to illuminate the cross hairs by shining a red 
flashlight down the front of the rifle scope.

5 UMi has a right ascension of 14h 27m.  This is almost exactly in a line 
with Polaris, which has a right ascension of 02h 33m, and the pole. So 
Polaris is on one side of the pole and 5 Umi is on the other side of the 
pole. You naturally won't be able to see 5 UMi in the rifle scope, but you 
will be able to see Polaris. 5UMi is also closer to the pole and in better 
alignment than either Epsilon Cassiopeia or Alpha Draco or Eta Ursa Major, 
which are other stars that are in rough alignment with Polaris and the pole 
that are sometimes suggested for this purpose.

Once you have the marked cross hair pointing at 5 UMi,  you then adjust the 
other cross hair by moving it with the adjustment on the rifle scope so 
that Polaris is centered on the cross hairs. Note that you do not disturb 
the polar alignment of the mount at any point in the rifle scope 
calibration.  The mount was drift aligned to start before you began the 
rifle scope calibration procedure, remember.

The next time you go out with the mount, you simply use the rifle scope on 
the mount and rotate it so that the marked cross hair points to 5 UMi and 
then adjust the altitude and azimuth of the mount so that Polaris is 
centered on the cross hairs.

There is no need to calculate the time of night, or the day of the year to 
orient the cross hairs, like needs to be done with some other polar 
finders, such as the one that used to be sold by Roger Tuthill.

I find this method is accurate enough to shoot 85mm focal length lenses for 
10 minute exposures without any further drift aligning.  For long-focal 
length shots at long exposures, accurate drift aligning is necessary after 
rough polar alignment with the rifle scope.

Jerry


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