Ad Astra Observatory

A,

The guy who wrote Gemini (Rene Goerlich) had this to say about Polar
Axis Correction, etc.

4 things of interest to note:
1) before 1st PAC, do a minimal pointing model
2) after 1st PAC, run a “big” pointing model

3) for photo work, run PAC more than once.
4) After a PAC, it automatically kills the PM and Synchs on the current
star.

This seemed to be my experience as well. (although I wasted some time
shutting off, going to CWD, and cold-starting. Apparently that’s
unnecessary due to 4)…).

J

Hello Group,

Maybe I can shed some light on PAC, Sync, Alignments, modelling, …

Gemini’s Sync[chronize] works differently, depending on the circumstances:
1) As long as a pointing model isn’t established yet, Sync simply shifts
Gemini’s internal coordinate system to the given coordinates.
2) Once a pointing model exists, Sync adapts the offset parameters IH and ID

so that the model points exactly to the given coordinate

There is another function available after Gemini was synchronized the first
time: The Reset Alignment menu point under Align Telescope. This will erase
the pointing model and adjusts the internal coordinate system to the given
object coordinate. This is comparable to the “Initial Alignment” of earlier
Gemini software levels (but without the now automatically solved East/West
question).

PAC internally calls this “Reset Alignment”. BTW, a Sync is therefore not
necessary after PAC, but it won’t hurt.

Why do I completely reset the pointing model, not only the A and E terms?
There are two important things to consider:
1) The user may want to further refine the pointing by doing more Additional
Aligns. For this, I need the collected alignment data, but these are not

longer valid after doing the PAC.
2) The errors from all the many imperfections are modelled into the terms you
know from the manual. But of course, the real life has far more imperfections
than a model has parameters 😎 That means, the A and E values reflect not
only A and E, but carry all the errors after a three star alignment. With more
alignments, more terms are calculated (maybe you watched the A and E values
changing after doinmg more Additional Aligns… new terms,
nonperpendicularities, flexure and so on get their share. Because of this,

keeping the other parameters would introduce new errors.

So what are the strategies I can recommend to use the PAC?
If you set up your mount in the field you are approximately off the pole in
the magnitude of a degree or three. This means, the polar alignment errors
exceed all other errors significantly. No need to build a sophisticated model
with many stars and derived terms here. Do a 3..5 star align (one side of the
mount, one hemisphere is enough), select a star near the intersection of

equator and meridian and do the PAC. Look through the finder while the mount
moves to be sure to adjust to the right object…

This will bring down you polar alignment error to the magnitude of 10..30
arcmin. This should already be fine for visual observing after doing some
Additional Aligns.

If you want to take photos, do some more Additional Aligns (about 10) and
repeat PAC. This reduces the offset to 0..5 arcmin, typically 0..3.

Ren�

In the Northern Hemisphere:

If you have an A:- (minus) then you have to move the front of your
scope towards EAST (Clockwise if looking from above), i.e. you are too
far WEST.

A:+ indicates you have to move the mount’s front to the West
(Counterclockwise if looking down from above).

E:+ indicates you have to raise the mount.

If you have an E:- (minus) then you have to lower the front of the
telescope, i.e. you are too HIGH. <cid:part1.02060300.03050004@jimbo.net>