From: "Ron Wodaski" <ronw@n...>
Date: Wed Oct 23, 2002 2:57
pm
Subject: Testing Paramount ME recommended quick polar alignment
method
I am writing up the
documentation for polar aligning the
The existing docs describe a quick alignment method that uses the
Test setup:
Polar alignment #1: Quick method in existing
docs.
I followed the technique almost to the letter. I carefully leveled the portable
pier, using an 8" carpenter's level to check how level I was on each of
the three legs. This took just a few minutes because the ATS pier is easy to
level by adjusting the leg supports.
I deliberately did not set elevation and azimuth carefully, on the thesis that
it did not matter since I was putting light scopes on the mount and I could
move it as much as I needed to.
Otherwise, setup was as usual. I homed the mount using
TheSky to make sure that site information was correct. (Always select your site
and set precise time before you connect to the mount and home, so that the
site/time info is available for the home operation!)
I forgot about using a star on the meridian for this, but I still got good
results. I used a star rather low, actually, because I was feeling lazy and didn't
want to have to squat down on the dew-wet grass to center the star. I used a
star about 30 degrees above the horizon.
It was quick work to center the star in the Sky90. I then stuck a video camera
in the Sky90 and did a 10-point mapping run with TPoint,
which told me that I was about a half-degree off in alt and az (32.8 and 30.2, to be exact). I like to be within
3/4 degree, so a half degree was great for rough. The quick alignment method
gets my vote.
The second part of this exercise was to do a precise polar alignment. The idea
was to use a small refractor with virtually no flexure (the Sky90, a 90mm f/4.5
(405mm focal length) with the reducer I was using) to do a simple mapping run
solely intended for refine polar alignment. The C8 was on for some additional
testing I'll get to in a moment. (The DSBS probably added some flexure, but the
weight of the scopes was light enough that this proved to be a non-factor.)
I should note that during the 10-piont mapping run, I saw something that I
expect to see when mapping with a small refractor: after the sixth point, stars
are always very close to the center of the frame. This happens because a short,
light refractor doesn't have many variables - pointing accuracy doesn't change
much as you move around the sky. This is the key to success with this method:
use a small, light refractor with a rock-solid focuser.
I made the adjustments indicated by the first mapping run, then
did another 10-point run. This second run confirmed that the
The combination of these two techniques - rough alignment using the Home
feature, and a single TPoint run of 8-10 points -
allows you to get set up and imaging in the field in a very short amount of
time. You should have no trouble being well-aligned and ready to go before
twilight ends. With experience, you could be polar
aligned in a few minutes. And I did this manually with a video camera; would be
faster with a scripted 10-point mapping run.
Note: I talked with Patrick Wallace, the originator of TPoint,
about the short-refractor technique. He said essentially that the math is
relatively insensitive to the short focal length used for polar alignment, and
he was not surprised that it generates very good results.
Now for the reason that the C8 was mounted. I put an
ST-8E into the C8 (used an Optec Widefield reducer as
well, for a focal length of about 1000mm) and tried slewing around, using the
last TPoint model created with the Sky90. Despite using a different scope for imaging,
at a higher focal length (2.5x longer), slewing was extremely accurate. Slewing
across the east where I had mapped with the Sky90 (no mirror lock down in
place), targets were very close to dead center. So if you are imaging in the
field and follow the first two steps, depending on the focal length of your
imaging scope, you may only need to:
* Rough align
* Refine alignment once with TPoint
* Do a final TPoint mapping run with the same scope
used for alignment
The final run in this case was 15 points, which pretty much put everything in
the eastern sky nearly dead center. Even though the mapping run was done with
the Sky90, the C8 was nearly perfect as well.
This was a lot of fun to test, given the results. Fog rolled in just as I was
finishing up the tests, so I didn't get any imaging done.
Ron Wodaski
author of The New CCD Astronomy
http://www.newastro.com