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Subject: [M]: Re: Location of Polaris
From: Tom Krajci
Reply To: mapug@shore.net
Date: Mon Nov 11 02:32:21 1996

>In north aligning my LX50, some confusion has set in regaring the
>actual location of the North Star. Please tell me
>where the north star "should be" in my field of view.

If you've got a computerized 'scope disregard this long post.

Richard, if you have access to star charts like Uranometria or Sky
Atlas 2000 (by Wil Tirion) or any other charts that show stars to
magnitude 7 or fainter - make a xerox copy of the area around polaris
and seal this in plastic for outdoor use. You'll see that polaris is
about 0.8 degree from the celestial pole. For casual visual
observing, centering right on polaris is often good enough. For
photo work I use this method:

Set the tube assembly to 90 deg declination as best as you can (and
lock it there!) and aim north with compass and mag variation data.
Once you see polaris in twilight, adjust the wedge/tripod as best you
can sighting along the tube for best azimuth and elevation. Next
comes the precise alignment part.

Rotate the fork in right ascension (it's still firmly locked at 90
deg dec!) so that the finder is at the top of the tube assembly.
Adjust the finder cross hairs so it's centered on polaris. Now
rotate the fork assembly through about 30-45 degrees and watch the
motion of polaris relative to the cross hairs:

In this case I'll asume you have a straight through finder (no right
angle prism) and that you rotate the fork assembly clockwise (I
assume you're standing behind, south of, the fork, looking north
toward polaris).

If, while you rotate the fork clockwise, polaris moves to the right
of the cross hairs your finder aim point is too high.

If polaris moves down your finder aim point is too far to the right.

Make the corrections to your finder aim point. Polaris will now no
longer be in the cross hairs of the finder, so adjust the wedge/tripod
to re-center polaris in the cross hairs. (DON'T change the dec
setting of the tube assembly - keep it locked)

Repeat this process of rotating the fork clockwise, applying the two
IF rules on observed drift motion, adjusting the finder, and
re-centering by moving the wedge/tripod assembly. The drift motion
will get smaller and smaller until you converge to. . .

The finder's cross hairs are parallel to the polar axis of the fork!
(Or pretty close) Now you have to point the whole schmoo to the
celestial pole, not polaris!

Remember that plastic coated chart of the polaris region? Look at
polaris and the nearby 5th, 6th, 7th mag stars and see where the
celestial pole falls in this pattern. Move the wedge/tripod assembly
to point the finder cross hairs as best you can toward the celstial
pole. You can probably get to within 0.2 (maybe 0.1) degrees of the
pole this way.

Tip: Make a laminated card that has the two IF rules and keep it handy
during this process. My mind often falters in the rush to set gear
up on a clear night and a checklist takes out the guesswork.

How accurate is this method to reduce declination drift? After a few
practice sessions you get good enough that for piggyback
astrophotography (up to 300mm telephoto) it keeps drift low enough to
be manageable every time you set up. After a few more practice
sessions you will find that it's good enough for astrophotography
through the 'scope. . .without using the drift method! On one 'scope
I can now consistently get declination drift to 1 arcsecond per
minute or less. . .and that means my right ascension drive errors are
the biggest source of guiding corrections.

I hope this wasn't too confusing. I'll try to explain it a different
way if questions come up.

Capt Tom Krajci
B-52 Intelligence Officer
"Intelligence: Ignored in peacetime, blamed in war!"
http://spur.barksdale.af.mil