Re: Stupid question about polar alignment
- From: Davoud <star@xxxxxxx>
- Date: Thu, 22 Dec 2005 12:09:22 -0500
Just Me:
> If my mount is properly aligned to Polaris, when Polaris is centered in
> view and I rotate about the RA axis, Polaris should stay centered, right?
> I seem to recall reading that, but when I was aligning to Polaris I
> never observed this.
1. Not a stupid question at all. It's Christmas. Be nice to yourself.
2. We in the northern hemisphere are lucky to have a bright star like
Polaris very close to the North Celestial Pole (NCP - the point located
at an infinite distance in space through which the northern end of the
Earth's axis of rotation passes.)
Polaris is not exactly at the NCP, however. With a declination of
approx. 89° 18' it is 42 arc minutes distant from the NCP. This doesn't
sound like much when the entire celestial sphere is 21,600 arc minutes
in circumference, but it is greater than the average diameter of the
full moon, which is about 30 arc minutes! It means that Polaris must
rotate around the NCP every 23h 56m 4.n seconds, like the rest of the
stars. You may easily observe this rotation in a telescope with a
sufficiently wide field view if the telescope has a an accurate polar
alignment.
There many articles on the WWW describing how to achieve true polar
alignment. Probably the best of these is the drift method, as it is
easy to understand and to perform. You start by locking your
telescope's declination at 90°, then you center Polaris by adjusting
your mount (not the telescope's right ascension or declination
controls). (If you are doing only visual observing you are finished,
but if you are prepared to spend a little more time, and get a better
approximate alignment, you can center Polaris and then move the mount
until it is aimed at a point about 1° from Polaris in the direction of
the star Kochab. See
<http://www.weasner.com/etx/ref_guides/polar_align.html>.)
If you are going to do deep-sky photography you need a more accurate
polar alignment than is provided by the procedures described above, but
performing those procedures will give you a starting point for the
drift method of polar alignment. One of many good how-to's on the drift
method may be found at <http://www.darkskyimages.com/gpolar.html>.
Davoud
--
usenet *at* davidillig dawt com
.
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