Polar Alignment Is Easy ... Really!
Using the "Star Drift"
In the following article, differing
directions for the southern hemisphere are indicated by braces [example].
If you have an
equatorial telescope, you may have experienced the pain of tracking error.
If you plan on doing long exposure astrophotography, you must be accurately
aligned on the north [or south] celestial pole. Having a drive corrector
isn't enough. Even if you keep a star right on the crosshair in your
reticle, if the mount isn't polar aligned you'll get some error from field
rotation. Polar finder scopes are useful tools and will get you very close
to the mark but imagine having your scope track "dead on" with no
declination corrections for 10 to 20 minutes! If you have periodic error
correction (PEC) you'll be free from right ascension corrections as well.
Polar alignment by the "star drift" method takes more time than using
a polar scope, but a simple procedure using two stars will get your mount RIGHT
ON the celestial pole. It's a simple method that does not require setting
circles or the knowledge of date and time. Another valuable aspect
of the two star method is that Polaris [or the south pole] need NOT be visible
Start by doing a quick and dirty eye-ball polar
alignment. The closer this initial alignment is, the faster you will complete
the task. Point the telescope at a star close to the celestial equator and
near the eastern horizon. Not too close to the horizon now ... you
don't want diffraction entering the equation. Track on the star and make
corrections in right ascension only, don't correct in declination yet.
Watch the star drift. An illuminated reticle makes this a lot
easier. If the star drifts south [north] in the field of view, then the
polar axis is too low (pointed below the pole—toward the horizon). If
the star drifts north [south], the axis is too high (pointed above the pole—toward
zenith). Make your adjustments to the altitude of the polar axis
until the drift becomes negligible
Now go to a star near the celestial equator again but
this time near the meridian. Guide only in right ascension as
before. If the star drifts south, then the polar axis is pointing east
[west] of north [south]. If it drifts north, then the axis is west [east]
of north [south]. This time adjust the azimuth of the mount until the
declination drift goes away. Do this a few more times until there's no
drift in declination from stars near the horizon and meridian.
There's an even easier way to remember which direction
to adjust. Forget all about north and south in the eyepiece.
Just remember that it's the altitude and azimuth of the mount that you are
adjusting. Astrophotographer Barry Gordon calls it the "altitudE is
Easy and aziMuth is Mad" method. The altitude adjustment is easy to
remember ... adjust it so the star moves back towards the center of the
field. The azimuth adjustment is maddening because you move the star
further away from the center, in the direction it's drifting.
It really is that simple—it just takes longer than
any of the other methods. But if you are taking shots over ten minutes in
length, it gets pretty boring constantly staring into an eyepiece.
Wouldn't you rather look every minute or two and make a tiny tweak rather than
being hunched over with your eyes straining and watering?
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