December 2001 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - December 2001 Issue

The Telescope Nut
by Jeff Baldwin

Polar Alignment

There are many ways to align a telescope's mount so that it will track the stars as well as possible, either for viewing convenience, finding a target, or for steady tracking during astrophotography. Here are a couple of methods, tricks, and ideas for doing this.

A Celestron or Meade Schmidt Cassegrain Telescope (SCT) will usually have a fork mount, either in alt-azimuth mode, or on a wedge that will allow polar alignment. However, some SCTs are sold on German Equatorial Mounts (GEMs). I'll explain the alignment process for the Fork Mount version. If you have a GEM then you can most likely make the transition.

Some scopes have the built-in finder scope inside the polar axis. If it really is aligned with the polar axis, simply point the crosshairs of this finderscope onto the North Celestial Pole (NCP) (see illustration) and lock it in. You're crudely aligned, and it will now work well enough for finding stuff using coordinates and for tracking visually. However, for astrophotography you need one more iteration.

Here is the area about the North Celestial Pole. Notice that the circle is only 2 degrees in diameter. You will see Polaris and two other stars form a triangle, and a fourth star near Polaris.
If lines are drawn to the base of the triangle farthest from Polaris from both Polaris and the star next to Polaris, the NCP is halfway between them.
The NCP is also halfway between Polaris and the opposing triangle base. If you put your finder scope's crosshair here, you're pretty close to Polar Alignment. From here on you will have to use star drift to increase your precision.

If you don't have the finder scope built in the polar axis, you can use your regular finder scope. However, you need to do something to it first.

Chances are your telescope is not perfectly perpendicular to the declination axis where it is mounted. For this reason, when you swing the telescope to 90° on your declination circle, the telescope, as well as the finder scope, may not actually be pointing toward the NCP. Point the telescope as close as I can to the NCP, then look through the finder scope. Take note of where the crosshairs are with respect to the NCP, and then rotate the telescope 180° around in Right Ascension (RA). Look through the finder scope again, and if it is pointing in exactly the same place, you know the finder scope is in fact aligned with the polar axis.

If the finder scope is pointing somewhere else, the midpoint of the two pointings is where the polar axis is pointing. Adjust the finder scope to this location and repeat until you can't see any difference at all, at which time the finder scope and the polar axis are aligned with each other (but not necessarily with the NCP). Now that you have them aligned, adjust the tripod until the finder scope is pointing at the NCP. Do this without moving anything in the mount.

The photographic iteration requires a little observing. You may have to correct a right-left error, and/or an up-down error. If there is a left-right error such that the polar axis is pointing to the left of the NCP, then when looking at a star as it crosses the meridian near the equator, it will appear to cross from south to north. This will happen minutely, so use a high-powered eyepiece with crosshairs and wait a few minutes. Likewise, if the star crosses north to south, the error is the opposite. Adjust the tripod until a star on the meridian near the equator does not cross the crosshairs. Now, if there is an up-down error such that the polar axis is higher than the NCP, a star observed in the east as it climbs above the horizon will cross the field of view from south to north. Likewise, if the polar axis is too low, the star will appear to cross from north to south. Adjust the tripod until the star tracks without drift. You may want to repeat the process after fixing both of these problems to make sure nothing happened.

Keep in mind that atmospheric refraction will alter the direction the telescope needs to point to a star. As the star changes altitude above the horizon, this refraction changes. As a result you will still have to correct the tracking during exposures.

Many people have auto-guiding and the telescope will track the object even if the alignment is off. However, if the alignment is off by much, the only perfect star image in the exposure will be the star that is guided. The others may appear as arcs in the photograph.

Also, some telescopes can't see the NCP, such as the one at Fremont Peak Observatory. If my memory serves me, the north pillar of that telescope and others obstruct the view to the NCP. That means that alignment will have to be performed by the photographic method.

Most clock drives are either stepper motors or synchronous motors. The rate at which they run may be different from the rate at which the Earth turns on its axis. If this is the case you will also need to control this rate during an exposure. Even atmospheric refraction may require this of you.

I'm sure this was as clear as mud. Go out and goof with your telescope. An evening of playing with a telescope's alignment and learning which way works best for you can be fun. Don't let it frustrate you, it's just all part of the hobby.

Clear Skies!...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.