Accurate Polar Alignment

Equatorial mountings should be aligned so that the polar axis of the telescope is parallel with the rotation axis of the Earth. Alignment errors larger than a fraction of a degree quickly reveal themselves in star images that drift north or south even during exposures of one or two minutes.

The small cross in each chart marks the location of the celestial pole.

If you have a portable telescope with a built-in polar axis alignment telescope, you can simply point that at the current location of the pole, and alignment is taken care of. For short exposures or for short-focus optics, a polar-alignment telescope makes this task quick and easy.

If, on the other hand, you have a telescope that is permanently mounted in a backyard observatory, you can adjust the polar alignment to within a gnat's whisker of perfection and then forget about it—only atmospheric refraction will disturb your tracking accuracy.

Observers with portable rigs that lack polar-alignment telescopes need to develop an efficient technique for setting up and checking alignment. Here is a procedure (for Northern Hemisphere observers) that works well with any mounting with setting circles:

1. Set the telescope so that the declination circle reads exactly +90°.

2. Adjust the polar alignment to center Polaris in the field of view.

3. While looking through a low-power eyepiece, rotate the telescope in right ascension. All objects in the field of view will appear to circle around some point. That point may be outside the field of view.

4. Move the telescope in declination only until that center of rotation is centered in the field of view. (Note: if the declination and polar axes of your telescope are not perpendicular, you will be able to center the rotation in declination but not in right ascension.)

5. Adjust the declination setting circle so that it reads exactly 90°. This calibrates the declination circle. Lock the setting circle adjustment so that it cannot change.

6. Point the telescope at a bright star roughly overhead. Depending on the date and time, convenient stars are:

8h30m — Muscida (o UMa) llh04m — Dubhe (a UMa) 13h 24m — Mizar (£ UMa) 17h 56m — Eltanin (y Dra) 21h 19m _ Alderamin (a Cep).

7. Set the polar circle to read the right ascension of the star. This calibrates that circle. Lock the circle adjustment so that it cannot change.

8. Set the telescope to the coordinates of Polaris (2h32m +89°.26).

9. Adjust the location of the polar axis to center Polaris in the field of view.

If the mount's setting circles are accurate, the polar axis is now aligned within perhaps 0.1°. Next, you must refine the alignment to eliminate trailing during long integrations. Use your CCD camera in its focus mode so that the image scale is large. Be sure that you know the orientation of images on the computer screen. Now use the drift method to fine-tune the polar alignment:

1. Center the telescope on a star on the celestial equator near the meridian.

2. Observe the motion of the star for several minutes. If the CCD software reads out the position to subpixel accuracy, so much the better.

3. If the star drifts south, move the north end of the polar axis west. If the star drifts north, move the north end of the polar axis east. After several repetitions, you will get a sense of how much correction it takes to change the drift. Quit when you have reduced the drift enough that it is difficult to measure.

4. Center the telescope on a star on the celestial equator rising in the east.

5. Observe the motion of the star for several minutes.

6. If the star drifts south, raise the north end of the polar axis. If the star drifts north, lower the north end of the polar axis.

After several repetitions, you will get a sense of how much correction it takes to change the drift. Quit when you have reduced the drift to a level that is difficult to measure.

7. Repeat fine-alignment steps 1 through 3 and 4 through 6 until the telescope tracks well enough that the drift is less than one-fourth of a pixel during your longest integration.

This technique calls for small adjustments in the polar axis alignment. Anything you can do to make the adjustments reliable and repeatable—such as attaching protractor scales on the adjustment knobs—will greatly speed the process.

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