Polar Alignment Using Drift Method

©2011 by Fred Espenak. All rights reserved.

    

In order for an astronomical telescope to track the Sun, Moon and stars, it must be mounted on an equatorial mount. There are several types of equatorial mounts but the most common are the German Equatorial Mount (GEM) and the Fork Equatorial Mount (FEM). The equatorial mount must be equiped with a motor or clock drive which turns the entire mount about its polar axis once every twenty-four hours.

When the polar axis is oriented parallel to the rotaion axis of Earth, the clock drive will compensate for and cancel out Earth's rotation so that the telescope will remain pointed at any astronomical object. A quick and easy way to achieve rough polar alignment is as follows.

First rotate the telescope's optical tube so that it is parallel to the equatorial mount's polar axis. If the mount has setting circles. this can be accomplished by rotating the telescope until the declination circle reads 90 degrees North (or 90 degrees South in the Southern Hemisphere). Next, make adjustments to the equatorial mount's altitude and azimuth settings so that the star Polaris appears on the cross-hairs of the telescope's finder scope (use the star Sigma Octanis in the Southern Hemisphere). This will produce a polar alignment good enough for visual use and short exposure photography.

If long exposure photography (several minutes or more) is planned, then a better polar alignment is required. The Drift Alignment method is one way of achieving this.

An eyepiece with illuminated cross hairs or reticle is needed for the Drift Alignment method. First perform a quick alignment using the proceedure described above. Next, choose a bright star within twenty degrees of the Celestial Equator and within an hour in RA of the Meridian. Put the star at the center of the cross hairs. Slowly ajust the telescope slow motion control in Declination and watch which way the star moves in the eyepiece. You must rotate the eyepiece so that the cross hairs are aligned with the north-south motion of the telescope. This may take several iterations between slow motion adjustments and eyepiece rotation.

The eyepiece cross hairs are now oriented with the north-south and east-west motions of the telescope and star. Put the star on the cross hairs and let the telescope track for ~five minutes without making any corrections. Ignor any drift in the east-west direction. If the star drifts NORTH with respect to the cross hairs, the telescope azimuth is pointing WEST of NORTH. Use the azimuth adjustment on the mount to turn it a small amount to the EAST. If the star drifts SOUTH with respect to the cross hairs, the telescope azimuth is pointing EAST of NORTH. Use the azimuth adjustment on the mount to turn it a small amount to the WEST.

After making a small azimuth correction, use the telescope RA and Dec controls to put the star back on the cross hairs and wait another five minutes. Once again, note any drift NORTH or SOUTH of the cross hairs and make correction to the telescope azimuth as described above. Repeat this process until the is no (or very little) drift in the NORTH-SOUTH direction. When this is accomplished, the azimuth of the telescope is now pointing to true NORTH.

Now it's time to test and correct the altitude of the telescope's polar axis. Choose a bright star within twenty degrees of the celestial Equator, and within twenty degrees of the eastern horizon. Put the star on the cross hairs and let the telescope track for five minutes without making any corrections. Ignor any drift in the east-west direction. If the star drifts NORTH with respect to the cross hairs, the altitude of the telescope's polar axis is too HIGH. Use the altitude adjustment on the mount to LOWER the polar axis altitude a small amount. If the star drifts SOUTH with respect to the cross hairs, the altitude of the telescope's polar axis is too LOW. Use the altitude adjustment on the mount to turn RAISE the polar axis altitude a small amount.

Repeat the drift test for altitude several times until the drift is eliminated (or very small). The telescope is now polar aligned. The last drift process for the telescope's polar axis altitude can also be performed with a star near the western horison rather than the eastern horizon. In this case, the HIGH/LOW corrections are opposite those given for the eastern horizon case.

If the telescope is set up south of the Equator, the telescope corrections in azimuth are all reversed from the Northern Hemisphere. The tables below summarizes the corrections in each hemisphere.


Polar Alignment - Northern Hemisphere
Star Direction Description
Meridian Star drifts NORTH, rotate telescope azimuth EAST
Star drifts SOUTH, rotate telescope azimuth WEST
Eastern Horizon Star drifts NORTH, adjust telescope altitude LOWER
Star drifts SOUTH, adjust telescope altitude HIGHER
Western Horizon Star drifts NORTH, adjust telescope altitude HIGHER
Star drifts SOUTH, adjust telescope altitude LOWER

Polar Alignment - Southern Hemisphere
Star Direction Description
Meridian Star drifts NORTH, rotate telescope azimuth EAST
Star drifts SOUTH, rotate telescope azimuth WEST
Eastern Horizon Star drifts NORTH, adjust telescope altitude HIGHER
Star drifts SOUTH, adjust telescope altitude LOWER
Western Horizon Star drifts NORTH, adjust telescope altitude LOWER
Star drifts SOUTH, adjust telescope altitude HIGHER


External Links On Polar Alignment