Mounts and Their Motions
A telescope on an altazimuth mount simply moves up-down (altitude) and left-right (azimuth). This means that the user often has to nudge the scope in two directions simultaneously to compensate for the Earth's rotation and follow a celestial object. An equatorial mount allows a scope to move in the celestial directions of north-south and east-west. As the Earth spins, an observer can easily track celestial targets by moving the telescope in only one direction (to the west).

For simple visual observing without setting circles, you don't need to align a telescope's equatorial mount very well on the north celestial pole. Just plunk it down so that the polar axis (the right ascension axis) is aimed at Polaris as best you can judge by eyeballing it. The mount will then do its job. For long-exposure astrophotography, however, the polar alignment must be a lot better.

How to Accurately Accomplish Polar Alignment

The "declination drift method" is the most accurate way to accomplish this. The method is straightforward, but it does require some time and patience.

    1. First, aim the mount's polar axis roughly at Polaris. Now point the telescope at a star that's somewhat above the celestial equator and as close to south as you can judge by looking opposite Polaris. Put in a high-power eyepiece. If the eyepiece has cross hairs, center the star on them. Otherwise put the star on the north or south edge of the field and defocus it a little. Turn on the clock drive, and ignore any east-west drift.
    2. If the star drifts south in the eyepiece, the polar axis is pointing too far east.
    3. If the star drifts north, the polar axis is too far west.
    4. Shift the polar axis left or right accordingly, until there is no more drift.
    5. Now aim at a star that's near the celestial equator low in the eastern sky.
    6. If the star drifts south, the polar axis points too low.
    7. If the star drifts north, the polar axis points too high.
    8. Again, shift the polar axis accordingly.
    9. Now go back and repeat from the beginning, because each adjustment throws the previous one slightly off. When all visible drift is eliminated the telescope is very accurately aligned, and you can take long deep-sky exposures.

If your eastern sky is blocked, you can use a star low in the west and reverse the words "too high" and "too low" in the above instructions. If you're in the Earth's Southern Hemisphere, reverse the words "north" and "south."

When followed, you will end up with accurate polar alignment to help your astrophotography - enjoy!




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July 24, 2014 at 12:08 pm

Mr. MacRobert's article outlines the drift method for polar alignment as it is universally quoted, where the drifts of stars in two equatorial fields are used to determine the direction towards which to move the polar axis to better align the it with the celestial pole. While most S&T readers understand this well, what may not so well known is that from the perspective of a tracking OTA with some polar-axis misalignment, the celestial sphere is rotating about an axis through the equator determined by the polar-axis orientation. The fact that no drift of the stars in a field near that axis of rotation will be seen is what requires a second equatorial field to align the polar axis. Once a field of view away from the equator is chosen, drift in that field is sensitive to both degrees of freedom of polar misalignment and one can think about polar alignment without moving between fields.

In fact, the best place to see star trailing due to misalignment is to view a field near the pole where the drift is present regardless of the direction towards which the polar axis is displaced. I know that this seems counterintuitive, but think about it for a moment. Imagine a telescope with the pole in its field of view. Without tracking, the OTA rotates with the earth and sees the field rotate about the pole at the sidereal rate. With tracking on and with the telescope perfectly aligned, the OTA is fixed with respect to the celestial sphere and one sees no rotation (or drift) of the field. But now add polar misalignment. Now the OTA sees a drift - not rotation - of the entire field, again because from the perspective of the OTA the celestial sphere rotates about an axis through the equator - far from the field. The pole is best because it is always 90 degrees from that axis of rotation ensuring drift in that polar field is almost uniform across it.

A more detailed mathematical treatment provides two more points. First, the axis of rotation through the equator is coplanar with the polar axis and the celestial pole, and thus the direction of the star trailing is perpendicular to that plane. Second, the rate of star trailing is the sidereal rate times the polar-axis misalignment angle expressed in radians. So from one measurement of the drift in a field of view, both the direction and angle of polar-axis miss-pointing can be determined with some degree of accuracy. I've done the math, as well tests, displacing my mount's axis in different directions and photographing the drift across the field while tracking. For a time I used such measurements to re-point my polar axis, and it worked well. It is not essential to view the pole itself, and in fact some cannot for reasons of geography, terrain, or landscaping. The essential point is that it is best to not use equatorial fields, but instead to use a single field well away from the equator. (In temperate latitudes a field near the zenith may work well enough.) One then simply observes the direction of the drift, moves the polar axis accordingly, and repeats as needed for accuracy. No second field and no slewing between iterations are required.

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