Polar Alignment Back | Up | Next

To track the motion of objects in the sky, we must compensate for the Earth's rotation. The easiest and best way to do this for astrophotography is with an equatorial mount that is correctly polar aligned. With an equatorial mount, we can compensate for the Earth's rotation by movement in only a single axis, the polar axis. But this axis must accurately point at the celestial pole.

What Is Polar Alignment?

Polar alignment means making the polar axis of an equatorial mount parallel to the Earth's axis of rotation by pointing it accurately at the north celestial pole in the northern hemisphere, or the south celestial pole in the southern hemisphere.

For the northern hemisphere, Polaris is reasonably close to the north celestial pole. It is about 42 arc minutes, or about 2/3 of a degree from Polaris on a line to Kochab. Polaris is relatively bright at second magnitude, as is Kochab, and Polaris is easy to find with the pointer stars in the Big Dipper.

For the southern hemisphere, Sigma Octantis is the closest bright star to the south celestial pole, about 1 degree from Sigma on a line towards Zeta Octantis. Sigma and Zeta are not that bright however, each being about magnitudes 5.5.

Rough Polar Alignment

To achieve rough polar alignment in the northern hemisphere, the polar axis of the mount can simply be pointed at Polaris. In the south it must be pointed at Sigma Octantis. This should be good enough for visual observations and photography with wide angle lenses and short exposures.

During the daytime or during twilight before Polaris can be seen, the mount can easily be roughly aligned by noting the latitude and magnetic variation from true north for the observing site. The mount's polar axis elevation is simply set to the latitude of the observing site. The mount's azimuth (its compass heading) is then aimed north, taking into account for the magnetic variation from true north for the observing site.

A similar procedure can be followed in the southern hemisphere, except, obviously, the mount's polar axis would point south at the south celestial pole.

The north celestial pole lies about 2/3 of a degree away from Polaris on a line between Polaris and Kochab.

When twilight deepens and it starts to get dark, you should be able to find Polaris without too much difficulty by using the pointer stars in the bowl of the Big Dipper. Polaris shines at about 2nd magnitude, so it's relatively easy to find.

You can easily get even more accurate polar alignment by simply aiming at a point 2/3 of a degree from Polaris to Kochab as seen in the illustration above.

There are also more accurate methods of polar aligning, such as drift aligning, for advanced astrophotographers.

Using a Polar Alignment Scope

Polar Alignment Scope
Polar Scope Reticle

Polar alignment is most easily accomplished by correctly using a polar alignment telescope that is built in to many of today's modern equatorial mounts. This is a very small refracting telescope that is located inside of the polar axis of the mount itself.

The reticle is rotated so that it is correctly oriented for the date and time. Then Polaris is placed on a reference mark on a reticle inside the polar alignment scope by adjusting the altitude and azimuth of the mount. The altitude is how high the scope's polar axis is pointing, and the azimuth is the direction east or west of north on the horizon that the scope's polar axis is pointing.

For the southern hemisphere, a reticle designed for the south celestial pole must be used.

Altazimuth Mounts and Polar Alignment

For an altazimuth mount, you can't really polar align it at all when it is in the altazimuth mode. Some of these mounts can be polar aligned by the use of a specially made wedge.

Computerized altazimuth mountings usually have an "alignment" function that involves locating, centering and syncing the scope and computer on 1, 2 or 3 stars, but this is not polar aligning. This only tells the computer where the scope is in relation to the sky so it can find objects from its database.

Depending on what part of the sky you are pointing at, and the size of the field of view of your scope or camera lens, you will only be able to use exposures of 30 seconds to a minute or so with an altazimuth mounting. After this length of time, your images will display "field rotation", where stars at the corners of the frame are trailed around the center of the frame. This is the advantage of an equatorial mount, as you do not have this problem.

Polar Alignment - The Bottom Line

Polar alignment is important for long-exposure deep-sky astrophotography.

By correctly pointing the mount's polar axis at the north celestial pole, you will be able to accurately track the stars as they move across the sky.

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