The Astronomical Unit


This page last modified 2012 December 24

Polar Alignment

In order to successfully track an object with a single motion, the polar (RA) axis of an equatorial mount must be parallel to the axis of rotation of the Earth. In what follows, it is assumed that the polar and declination axes are mutually perpendicular and that the optical axis of the telescope and the declination axis are mutually perpendicular.
There are essentially three levels of accuracy in polar alignment:

Rough Method

This is the simplest of all, but has the advantage of being rapid and is suitable for visual observing.

There is little point in offsetting towards the celestial pole, since it is likely that the error induced by aligning to Polaris will be exceeded by error in judging when the telescope and polar axis are parallel. The telescope will be sufficiently well aligned for visual tracking for shortish periods, but objects will appear to drift in declination.

If you are setting up your telescope somewhere from where Polaris is not visible, you will need to do the following:

Intermediate Method

This is suitable for mounts with setting circles or polar-alignment scopes, such as bore- scopes. A bore-scope is a small telescope, usually with an illuminated reticle, inside the polar axis of a German equatorial mount. Some other mounts have detachable sighting scopes. For all these, follow the manufacturer's instructions, as the exact method of set-up will be specific to the mount and the alignment scope. The general principle of these alignment scopes is the same: they are exactly parallel to the polar axis and are sighted onto Polaris. The scope either has rotatable reticle or is itself rotatable, and has a marker which is aligned with a sidereal time scale (or local mean time and date scales) on a bezel. Polaris is then centred in the appropriate place in the reticle.

For mounts without alignment scopes, but with setting circles, the method is an enhancement of the Rough Method:

If you do this carefully, this method should allow long periods of visual observing and will be sufficiently precise for piggy-back photography for up to exposures at least as long as 30 minutes with lenses up to 200mm focal length. It will also allow the setting circles to be used to find objects.

Accurate Method

If you wish to do prime focus photography through your telescope, or if you are setting up the mount permanently, you should take the time to polar align it as accurately as you possibly can. Firstly, align the mount as accurately as you are able, using the appropriate method above. The instructions given are for the northern hemisphere and assume that you have a high-power guiding eyepiece, giving about 200x magnification.

Correct any Index Error of the Declination circle

Correct the Altitude of the Polar Axis

If you try to drift a star near the meridian, drift will be neglegible.

Correct the Azimuth of the Polar Axis

If you try to drift a star near the horizon, drift will be neglegible.

After correcting the azimuth, check the altitude again, and so on until the mount is properly polar aligned.

NB: For drift alignment, only north-south drift is relevant.