Polar Alignment

Polar alignment is the act of aligning the rotational axis of a telescope’s equatorial mount or a sundial’s gnomon with a celestial pole to parallel Earth’s axis.

There are a variety of alignment methods and equipment to help amateur astronomers to assist with polar alignment of their telescopes. There are a variety of different methods and equipment that work for everyone differently. As there are a variety of techniques, which vary based upon cast and expertise, there are a different ways to go about this.

Polar Alignment Methods

Aiming At The Pole Stars is done by aligning the telescope’s mount polar axis in parallel to the Earth’s axis through visually sighting Polaris. There are alternatives for the southern hemisphere, where Polaris might not be available.

Rough Alignment Method In the Northern hemisphere, rough alignment can be done by visually aligning the axis of the telescope mount with Polaris. This method can sometimes be adequate for general observing through the eyepiece or for very wide angle astro-imaging with a tripod-mounted camera; it is often used, with an equatorially-mounted telescope, as a starting point in amateur astronomy. That said, there are a few ways to improve the accuracy of this method.

Polarscope Method is great when visually observing and performing short exposure imaging. This method uses a polariscope, which is a low magnification telescope mounted co-axially with the mount and is adjusted to maximize the accuracy of this alignment. Then, a special reticle is used to align the mount with Polaris, which can be simplified using computer apps which calculate the correct position of the reticle.

Drift Alignment Method allows the operator to refine the polar alignment after a rough alignment is done and is based on attempting to track stars in the sky using the clock drive. This means that any error in the polar alignment will show up as the drift of the stars in the eyepiece/sensor. But after this alignment is done, adjustments are then made to reduce the drift, and the process is repeated until the tracking is satisfactory. For the polar axis altitude adjustment, one can attempt to track a star low in the east or west. For the azimuth adjustment, one typically attempts to track a star close to the meridian, with declination about 20° from the equator, in the hemisphere opposite of the observing location.

Plate Solving is great for telescopes that are used with an imaging camera connected to a computer because it is possible to achieve very accurate polar alignment by approximate alignment it, then identifying the exact field of view when aimed at stars near the pole. Once complete, the telescope is then rotated ninety degrees around its right ascension axis and a new ‘plate solve’ carried out. The error in the point around which the images rotate is compared to the true pole is calculated automatically and the operator can be given simple instructions to adjust the mount for close polar alignment.

Alignment Equipment

Crosshair Eyepiece is an ordinary ocular with the only difference that it has a crosshair for aiming and measurement of the angular distance. This is equipment is useful for observers who are polar aligning using of the methods above, but is especially in drift.

Auto Guiding Systems

Dedicated Polar Scope is a small telescope usually with an etched reticle that is inserted into the rotational axis of the mount.

References and Further Reading




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