Polar alignment

[1] Polaris is approximately three-quarters of a degree from the North Celestial Pole, and is easily seen by the naked eye.

σ Octantis, sometimes known as the South Star, can be sighted in the Southern hemisphere to perform a polar alignment.

In the Southern hemisphere or places where Polaris is not visible, a rough alignment can be performed by ensuring the mount is level, adjusting the latitude adjustment pointer to match the observer's latitude, and aligning the axis of the mount with true south or north by means of a magnetic compass.

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.

For example, instead of reading the latitude scale directly, a calibrated precision inclinometer can be used to measure the altitude of the polar axis of the mount.

An alignment suitable for visual observation and short exposure imaging (up to a few minutes) can be achieved with a polar scope.

A special reticle is used to align the mount with Polaris (or a group of stars near the polar region) in the Southern Hemisphere.

While primitive polariscopes originally needed the careful adjustment of the mount to match the time of year and day, this process can be simplified using computer apps that calculate the correct position of the reticle.

A new-style northern-hemisphere reticle uses a 'clock-face' style with 72 divisions (representing 20-minute intervals) and circles to compensate for the drift of Polaris over around thirty years.

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.

For telescopes combined with an imaging camera connected to a computer, it is possible to achieve very accurate polar alignment (within 0.1 minutes of arc).

The error in the point around which the images rotate compared to the true pole is calculated automatically and the operator can be given simple instructions to adjust the mount for a more accurate polar alignment.

[4] From the difference between the right ascension and declination of the telescope encoder and the second's star position, the elevation and azimuth error of the polar alignment can be calculated.

is Polar error in azimuth The inverse can be calculated if the above formula is written in matrix notation.

So the polar error expressed in Δe and Δa can be calculated from the Δα and Δδ between the telescope encoder and the second reference star.

Exposure is set at "B" (Bulb) and an image is taken while the camera is slowly turned around the polar axis.

In addition, we need the current right ascension and declination of the 3 stars, the longitude and latitude of the observatory, and the date and time the images were taken.

A small telescope usually with an etched reticle is inserted into the rotational axis of the mount.