Polar alignment

Polar alignment is the act of aligning the rotational axis of a telescope's equatorial mount or a sundial in parallel with that of the Earth.

Alignment methods

The method to use differs depending on whether the alignment is taking place in daylight or in night. Furthermore, the method differs if the alignment is done in the Northern Hemisphere or Southern Hemisphere. The purpose of the alignment also must be considered. For example, the demand for accuracy is much more significant in astrophotography than in occasional stargazing.

Aiming at the pole stars

In the Northern hemisphere, sighting Polaris the North Star is the usual procedure for aligning a telescope mount's polar axis parallel to the Earth's axis.^[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 polar alignment. At magnitude +5.6, it is difficult for inexperienced observers to locate in the sky. Its declination of -88° 57′ 23″ places it within 1° 2′ 37" of the South Celestial Pole. An even closer star BQ Octantis of magnitude +6.9 lies 10' from the South Pole as of 2016. Although not visible to the naked eye, it is easily visible in most polar scopes. It will be closest (9') to the South Pole in the year 2027.

Location of σ Octantis in the southern sky, with dashed lines as guides to the approximate location.

Rough alignment method

In the Northern hemisphere, rough alignment can be done by visually aligning the axis of the telescope mount with Polaris. In the Southern hemisphere or places where Polaris is not visible, a rough alignment is 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 (after taking the local magnetic declination into account). This method can sometimes be adequate for general observing through the eyepiece or for very wide angle astro-imaging with a tripod-mounted camera. This method is often used by newcomers to amateur astronomy equipped with an equatorially-mounted telescope.

For astro-imaging through a lens or telescope of significant magnification, a subsequent drift alignment is necessary to refine the rough alignment.

There are ways to improve the accuracy of this method. 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. If the setting circles of the mount is then used to find a bright object of known coordinates, the object should only mismatch in azimuth, so centering the object by adjusting the azimuth of the mount should complete the polar alignment process. Typically, this provides enough accuracy to allow tracked (i.e. motorized) telephoto images of the sky.

Drift alignment method

A rough alignment is performed, then refined by pointing at different stars and observing any drift that occurs. The mount is then adjusted according to the direction of the observed drift.

The procedure is as follows (hemisphere independent):

Step 1 Point the telescope at a star in the meridian opposite the pole location (e.g. in the North from the Southern hemisphere) that has a declination less than 30 degrees from the equator.

Step 2 Rotate the guider CCD or reticule eyepiece in the telescope so that moving the mount in Right Ascension causes the star to follow a crosshair line very accurately. Consider this line to be the ‘horizontal’. If the mount’s tracking is switched off for a few minutes the star should follow this line.

Step 3 Position the star exactly on the centre of the cross hairs.

Step 4 Leave the mount tracking and note the drift direction of the star away from the crosshair centre.

Step 5 While watching the position of the star, put a hand on the front of the scope and push gently either up or down - enough to move the star visibly. Figure out if you need to push UP to re-centre the star or DOWN.

Step 6 If you need to push UP, use the azimuth adjustment to rotate the whole mount with the side facing the pole in the direction EASTward (imagine looking at the mount from above). If you had to push DOWN, rotate the side of the mount facing the pole WESTward. Start with a small rotation to get a sense of how much change the rotation will produce.

Step 7 Go back to step 3. Keep repeating steps 3 to 7 until the star does not drift up or down for at least 5 minutes. (Remember, sideways drift is not important).

Step 8 Now point the scope at a star low in the West.

Step 9 Centre the star perfectly and let the mount track until a drift is observed.

Step 10 Put a hand on the front of the scope and give a gentle push. If you have to push the scope UP to re-centre the star, then you must increase the angle that the polar axis makes with the horizontal. If you need to push DOWN then the angle must be reduced. (It is probably wise to figure out which way the adjustment knobs move your mount in daylight before you do the actual drift alignment. It is quite easy to get confused in the dark and go the wrong way.)

If the Western part of the sky is obscured, perhaps by trees or a building, then use a star in the East. Use the same procedure as for the West, but reverse the correction. If you have to push the scope UP to re-centre the star, then you must decrease the angle that the polar axis makes with the horizontal. If you need to push DOWN then the angle must be increased.

You should then repeat the whole procedure until no drift is seen for 10–20 minutes.

Equipment used in polar alignment

Crosshair eyepiece

A 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 useful in any type of polar alignment, but especially in drift.

Auto guiding systems

Dedicated polar scope

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

References

^ Michael A. Covington (1999). Astrophotography for the amateur. ISBN 978-0-521-62740-5.

External links

[1] Michael A. Covington (1999). Astrophotography for the amateur. ISBN 978-0-521-62740-5.

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