Polar alignment

Polar alignment is the act of aligning the rotational axis of a telescope's equatorial mount in parallel with that of the Earth. There are various ways to achieve this.

Alignment Methods

The method to use differs depending on if 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 and σ. 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.

Aim at a Pole Star method

Northern Hemisphere

Sighting on Polaris the North Star is the usual procedure for aligning a telescope mount parallel to the Earth's axis.

Southern Hemisphere

σ Octantis the South Star

Illustrates how to find σ Octantis, the South Star.

Drift Alignment

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 for the Northern Hemisphere is as follows:

Step 1 Point the telescope at a star due South that is close to declination 0 degrees.

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 clockwise (imagine looking at the mount from above). If you had to push DOWN, rotate the mount anti-clockwise. Start with a small rotation to get a sense of how much change the rotation will produce.

Step 7 Go back to step 4. Keep repeating steps 4 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.)

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.

Dedicated Polar scope

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

See also

• Setting circles
• Inertial guidance system
• Tpoint

External links

• Polar alignment - Starizona
• How to polar align your equatorial mount - Celestron
• Drift method of polar alignment - Astrosurf
• LX200 polar alignment procedure - Philip Perkins