Drift Alignment is an accurate method of Polar Alignment
to align to the Celestial Pole. It can be used for both northern hemisphere polar alignment and southern hemisphere polar alignment
, and is usually performed after Rough Polar Alignment.
A rough polar 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:
- Point the telescope at a star due South that is close to declination 0 degrees.
- 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.
- Position the star exactly on the centre of the cross hairs.
- Leave the mount tracking and note the drift direction of the star away from the crosshair centre.
- 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.
- 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.
- 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).
- Now point the scope at a star low in the West.
- Centre the star perfectly and let the mount track until a drift is observed.
- 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.