Home | Astrophotos
Drift Method of Polar Alignment

Drift alignment seems to be a major stumbling block for amateurs who want to move up to serious astrophotography (i.e. photography through a telescope). As a result you'll find many writeups on this subject on the internet. Personally, I find many of these not to my liking because they involve a stream of references to up/down, left/right with inadequate context as to the type of equipment used, position of the observer, etc. If you prefer a string of instructions over looking at a map when trying to find an address perhaps these will suit you, but I strongly suggest you read them while sitting at your own scope. If you prefer looking at a map as I do, read on. I hope this is helpful to you.

One reason why I think there is considerable confusion about drift alignment is because one needs to first take the time to gain a basic understanding of the geometry of tracking an object across the sky. The following description assumes you already have a basic understanding of why an equatorial mount is necessary for tracking the movement of the stars (or rather the apparent movement due to the earth's rotation).


The first thing to note is that polar axis misalignments involve two components:

  • The mount's polar axis may be off in azimuth, pointing E or W of the true pole.
  • The mount's polar axis may be off in altitude (elevation) and pointing higher or lower than the true pole.
The basic idea behind drift alignment is to correct each error, one at a time. To do that, we need to find a situation in which we can observe the path of a star such that the mis-tracking is entirely due to one of these errors. So first consider the cases of error in each axis separately:


In practice, one of the confusing aspects of actually performing drift alignment is the potential for the optical system and your observing position to affect which way you think the drift is occuring, so here are some practical tips:

  • Get approximately aligned using one of the alternate methods (e.g. polar finder).
  • Use a high magnification to reduce the drift time and increase your accuracy.
  • Lock your mirror If you must use a scope with a movable mirror in order to avoid false drift movement.
  • Align your reticle eyepiece so that the cross-hairs are aligned with your mount's RA and Dec axes, not along the path of drift of a star when the mount is not running! Slew in RA and set the direction of the crosshair to this direction as closely as possible. Once this is done, lock it in place for the rest of the testing.
  • Determine N vs. S by nudging the scope toward the pole while looking in the eyepiece. You may find it helpful to put some tape on the eyepiece to indicate north for drift observations.
  • Familiarize yourself with your altitude and azimuth adjustment screws, labeling them with N, S, E, W, if necessary. This will avoid the frustration of adjusting in the wrong direction.

One thing to notice is that the altitude adjustment is especially simple -- when viewing a star on the eastern horizon, if you see either north or south drift, turn the altitude adjustment screw so that the star image N-S drift is cancelled out. You can ignore E-W motion of the star during drift as well as during your adjustment. This easy situation occurs because the altitude adjustment axis is (almost always!) such that the mount is tilted in the N-S direction. This is also an argument for setting your mount up with reasonable care given to leveling.

Another thing to note, especially for the altitude adjustment, is that if you are far from perfect alignment, the star will drift rapidly. In that case, make your adjustments immediately. If your goal is to get no drift for 5 minutes, but you get noticeable drift in 10 seconds, estimate where the star would be in 5 minutes and move the star that distance back and past the E-W reference line.

The theoretical discussion of the geometry of drift alignment unfortunately is also subject to real world problems:

  • The ideal point for altitude error testing (the eastern horizon or alternatively, the western horizon) may not be visible and is not desirable anyway due to atmospheric refraction and extinction.
  • There may not be a suitably bright star at the best point for either azimuth or altitude alignment drift testing.
In either case, selection of a star closer to the pole will decrease the rate of drift and increase the amount of time you should watch the star.

Selecting a star off the meridian (azimuth alignment) or eastern horizon (altitude alignment) will cause error from the other axis to contribute to the drift you observe. To get around this, make adjustments in one axis, then adjust the other axis. Repeat the adjustments as many times as necessary as your drift observations will not be as independent of the other axis as we would like.

Getting Comfortable with Drift Alignment

Probably the most common reason for frustration with drift alignment is that amateurs don't practice it enough. Waiting until the stars come out on a moonless night which is perfect for imaging is the wrong time to try drift alignment for the first time! Drift alignment can be practiced in your back yard under the poorest conditions!

Here is my suggested procedure for getting familiar with drift alignment:

  • Read (and understand) the theoretical discussion above!
  • Align your mount as closely as possible using one of the alternative methods.
  • Misalign your altitude or azimuth by a known amount (say, 5 degrees), and practice getting back in alignment using the drift method.
  • Do it again and again!
  • Write down your personal set of instructions for use out in the field.

Another complaint about drift alignment is that it takes too much time. If you are sitting there just waiting for drift to become noticeable, that may be true. But generally, after the initial gross adjustments, you can overlap your waiting time with doing other setup tasks (loading cameras with film, etc.). With a properly planned workflow, drift alignment need not be a big burden.

More Reading:

Copyright 2001 by David A. Kodama