Polar Alignment for Astrophotography
by Matt BenDaniel
Accurate polar alignment helps produce pinpoint stars in your images.
This is especially important for long exposures. Here is how I drift align
without even a polar alignment scope or a pole star. I try to do the first
few steps before sunset.
- If the mount has an azimuth adjuster, set it into the middle of its
- Adjust the mount legs in azimuth to align the mount RA shaft exactly
north (or south in the southern hemisphere). If it is not yet twilight,
use a magnetic compass, but make sure to account for magnetic declination
Geomagnetism). It is very important that you get the azimuth adjustment
to within a degree or so, because if the azimuth adjuster runs out of
travel while you're drift aligning, it will cost you some time.
- Make sure the mount is level**. If you have to move the mount to level
it, then re-check the azimuth.
- Adjust the approximate altitude of the mount (the RA shaft) to the
latitude (e.g. 42 degrees). The mount is now roughly aligned, probably
well enough for visual observation.
- Install your illuminated reticle eyepiece. Focus it approximately
on some distant object.
- Slew the scope to a point at zero declination at transit. This is
- Wait for twilight. Turn on the drive.
- Optional: if you have a polar scope, you can use it now to obtain
a fairly accurate alignment.
- To drift align, center any not-too-bright star near Position A.
- Make sure the reticle is square, by rotating the eyepiece until the
star moves parallel to one of the lines (that would be the North-South
line) when you slew the scope in DEC. Tighten the set screw that holds
the eyepiece in.
- Focus. If the reticle has an adjustable focuser, make sure the star
and reticle are both in focus. If the focuser has a lock down, tighten
- Using the hand control at low speed, slew the star in DEC until it
is precisely bisected by the East-West reticle line.
- If the scope is in perfect alignment, the star will not drift from
the E-W line even over a 15 minute period. However, you are not aligned
yet, so it will drift N-S. Ignore any E-W motion.
- After observing the N-S drift, adjust the azimuth a lot in one direction.
Now observe the drift again. If the drift is more rapid in the same
direction, you adjusted the AZ in the wrong direction.
- Keep adjusting the azimuth until the star does not drift over a period
of one minute.
- Now find a star +30* degrees DEC at +/- 6-hour angle. This is Position
- Slew in DEC until the E-W line again bisects the star.
- Adjust the altitude until the star does not drift N-S for five minutes,
or 15 minutes if you want high accuracy.
- Now go back to Position A and do a five-minute drift alignment, or
15 minutes if you want high accuracy.
- Make sure AZ and ALT are locked down.
Drifting Aligning with an ST-4
You can use this method even without a polar alignment scope, line of sight
to pole star, or a guiding eyepiece.
- Do rough polar alignment and/or use polar scope.
- Make sure that ST-4 is focused.
- Identify whether DEC is ST-4's X or Y.
- On ST-4 chip center a transiting star near 0 degrees DEC [Position
- Press calibrate then interrupt (to disable relays).
- Press track.
- Observe DEC for one minute.
- If DEC has trend, adjust mount azimuth. If star goes off chip, repeat
- Repeat steps 7 and 8 until no DEC trend.
- Center star at +30* degrees DEC at +/- 6 hour angle [Position B]
- Press track and watch DEC.
- Adjust mount altitude until no DEC trend.
- Re-check drift at Position A.
- Make sure AZ and ALT are locked down.
*In Southern hemisphere, use -30 DEC
**A level mount is important for efficient drift alignment. If a mount is not
level, adjustment of either AZ or ALT will affect the other. If the mount
is not level, it means that after you perfectly adjust the ALT, adjustment
of AZ will skew the ALT. If the mount is perfectly level, you should be
able to adjust the ALT then the AZ, and you're done. It takes a little time
to level the mount, but that can be done before sunset. The important thing
is to finish drift alignment before the end of twilgiht, so that you do
not lose dark sky time.