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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.

  1. If the mount has an azimuth adjuster, set it into the middle of its range.
  2. 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 (see 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.
  3. Make sure the mount is level**. If you have to move the mount to level it, then re-check the azimuth.
  4. 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.
  5. Install your illuminated reticle eyepiece. Focus it approximately on some distant object.
  6. Slew the scope to a point at zero declination at transit. This is Position A.
  7. Wait for twilight. Turn on the drive.
  8. Optional: if you have a polar scope, you can use it now to obtain a fairly accurate alignment.
  9. To drift align, center any not-too-bright star near Position A.
  10. 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.
  11. 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 it.
  12. Using the hand control at low speed, slew the star in DEC until it is precisely bisected by the East-West reticle line.
  13. 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.
  14. 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.
  15. Keep adjusting the azimuth until the star does not drift over a period of one minute.
  16. Now find a star +30* degrees DEC at +/- 6-hour angle. This is Position B.
  17. Slew in DEC until the E-W line again bisects the star.
  18. Adjust the altitude until the star does not drift N-S for five minutes, or 15 minutes if you want high accuracy.
  19. Now go back to Position A and do a five-minute drift alignment, or 15 minutes if you want high accuracy.
  20. Make sure AZ and ALT are locked down.
  21. Done


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.
  1. Do rough polar alignment and/or use polar scope.
  2. Make sure that ST-4 is focused.
  3. Identify whether DEC is ST-4's X or Y.
  4. On ST-4 chip center a transiting star near 0 degrees DEC [Position A]
  5. Press calibrate then interrupt (to disable relays).
  6. Press track.
  7. Observe DEC for one minute.
  8. If DEC has trend, adjust mount azimuth. If star goes off chip, repeat step 4.
  9. Repeat steps 7 and 8 until no DEC trend.
  10. Center star at +30* degrees DEC at +/- 6 hour angle [Position B]
  11. Press track and watch DEC.
  12. Adjust mount altitude until no DEC trend.
  13. Re-check drift at Position A.
  14. Make sure AZ and ALT are locked down.
  15. Done

*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.

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