Polar Alignment with a CCD Imager

This is not for the faint of heart. You need to know how to use a CCD imager and your telescope. I write this here, because there are so many websites with INCORRECT information about this process. Some miss the point completely. All this does is mess with your head and mess with your gear. This method works. Trust me.

Polar Alignment with CCD

This is for permanent installations, as it can take up to 3 or 4 hours to complete the whole process to the point where your mount is perfectly polar aligned. The result is that you no longer have to worry about declination drift during imaging. You still have to worry about periodic error from your mount’s drive, but any good autoguider will take care of that. These instructions assume that you know how your CCD is aligned on the scope (which way is N, E, S, W on the images). In all cases, you will be monitoring North-South changes in a star’s position on your CCD images. IGNORE any east-west drift.

Polar Axis Altitude Alignment:

1. Manually polar align your mount to the best of your ability. Some mounts come with polar alignment scope in the polar axis shaft. Use it! This will actually get you close enough to take 5 to 10 minute integrations without doing the rest of this list! You will depend on your autoguider.

2. Aim your telescope at a star low on the eastern horizon and on the celestial equator (close is good). Faint stars are ok. You do not want them blooming.

3. Take 15 30-second long integrations of that star. Some people prefer to take one 5 minute long shot of the stars to see its path, but it works just as well to see the star’s position move due to drift via snapshots. DO NOT move the mount in any way during these shots. Just let the RA motor do its job.

4. Now to fix your drift! In this step you move the mount’s permanent polar axis depending on the way the star has drifted on your images.

a. If the star drifted north on your images, then move the mount’s polar axis down a tiny(!!) bit.

b. If the star drifted south on your images, then move the mount up a tiny(!!) bit.

5. Repeat steps 2 through 4 until you see NO DRIFT in 5 minutes. Want better? Go for longer. You will find that you can use the centroid tool in your image processing software and get excellent results in about 30-40 minutes.

Polar Axis Azimuth Alignment:

1. Aim your telescope at a star on the meridian and on the celestial equator.

2. Take 15 30-second long integrations of that star. Some people prefer to take one 5 minute long shot of the stars to see its path, but it works just as well to see the star’s position move due to drift via snapshots. DO NOT move the mount in any way during these shots. Just let the RA motor do its job.

3. Now it is time to fix your polar alignment’s altitude to perfection! Be sure to make VERY SMALL adjustments to the polar axis at this time.

a. If the star drifted to the north in your images, then slightly move the mount to the east.

b. If the star drifted to the south in your images, then move the mount to the west.

4. Repeat steps 2 and 3 above until there is NO DRIFT in your 5 minute series.

Congratulations! Your mount is polar aligned. You will likely not need to adjust this again until you swap out telescopes, have an earthquake (more common than you think!) or someone fiddles with a knob or two on your mount (also not all that uncommon as you think).

About johnb

- Director of Grainger Observatory, Phillips Exeter Academy. - Variable-star-crazed astronomer, but have done research in other areas. - Drummer, archer, pilot, chef, friend, pet owner, husband, father, Train-nut.
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