Polar Alignment

Unless you are lucky enough to live in a dark location and have a permanent observatory you will have to accurately polar align your telescope each and every time you go out.

Why?

An accurate polar alignment is required to eliminate “field rotation”. A perfectly guided image will still show error is the mount is not very accurately polar aligned. Field rotation shows up in an image as curved stars on the outer edges of the image. This is not ot be mistaken for coma, usually seen in fast camera lenses, in which the stars are bloated like little UFO-like things. A well aligned mount will not show field rotation.

How?

Once you have read through the following detailed procedure you may wish to print out the Quick Guide to Drift Alignment to have with you in the field.

Note: The drift method of polar alignment requires the use of an illuminated crosshair eyepiece of the ability to place crosshairs on an image if using a CCD rather than your eyes. A simple double crosshair (shown below) works perfectly. Most CCD control programs have crosshairs available

Above: View through a double crosshair illuminated reticle eyepiece. Rotate the eyepiece so that the crosshairs are parallel to the east-west motion of the star.

The telescope mount should be already roughly polar aligned either using a polar alignment bore scope or sighting the axis on Polaris.

Azimuth Adjustment

The drift alignment requires that you let the telescope track on two different stars in specific locations in the sky. Watching how the stars drift relative to the reticle in the crosshair eyepiece tells you how far the mount is offset from true celestial north and in which direction.

Pick a star near the meridian, just north of the celestial equator (due south, between about 60°-70° above the horizon from the U.S.). Select a star that is reasonably bright but not too bright (about magnitude 3-4). Be sure that no other similar stars are in the field of view, as you do not want to get confused as to which star is which.

Aim the telescope to this star. Rotate the diagonal until the eyepiece is oriented so that you are standing on the north side of the telescope when looking into the eyepiece. This step is not absolutely necessary but will make the following procedure easier.

The crosshairs of the eyepiece must be aligned with the north-south and east-west directions. Center the star in the eyepiece. Use the mount's hand-controller to move the star east and west (roughly left and right) in the eyepiece. You should see that the star's motion is not perfectly parallel to the horizontal lines in the eyepiece. Rotate the eyepiece and check the east-west motion again. Repeat until the crosshairs are properly aligned.

Once the crosshairs are oriented, place the star on one of the lines east-west (approximately horizontal) lines. In other words, the star image should be bisected by one of the horizontal lines as shown below. Do not place the star between the lines, as it will not provide enough accuracy for the following steps.

Above: Place star on east-west line.

Let the telescope track for a minute or so. You will see the star begin to drift off of the line. It will drift either north (above the line) or south (below the line). Ignore any east-west (left-right) drift….that’s Periodic Error

Newtonian telescope users must reverse the following directions

If the star drifts up, use the mount's azimuth adjustment knobs to move the mount so that the star appears to move right in the field of view.

If the star drifts down, use the mount's azimuth adjustment knobs to move the mount so that the star appears to move left in the field of view.

Star drifts up Adjust mount to move star right

Use the hand-controller to move the star back onto the horizontal line.

Let the star drift again. You should notice that it takes longer for the star to begin drifting off the line. Repeat the azimuth adjustments, placing the star back on the crosshair again when finished.

Continue letting the star drift and making adjustments until the star takes about 5 minutes to drift off the line. Again, ignore any left-right motion. Once the star stays bisected by the line (not just close to the line) for 5 minutes without any drift, your mount is accurately aligned in azimuth. Now you just need to adjust the mount in altitude.

Altitude Adjustment

Pick a second star in the east, about 20° above the horizon, near the same declination as your first star (near the celestial equator). In other words, move the telescope mostly in right ascension to select the second star. If there are any obstructions on your eastern horizon, it is possible to achieve an accurate alignment using a star up to about 50° above the horizon.

If you do not have an unobstructed view to the east, a star in the west can be chosen. You must reverse the adjustments below, however, if you use a star in the west.

Rotate the diagonal so that you are now standing on the south side of the telescope when looking in the eyepiece. Again, this just makes the adjustments easier.

Orient the crosshairs again as you did above, so that the horizontal crosshairs are parallel to east-west motion and the vertical crosshairs are parallel to north-south motion.

Place the star on one of the horizontal lines.

Let the star drift. You should notice some drift after only a minute or so unless you initial rough alignment happened to be very good.

The following directions are the same for all telescopes

If the star drifts up, use the mount's altitude adjustment knobs to move the mount so that the star appears to move down in the field of view.

If the star drifts down, use the mount's altitude adjustment knobs to move the mount so that the star appears to move up in the field of view.

Star drifts up Adjust mount to move star down

Use the hand-controller to move the star back onto the horizontal line.

Let the star drift again. You should notice that it takes longer for the star to begin drifting off the line. Repeat the altitude adjustments, placing the star back on the crosshair again when finished.