Polar Aligning your Mount for Imaging
Accurate polar alignment is critical for CCD imaging. Field rotation, a trailing of stars near the edges of an image, can result from improper polar alignment. Even short exposures such as planetary images can benefit from good polar alignment as there will be less drift from one image to the next during a sequence, keeping the object in the small field of view and making registration of the images for stacking easier.
Most modern goto equatorial mounts have a handy polar alignment routine built in. This eliminates the need to do a long drift alignment procedure and eliminates the need for a polar alignment scope (though it can still be handy to have). Below we describe rough polar alignment and the use of a polar alignment scope, which would be the usual setup to do before using the computer alignment routine in the mount.
We also describe doing a drift polar alignment as this gives very precise alignment for a permanently mounted telescope. But this routine is rarely used anymore and computerized methods are very accurate and easy.
This is simply a matter of getting the telescope oriented in basically the right direction.
Point the right ascension axis roughly toward Polaris.
Above: The two stars in the end of the Big Dipper's handle always point toward Polaris, the North Star. This chart shows the position of the Big Dipper and Cassiopeia in the evening in early summer. As the seasons progress these constellations will rotate from month to month in a counterclockwise direction around Polaris.
Level the tripod. While this is not strictly necessary, it will be very helpful.
Make sure the latitude scale on the mount is set correctly. There is no need to be extremely accurate here. Within 1 or 2 degrees is close enough. Your latitude can be found on many local maps or from smartphone apps.
Most German equatorial mounts have a hole where a polar scope mounts that you can simply look through even without the polar scope in place. If you sight Polaris through this hole and reasonably center it, you will be doing pretty well.
It is not recommended that you adjust the mount by centering Polaris in your telescope's finderscope. For this to be accurate, the telescope must be accurately aligned with the polar axis of the mount, which is unlikely. It is better to sight directly through the mount axis.
Polar Alignment Scope
With new computerized polar alignment routines, a polar alignment scope is much less necessary than it used to be. In fact, you can do a perfectly good alignment without one. But if you have one, it will get you closer to begin with. This will then require fewer iterations with the computer alignment method, making the overall setup quicker.
A polar alignment scope works by showing the offset from Polaris to true celestial north, which is about 3/4 of a degree. By aligning the mount using a polar scope, the telescope can track sufficiently for many types of photography. However, the pointing accuracy of the mount is dependent on the polar alignment as well, so have a precise alignment is beneficial in that regard as well.
Above: The view through a typical polar alignment scope. Polaris is centered, which is not where it needs to be.
Above: Polaris really needs to be in the position shown above, offset slightly from true celestial north.
The accuracy of this alignment can be improved further by rotating the mount in right ascension to match up the second alignment star seen at the top of the diagram above.
Above: Both Polaris and the nearby alignment star are in their proper positions. A very accurate alignment has been achieved.
Computerized Alignment Routines
Most goto equatorial mounts have a polar alignment routine built into the hand controller functions. The Celestron mounts, for example, have an All-Sky Polar Alignment technique that does not require being able to see Polaris (although that is useful for the initial rough alignment). After doing a normal star alignment, the mount is synced on a star near the meridian in the south and the computerized routine then offsets the telescope such that centering the star by adjusting the altitude and azimuth of the mount results in an accurate polar alignment.
The Astro-Physics mounts use an iterative routine, pointing between Polaris and another alignment star, adjusting the mount on Polaris each time and re-centering the other star with the hand controller, until sufficient accuracy has been achieved. Any such routine will get your mount polar aligned to very high accuracy.
Declination Drift Alignment
Drift aligning an equatorial mount was a necessary procedure before the advent of computerized mounts. Anymore, it is not normally required. It can be a useful procedure for accurately aligning a permanently mounted telescope, but computerized routines are more often used with perfect results. Still, we describe the procedure below.
Once you have read the directions below, you may wish to print out the Drift Alignment Quick Reference to have with you in the field. The drift method of polar alignment requires the use of an illuminated crosshair eyepiece. A simple double-crosshair (shown below) works perfectly, although a fancier eyepiece such a Celestron's Micro Guide eyepiece will work fine as well.
Note: These instructions apply to telescopes which use diagonals such as SCTs, RCs, and refractors. Newtonian-style telescopes will need to reverse certain directions which are noted.
Above: The view through a typical illuminated crosshair eyepiece.
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.
Above: Rotate the eyepiece so that the crosshairs are parallel to the east-west motion of the star in the telescope.
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.
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.
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 steps are the same regardless of the type of telescope used
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.
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 polar aligned. You are ready to begin imaging the heavens!