GEM Alignment

GEMs must be at least casually polar aligned if they are to track the stars and (maybe) go-to accurately. The closer the mount's RA axis points to the true pole, the more precise its tracking (but not necessarily go-to) will be. The major symptom of poor polar alignment is constant object drift in declination (north/south) in the eyepiece. There are three basic methods of polar alignment: boresight, borescope, and software assisted.

The first step for polar alignment for Northern Hemisphere observers is always locating Polaris as it provides a prominent signpost to the true North Celestial Pole. Since Polaris is the end star in the Little Dipper's handle, that constellation can be used to find it if light pollution has not made dim Ursa Minor totally invisible. Another way to locate Polaris is by its altitude and azimuth. Using a compass, look north. Polaris will be in that direction and at an altitude equal to the site's latitude. If the latitude is 40° , Polaris will be 40° above the horizon. It will be eminently noticeable, since second-magnitude yellowish Polaris is the only bright star in the area.

Some GEM mounts, like those sold by Celestron, do not need overly precise polar alignment to yield good go-to performance. Just sighting Polaris through the hollow bore of the RA axis (with no polar scope installed) is good enough to ensure excellent go-to accuracy, and more precise polar alignment will not improve go-tos. Tracking will not be as good as it would be with a more precise polar alignment, but that is not usually a problem for visual observing.

To perform a boresight alignment, set the telescope up with the RA axis pointed north and elevated to an angle that matches the site's latitude. Altitude is adjusted on most GEMs with the aid of a latitude scale on the side of the mount and a simple bolt arrangement: Tighten a bolt on the "south" side of the mount and loosen one on the "north" side to raise the RA axis; do the reverse to lower it. Before beginning, remove any caps blocking the polar bore (usually one on each end). It is almost always also necessary to move the mount in declination until the telescope tube is perpendicular to the polar axis to open a hole in the intruding declination shaft. Sight through the south end of the bore and adjust altitude (with the two bolts) and azimuth (usually via a pair of knobs) until Polaris is centered. If the mount is way off in azimuth, it may be necessary to lift it and the tripod bodily and turn it until Polaris is visible in the bore before doing finer adjustments with the azimuth adjustment knobs.

A few German mounts, including Celestron's CGEs, do not have hollow polar bores. What do you do then? Sighting along the polar axis or centering Polaris in the main scope's finder with it set to a declination of 90° and the counterweights "down" is "good enough."

Not all GEMs deliver good go-to accuracy without a more accurate alignment than that provided by the boresight method. One way to get better accuracy is with a polar alignment borescope (Plate 47). This small refractor fits into the hollow bore of the RA axis (or is mounted beside it). The borescope will have a reticle similar to that in Plate 48. Move the mount in altitude and azimuth until Polaris is in the little circle provided for it on the reticle and the mount is polar aligned, just like magic—simple.

Unfortunately, it is not quite that simple. Polaris circles the actual North Celestial Pole once every 23 hours 56 minutes and 4 seconds at a distance of about two-thirds of a degree. For that reason, the polar scope must be rotated (usually by moving the mount in right ascension) until the Polaris marker is in the proper position for the date and time of day when the alignment is performed. That is done by rotating the mount in RA until the proper day and time line up on a pair of graduated scales. Before setting date and time, these dials must be calibrated for the user's longitude; see the mount's instruction manual for details. Some polar scopes use a simpler if somewhat less-accurate method of calibration: The reticle will show the positions of several constellations near the pole. Turn the scope in RA until the constellations are in roughly the correct positions, and the polar scope will be ready for use.

If these instructions sound way too complicated, there is a trick you can use that eliminates the need to calibrate the polar scope. Using this method, the date/time

Plate Polar Alignment

Plate 47. (Polar Reticle) Placing Polaris in the proper spot on the polar alignment scope's reticle yields an alignment good enough for most purposes. Credit: Author.

Plate 48. (Polar Scope) The eyepiece end of a German equatorial mount's polar alignment borescope. Credit: Author.

scales are not needed. The key is a small PC (Windows) program, PolarFinder. Given location (longitude) and time, it will display a graphic that shows where Polaris should be placed. See Appendix 2 for further information.

How does alignment accuracy compare to that done by calibrating the polar scope for time and date? There does not seem to be any noticeable difference, and actually this method is more accurate than the line-up-the-constellation-figures routine.

A go-to scope has a computer. Can't that help in polar alignment? Indeed, it can be. Some GEMs, most notably Celestron's CG5 and CGE mounts, include a built-in polar alignment software routine. Select this from the HC utility menu following a successful go-to alignment, and the mount will slew to the place it thinks Polaris would be if the mount were precisely polar aligned. The user is then instructed to adjust the mount's altitude and azimuth (not RA and declination) until Polaris is centered in the crosshairs of the finder and in the main scope's eyepiece. When that is done, the mount is polar aligned. This method yields an alignment that is at least as good as a borescope alignment and maybe a bit better. This routine should be more than adequate for guided charge coupled device (CCD) imaging at short focal lengths. The only catch is that since the mount head has been moved, the go-to alignment must be redone following the polar alignment.

The preceding polar alignment methods ensure accurate go-to performance and will deliver tracking good enough for all visual use. If serious imaging is the goal, however, most amateurs use a much more accurate method of alignment called declination drift. Drift alignment takes a minimum of a half hour to perform and is only needed for imaging. See Chapter 11 for instructions.

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