Celestron CGEM - 925 Computerized Telescope CGEM Series Manual - Page 37

Polar Aligning the Mount Latitude Scales The easiest way to polar align a telescope is with a latitude scale. Unlike other methods that require you to find the celestial pole by identifying certain stars near it, this method works off of a known constant to determine how high the polar axis should be pointed. The CGEM mount can be adjusted from 15 to 70 degrees (see figure 5-3). Figure 5-3 Latitude Scale The constant, mentioned above, is a relationship between your latitude and the angular distance the celestial pole is above the northern (or southern) horizon; The angular distance from the northern horizon to the north celestial pole is always equal to your latitude. To illustrate this, imagine that you are standing on the north pole, latitude +90°. The north celestial pole, which has a declination of +90°, would be directly overhead (i.e., 90 above the horizon). Now, let's say that you move one degree south - your latitude is now +89° and the celestial pole is no longer directly overhead. It has moved one degree closer toward the northern horizon. This means the pole is now 89° above the northern horizon. If you move one degree further south, the same thing happens again. You would have to travel 70 miles north or south to change your latitude by one degree. As you can see from this example, the distance from the northern horizon to the celestial pole is always equal to your latitude. If you are observing from Los Angeles, which has a latitude of 34°, then the celestial pole is 34° above the northern horizon. All a latitude scale does then is to point the polar axis of the telescope at the right elevation above the northern (or southern) horizon. To align your telescope: 1. Make sure the polar axis of the mount is pointing due north. Use a landmark that you know faces north. 2. Level the tripod. There is a bubble level built into the mount for this purpose. 3. Adjust the mount in altitude until the latitude indicator points to your latitude. Moving the mount affects the angle the polar axis is pointing. For specific information on adjusting the equatorial mount, please see the section "Adjusting the Mount." This method can be done in daylight, thus eliminating the need to fumble around in the dark. Although this method does NOT put you directly on the pole, it will limit the number of corrections you will make when tracking an object. It will also be accurate enough for short exposure prime focus planetary photography (a couple of seconds) and short exposure piggyback astrophotography (a couple of minutes). Polar Align using the Hand Control The CGEM mount has a polar alignment function called "All-Star" polar alignment that will help you polar align your telescope for increased tracking precision and astrophotography. This feature allows you to choose any bright alignment star to assist in accurately aligning your telescope's mount with the North Celestial Pole. Before using the Polar Align feature, the scope must first be roughly pointed towards North and should be aligned with two stars in the sky. See the "Latitude Scale" section for help with finding north and adjusting the mounts latitude. Once your telescope is aligned on two stars and at least one additional calibration star, slew the telescope to any bright star in its Named Star database list. For best results choose a polar alignment star that is high in the sky and near the Meridian. Try to avoid stars that are close to the west/east horizon, directly overhead or too near the celestial pole. Once there, press the Align button and use the Up/Down buttons on the hand controller to select Polar Align from the list. The Polar Align feature has two options: Align Mount- After performing a two star alignment and slewing your telescope to any bright star in the telescope's database, select the "Align Mount" option. The telescope will then re-slew to the same star. 37