Two equatorial mount designs, the German equatorial and the equatorial fork mount, account for the vast majority of equatorial mounts in use by amateur astronomers. Let's talk about how to polar align each. We'll start with the German equatorial mount.
The German Equatorial Mount |
You'll recall that declination is similar to latitude and that right ascension is the same as longitude. If we could float high above the North pole, we would see an Earth that spins in a counterclockwise direction completing one rotation every 24 hours. As a result, the stars seem to move east-to-west across the sky to an earth-bound observer. The motorized drive at the back end of the right ascension axis is there to allow the telescope to track objects as they move westward across the sky. But its success depends upon accurate polar alignment. |
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Step 1: Set the Scope to Your Latitude |
Before setting the tilt, I check to make sure the pier is level. I use a Sears "Craftsman" Torpedo Level, which has a magnetized bar along one side. If the pier is level, then I can accurately set the tilt of the mount in broad daylight using a protractor. I use a Sears "Craftsman" protractor with a magnetic base to set the tilt of my mount. When the protractor reads 35 degrees (Flagstaff's latitude), I lock the mount in place. This can be done at home during the day, which is much easier than at night in the dark in some remote field. It's worth repeating that the pier/tripod needs to be level in order for this step to work as described. As long as you level the mount, you never need to change the tilt when setting up at your regular observing site. |
Step 2: Setting Up the Mount |
When the sky becomes dark enough that stars begin to appear, look for Polaris. Polaris is a 3rd magnitude star at the end of the Little Dipper's handle. You will find it in the same location every night. Look to the North. Polaris will be at an altitude above the horizon equal to your local latitude. Make a fist and extend it to arm's length. A fist held at arm's length covers about ten degrees of sky. Suppose your latitude is 40 degrees. Polaris will be about four fists above the northern horizon. After finding Polaris, manually reposition the telescope so the right ascension axis is pointing in that direction. You're almost home. |
Step 3: Align With the Celestial North Pole |
Polaris is flanked by two fainter stars. Together, this group forms a triangular pattern with Polaris being the brightest star along the short leg. The triangle spreads across two degrees of sky. The cross hairs in the diagram are centered at the location of the celestial North pole. Notice its position with respect to Polaris. If you don't see this when looking into the cross hairs, don't be alarmed. You just have a few minor adjustments to make. Do not move the telescope in right ascension or declination to finish the alignment. The mount may have a lock screw that, when loosened, allows you to move the whole mount around the horizon. If not, you'll just have to grab a couple of the mount legs. Rotate the mount around the horizon until Polaris is visible near the cross hairs. Make sure the declination still reads 90 degrees and the optical tube is sitting directly above that north-pointing leg. If the cross hairs are centered on a point close to the illustrated position, then the polar alignment is accurate enough that a clock drive will move the telescope to track objects at high magnification for several minutes at least. This process may take 15- to 30-minutes the first few times you go through it. However, polar alignment will soon become second nature. I spend about five minutes doing this procedure and often am able to use my Newtonian's clock drive to track planets at nearly 400X for 30 minutes or more. If you need extremely accurate polar alignment, then visit my page with instructions on how to use the declination drift method. |
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