Polar Alignment


Polar Aligning an Equatorial Fork Mount

Schmidt-Cassegrain TelescopeEquatorial Fork Mount The illustration at left shows an SCT on an equatorial fork mount. The directions for polar aligning this mount are the same as for a German equatorial mount with a few minor changes. Commercial SCTs are among the most popular amateur telescopes in the world. Their compact size, moderate aperture, and versatility as both visual observing and astroimaging platforms make SCTs suitable for a wide range of astronomical uses. The illustration at right points out the various functions of an equatorial fork mount.

The equatorial fork mount has two axes of motion. Like the German equatorial mount one axis is in right ascension and the other in declination. The declination axis runs through the telescope optical tube. The U-shape of the fork allows the telescope to be connected to the mount in two locations. The telescope moves through declination by tilting up and down between the fork arms. The right ascension axis runs upward through the arms of the fork mount. A motor drive is mounted beneath the fork. The telescope moves in right ascension by pivoting around the horizon at the juncture between the fork and the motor drive. Finally, the tilt of the mount is set where the motor drive and fork join the tripod. This device is called an equatorial wedge. Set at the proper tilt, the mount is aligned with the Earth's axis of rotation.

Step 1: Set the Tilt to Your Latitude

Fork equatorial mount tilt angle The first step in polar alignment is to set the tilt of the mount to your local latitude. This aligns your telescope parallel to Earth's axis. The illustration at left shows that angle. Many equatorial mounts have an adjustment knob allowing you to easily adjust the tilt angle. Also, look for a scale on the mount indicating the tilt angle.

Before setting the tilt of the mount, I check to make sure the tripod is level. I use a Sears "Craftsman" Torpedo Level which has a magnetized bar along one side. If the tripod is level, then I can accurately set the tilt of the mount in broad daylight using the gauge affixed to the mount or 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 some remote field. It's worth repeating that the tripod (or pier as the case may be) 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: Pointing North

Fork equatorial mount alignment with due North The next step in polar alignment is to setup the telescope. Position the mount so the right ascension axis is pointing North. This can be done during twilight. No need to use a compass for this step. Just get the telescope's right ascension axis pointed roughly North.

When the sky becomes dark enough that stars are appearing, 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 elevation above the horizon equal to your local latitude. Suppose your latitude is 40 degrees. Make a fist. A closed fist held at arms length covers roughly ten degrees of sky. So, Polaris will be four fists above the northern horizon. After finding Polaris, manually move the telescope so the right ascension axis is pointing in that direction.

Step 3: Align With the Celestial North Pole

Finder scope view of Polaris The final step is to align the telescope with the celestial pole. First, rotate the telescope in declination to 90 degrees. This is the declination of the celestial north pole and the optical tube should be pointing toward Polaris. Second, rotate the telescope in right ascension until the finder scope eyepiece is accessible. This may be next to impossible unless your scope has a right angle finder.

Look into your finder scope. Hopefully, the view will be similar to the illustration at left. 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.

If you don't see this or if the finder scope is inaccessible, don't be alarmed. You can sight along the optical tube to achieve a rough polar alignment good enough for visual observing. In either case, do not move the telescope in right ascension or declination to finish the alignment. Your telescope 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, and rotate the mount around the horizon until Polaris is visible in the cross hairs or the telescope appears pointed in that direction. Make sure the declination still reads 90 degrees. If the finder is 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. If you need extremely accurate polar alignment, then visit my page with instructions on how to use the declination drift method.

Polar Alignment, Page 2 Polar Alignment, Page 1


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Revised: February 11, 2002 [WDF]