Celestron CG-4 Equatorial Mount

My notes on how to use this mount.

Setup is relatively simple, but much more involved than altizumuth mounts we might be used to.

Balancing

First, we balance in the right ascension, then in declination.

RA balancing is necessary for accurate tracking when using motor. It also eliminates undue stress on the mount.

DEC balancing is necessary to prevent sudden motions when the DEC clamp is released.

Right Ascension

  1. Release the RA clamp (the lower clutch), and position the telescope off to one side of the mount. The counterweight bar should be horizontal on the opposite side of the mount.
  2. Release your hold on the telescope - gradually - to see which way the telescope roles (to one direction or the other)
  3. Move the counterweight as necessary to balance the telescope (remains stationary when the RA clamp is released).
  4. Tighten locking screw to hold counterweights in place.

Declination

  1. Release the RA clamp and position the telescope off to one side of the mount - basically, same start as when balancing in RA.
  2. Lock the RA clamp to hold the telescope in place.
  3. Release the DEC clamp, and rotate the telescope until the telescope is parallel to the ground
  4. Gently release hold on the telescope to see which way it rotates. As with before, don’t let go entirely.
  5. Move the telescope on the mounting bracket in either direction until the telescope doesn’t move, as tested in part 4.
  6. Tighten the mounting bracket screws.

Polar Alignment

Now we get to the part of what makes equatorial mounts actually different than altizumuth mounts. This is necessary to track the stars correctly.

The goal is to place the telescope’s axis of rotation parallel to the Earth’s axis of rotation. This is done by moving the telescope vertically (altitude) and horizontally (azimuth), not in RA or DEC.

Note that the mount can really only be adjusted between 20 and 60 degrees.

There are a few ways to do this.

Latitude scale

This is the easiest way to align a telescope. It also can be done in daylight, because it only requires that you know which way is (true) north, and your latitude (degrees above the equator). This is also the least accurate, but it gets close enough for short exposure astrophotography.

  1. Make sure the polar axis of the mount is pointing due north.
  2. Level the triod (there’s a bubble level built into the mount for this purpose)
  3. Adjust the mount in altitude until the latitude indicator points to your latitude.

Pointing at Polaris

This is conceptually simple. Polaris is less than a degree away from the celestial north pole, so you use Polaris as a stand-in for the celestial north pole. It’s about as accurate as the latitude scale method.

  1. Make sure the polar axis is pointing north.
  2. Loosen the DEC clutch nob and move the telescope so that the tube is parallel to the polar axis. When this is done, the declination setting circle will read +90 degrees. If the declination setting circle is not aligned, move the telescope so that the tube is parallel to the polar axis.
  3. Adjust the mount in altitude and/or azimuth until polaris is in the field of view of the finder.
  4. Center polaris using those same altitude/azimuth controls. Do not move the telescope in RA or DEC.

Declination Drift

This takes the longest amount of time, but produces the best results. In this, you’re looking at two stars to see how much they drift in declination over time, which tells you how out of alignment you are from the polar axis. Because this takes a while, you should first get a rough alignment (using either latitude scale or pointing roughly at a polar axis).

The idea here is to choose two bright stars - one near the eastern horizon and one due south near the meridiam. Both should be near the celestial equator (0 declination).

For the southern star, choose one within half a degree of the meridian, and 5 degrees of the celestial equator. If the star drifts north, the polar axis is too far east. If it drifts south, the polar axis is too far west.

Once that star no longer drifts, we move on the the easter star. This should be 20 degrees above the horizon and within 5 degrees of the celestial equator. If it drifts south, the polar axis is too low. If it drifts north, the polar axis is too low. Adjust the latitude scale to fix this.