How to Point a Telescope

It was a lot simpler in Galileo’s day.

Up and down, left and right, manually controlled—simple, right? Yes, but simpler was not always better, at least until computers came along. But we’re getting ahead of ourselves.

There are two issues to be dealt with here.

  1. Pointing the telescope accurately.
  2. Following the apparent movement of celestial objects so that they remain in view.

Accurate pointing–The first is not a big problem if you are not working with a high-magnification view. A pair of 7 x 50 binoculars which magnify 7 times (the 50 is the diameter of the light-gathering lens in millimeters) can be hand-held and pointed with relative ease. A typical field of view for these binoculars is 7 degrees, and this is 14 times the width of the full moon.

A larger instrument clearly cannot be hand-held, and larger light-gathering elements (whether a lens or a mirror) allow higher magnifications. Here is the view afforded by a medium-power eyepiece in combination with the 20-inch Gilbert telescope at Lynchburg College’s Belk Observatory. This field is about ¼ of a degree across and is indicated by the red circle beside the moon.

FOV for Gilbert

You would be amazed at how difficult it can be to manually aim a big telescope with such a small field of view at a big and obvious target like the moon. Believe me—I’ve done it, and it was anything but simple. Some of the cameras we can attach to the telescope have even smaller fields of view, and accurate pointing is essential for their use.

So…we need a way to mount a big telescope so that it can be moved both in large increments from one side of the sky to the other AND in very small steps that can allow us to “fine-tune” our aim. The mount needs to be massive enough so that it does not shake with every touch or passing breeze.

Accurate tracking—Looking at a celestial object through a telescope is not like pointing it at a distant mountain top. The mountain top does not move. The stars and planets do, or rather they appear to move because of the Earth’s rotation around its axis. If you are facing north in Virginia, the stars appear to circle counter-clockwise around a point very near Polaris, the North Star.

And for a gorgeous time-lapse video of “moving” stars and the Milky Way, check this one shot at the Very Large Telescope site in Chile:

There are two basic types of telescope mount that can compensate for the Earth’s rotation by following celestial objects across the sky: equatorial mounts and altitude-azimuth (altazimuth) mounts. Let’s take the equatorial mount first. Here is a design where its features are most easily seen.

Here are the key points to notice:

  • There are two axes around which we can rotate: declination and RA. Don’t worry for now about what those stand for, but if you want to know, go here.
  • These two axes are at right angles to each other.
  • The RA shaft points toward a celestial pole, the point around which stars appear to rotate. In the northern hemisphere, this is very close to Polaris.
  • If we aim a telescope mounted here to a star, we will only need to move around one of these axes, the one represented by the RA shaft, to track the star all night long.

What looks like a fairly complicated arrangement can actually simplify matters by only requiring movement along one axis. The great telescopes built in the twentieth century through 1948, the installation year of the 200-inch (5-meter) Hale Telescope at Mount Palomar, all used some variant of an equatorial mount. The Margaret Gilbert Telescope at the Lynchburg College Belk Observatory also employs an equatorial mount. The counterweights you see on the declination shaft help balance the entire assembly for easy movement.

Gilbert

So why would you use an altazimuth mount for a research-quality telescope? To track across the sky, you need to constantly move up (or down) AND right (or left): movement along TWO axes. More complicated, right?

Not for a computer.

As telescopes get larger, the advantages of lighter and less bulky mounts become ever more obvious, and the increasing ability of computers to precisely aim these scopes can easily deal with the complications of moving in two axes. This model of the 10-meter Keck Telescope in Hawaii shows the type of mount found in all large telescopes built since the 200-inch Hale. What may not be obvious from this image is that the entire assembly can rotate. That is the movement in azimuth; tilting the telescope moves it in altitude.

This design is increasingly used in relatively inexpensive mounts available to amateur astronomers. The mount below costs about $1100. It rotates in azimuth and tilts in altitude to allow a small telescope to stay locked on a moving celestial target for hours at a time. I can attest that it works quite well; this is the mount I recently purchased for my personal telescope!

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Neal Sumerlin

Neal Sumerlin, retired Professor of Chemistry and founding Director of the Belk Observatory at Lynchburg College

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