A Quick Tour of an Equatorial Mount

Setting Up an Equatorial Mount: A Quick Tour

To begin our discussion of setting up an equatorial mount, we will have a little tour of a typical mount. There are many kinds of equatorial mounts, but most share some common features such as those found on this Stellarvue M4 mount. We’ll use this M4 as a guide for most of this article, occasionally showing differences on other mounts.
Equatorial mounts (EQ mounts for short) are immediately recognizable by the way they hold the telescope tube at an angle tilting upward, and by the shaft, holding a counterweight, that hangs under the mount at right angles to the telescope tube.

If you draw an imaginary line through the mount at a right angle to the counterweight shaft, you define the polar axis. This part of the mount will be aligned to point to the North Celestial Pole (NCP), and it does not move, even when the telescope is moving in the other directions in which it can.

What seems like extra complexity in EQ mounts will become simpler if you remember that the whole point of an EQ mount is to get, and keep, this axis pointing toward the NCP. We’ll discuss how to do this in a later article.

EQ mounts can move the telescope in two directions (both of which trace circular paths, and neither of which is similar to the directions we normally use on Earth). First, it can rotate around the polar axis. Such motion is called Right Ascension (RA for short).

Next, it can rotate around the direction defined by the counterweight shaft. This motion is called Declination.

Summary: Move the telescope; if the counterweight is moving, that is Right Ascension. If the counterweight is not moving, that is Declination.

By combining these two motions, the telescope can be pointed anywhere in the sky.
Somewhere on the mount there will be knobs which, when tightened, prevent the scope from moving in these directions. One to lock RA motion, one to lock Declination motion.
On some mounts (all non-motorized mounts and some motorized ones) there will be two flexible shafts with knurled knobs. These are slow-motion controls for the RA and Dec motions. The mount can be moved by these controls even when the locking knobs shown above are tightened – those are only for locking big “by hand” movement. In fact, the slow-motion controls are supposed to be used with the other locks tightened.
The above RA and Dec controls are for moving the telescope on the mount. There are usually also two controls for moving the mount itself, used when polar aligning it. These controls may be harder to access since they are used less often. There will be one set of knobs for moving the mount left and right a small amount, and another set of knobs for raising and lowering the elevation of the polar axis.

There is usually also a scale somewhere showing the current elevation of the polar axis in degrees. The elevation is usually set to your latitude on Earth (or close to that, adjusting for lack of levelness in the mount), so this is generally called the “Latitude Scale”. We’ll discuss this when we discuss polar alignment.

Again, these controls are not used to point the telescope, they are used to align the mount. You’ll use them infrequently – typically only when you set up your mount the first time, or if you disassemble it or move.

There may be additional knobs to lock the mount against motion in these directions after it is adjusted.
Finally some mounts will have one or two small DC motors used to smoothly drive the RA and possibly the Declination motions. Usually these are optional on smaller mounts and standard on high-end mounts.

5 comments

  1. This was a great article. I have never worked with an EQ before, but just got one. I thought I understood how they worked until I put it all together and mounted the scope and then said “wait a minute…” This cleared up everything! Very well done.

    Reply

  2. Great article. Nothing comes close to explaining what is, at the end of the day, a simple concept.

    Reply

  3. Hi: Found your site Googling “RA Motor”. I have an RA motor setup exactly like the one in your last photo (circled in green). Can you tell me the function of the two plugs, the one on top with male pin and spring clip, and the one below with multiple tiny female pinholes? What kind of power does it take 9V/12V? What is the tech nomenclature of the cable/cables that go with the motor? Any other info like manufacturer, original setup, etc? Mine is on a LIN Optical EQ. Thanks. Mike/moreas@ij.net

    Reply

    1. Hi Mike, where did you get that RS motor? I’m looking for a motor as well for my Stellarvue M4 mount (the same mount in the picture).

      Reply

  4. The moving pictures are just what I needed! I agree that all telescope manuals should just reference you articles on getting started. I’ve already added you to my favorites for clear, concise information. Thank you again and keep up the good work.

    Reply

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