The Lake County Astronomical Society

Mounts: Equatorial vs Altazimuth

Jack Kramer

This article probably won't resolve the question of which is the "best" mounting for your purposes. But it brings to the table several of the differing viewpoints. Experienced observers agree that the mounting functionally accounts for a large part of the overall system -- even the best of telescopes is of little worth if it's on a poor mounting. Each design has advantages and disadvantages. The traditional German equatorial mount (GEM) is seen less often these days as Dobsonians, which are altazimuths, have taken front stage. The term "equatorial", applies to any mount having an axle that can be aligned toward the celestial pole. Although the easy-to-use fork mounts are generally supplied with Schmidt-Cassegrain telescopes (SCT's), the GEM is offered as an alternative on some models. The nice thing about the short-tubed SCT's is that they're adaptable to virtually any type of mounting. Fork mounts were always equatorials, but nowadays many are of an altaz configuration; the altaz versions with dual-axis drives and digital controls behave like very intelligent equatorial mountings. There are persuasive arguments in favor of all the different mountings, and the once clearcut distinctions between the different designs are nowadays somewhat blurred. Here are some pros and cons, related to the various ways a mounting might affect your observing.

Movement: With an equatorial, you can follow a celestial object as it moves through the sky by moving the telescope in only one direction -- right ascension. With an altazimuth, you move the scope in an east-to-west direction (azimuth) and slightly up-or-down (altitude). This isn't a big deal when observing an object that's low in the sky, but it becomes more of an inconvenience as you head up toward the zenith. In the area directly overhead, you can't locate or follow an object by a simple east-west-up-down motion, so with an altazimuth you have to take your nearly vertical telescope tube and rotate it about a horizontal axis. That's more difficult than it sounds. With a copyscope mounted on a camera tripod, I simply avoided aiming at objects overhead. It gives a whole new meaning to the term "well-placed object". But the first time I observed Polaris (a fine double star) it was with the LCAS club scope -- a Dobsonian -- because it was simply too awkward to point my own equatorially-mounted scope toward the polar region. While there are no blind spots with any of the popular designs, there are some areas of the sky that all observers tend to avoid due to the difficulty in aiming their scopes. (And see below.)

Eyepiece position: With a larger Newtonian telescope on an equatorial mount, there has to be some way to rotate the tube. In the Newtonian GEM configuration, a telescope of eight inches or larger can end up in such an awkward position that it becomes impossible to comfortably position yourself at the eyepiece. One of the advantages of the Dobsonian is that the eyepiece is always in a convenient position. The fork mounting has its own awkward spot when the scope is aimed toward the polar region. Here you end up with the eyepiece positioned squarely between the forks while you try to get your head close enough to see through it.

Astrophotography: Astrophotography requires a motor drive. It always used to require an equatorial mount, too. The altazimuth configuration results in "field rotation", which makes the star images come out as short lines, rather than points. This is caused because celestial objects describe an arc as they move through the sky, whereas an altaz-mounted scope moves only at right angles (up-down-right-left). However, in recent years, sophisticated CCD cameras on the market have compensated for the old bugaboo of field rotation.. So this may no longer be an impediment to CCD work with a motor-driven altaz mount. On the large altazimuth mounted observatory telescopes, the instrument clusters rotate to compensate for field rotation.

Clock (motor) drives: Good clock drives add expense to the telescope package. Don't be misled by ads for inexpensive scopes that boast a motor drive so they're "all set for astrophotography". These may be okay for visual work, but the precise tracking needed for photography requires that even good drives have correctors, either built in or as a separate unit, and have the ability to be slued ("sped up") on both axes while tracking. An incident occurred quite a number of years ago when a manufacturer began marketing an inexpensive Newtonian telescope on a GEM with clock drive. Astrophotos taken with that telescope were published in ads, but what they didn't tell you was that when the photos were taken, the telescope wasn't on the mounting that comes with the scope. In fact, that drive was so anemic that it was barely adequate to follow objects visually. The ads were subsequently withdrawn and the mountings redesigned for improved performance.

Visual Tracking: A motor drive is necessary for photography, with the exception of "snapshots" of bright objects like the moon and sun. But even for visual observing at higher magnifications, the narrower field of view makes a motor-driven equatorial especially welcome, otherwise you may have to nudge the scope along a couple of times per minute. Each time it's moved, it takes a while for the image to settle down. Not only does that break your concentration, but you may have missed a moment of excellent seeing. (Even in very clear skies, cells in the atmosphere can substantially alter viewing conditions from moment to moment.) According to ALPO (the Association of Lunar and Planetary Observers), dedicated planetary observers consider a clock drive essential. You may find that a decent manual slow-motion control will be perfectly acceptable for visual observing. However, your experience may be the same as mine. When using a 4" refractor for lunar and planetary work, the image quality permits routine use of high magnification, with resulting narrower field of view. I discovered that objects went sailing through the field pretty quickly, and that made a dual-axis drive just about a necessity for this scope.

Stability: Many observers get along quite nicely with well-made Dobsonians. This is the most stable mounting design, so images settle down quickly whenever the scope is moved. Stability is especially important when a breeze afflicts scopes with a case of the "shudders". Some new generation amateur and observatory telescopes are mounted on altazimuths (glorified Dobsonians), with digitally-controlled drives on both axes. They can follow objects with precision, while taking advantage of the stability of the altaz design. I have yet to see a reasonably portable GEM that allows the image to settle down as quickly as it does in a Dob.

Finding objects more quickly: A primary consideration in any mounting is how easy it will be to use, since you'll be spending a lot of time handling it. According to the late Walter Scott Houston:

Some years ago I kept a record of how long I and others spent searching for variable stars compared to the time we spent making magnitude estimates. I was amazed to discover that roughly 90 percent of our time was devoted to the search. It made little difference whether the observer used an altazimuth or equatorial mounting since even an equatorial with setting circles usually just puts you close to the object of interest. You still must star-hop to center the field in the eyepiece.

Scotty's point was the importance of becoming proficient in finding your way around the sky. This also applies to deep sky objects that appear as barely perceptible hazy spots; we may spend more time searching for the object than observing it.

Prolonged observation: When examining the features on a planet or the structure of a bright galaxy, we spend more time observing the object than locating it. So it becomes important to have a mounting that's easy to use both to locate an object and to follow it over a period of time. For star-hopping with an equatorial, it's quite simple to locate a brighter object at about the same declination as the target object, then just move a bit in right ascension (or "left ascension", as the case may be). Of course, this again gets into the area of personal preference for the convenience of single-axis movement with an equatorial. However, near the celestial equator, there's not much difference between the movement of an altaz or an equatorial.

Construction and setup: It has to be said that Dobsonian mountings are easier to build and much lighter in weight than equatorials. Add to this the fact that the materials to make a Dob are less expensive and are readily available. With a reflector larger than 10", a Dobsonian is eminently practical. With a larger equatorial, you have to disassemble the mounting into various parts for transportation. The time required for setting up a scope on a Dob is minimal. There's a lot to be said in favor of measuring setup time in seconds!

Conclusion (sort of): Despite the greater cost of the equatorial mounting, it can be a worthwhile investment, especially if you intend to do astrophotography or CCD imaging. This applies to the digitally-controlled SCTs on altaz mounts, as well. For larger scopes, there are size and weight considerations that clearly favor the Dobsonian. A large equatorial mount is just plain HEAVY! There are some large, commercially-made telescopes on split ring equatorial mounts that are quite manageable, but they're also quite expensive. While newcomers to the hobby may be mystified by the configuration of the GEM in particular, it generally proves to be user-friendly once people become accustomed to it. The simplicity of the Dobsonian, however, requires no such period of acclamation.

Be aware that you don't have to buy the whole package at once. Especially if you're on a limited budget, you could start out with a Dobsonian, then remount the scope on an equatorial if you get into astro-imaging or prolonged, high magnification observing. Just make sure whatever mounting you choose is stable and operates smoothly. The choice of design all boils down to the fact that nothing's perfect!