TAS Telescope Mount Buying Guide
By Max Corneau
Hopefully if you are one of those special people with a newfound interest in amateur astronomy and have gotten this far…you know the
Golden Rule of Telescope Purchase. For those of you starting here, the Golden Rule is:
When you buy a telescope, remember that it’s really a system of two parts, the mount and the telescope (optical tube – OT) itself. The most important and unfortunately overlooked aspect of the telescope system is the mount, for it determines how the optics are applied against your search of the night sky.
Mounts come in several varieties ranging from your arms holding a pair of binoculars to a fully computerized, remotely operated system. Forget that last part if you are just getting into this hobby. Before we look closely at the mount, consider that the mount itself determines what your optical tube can be used to see or even what you can eventually image with a camera. Telescope mounts come in the following varieties from least complex to most complex.
For our purposes, both the fork mount and equatorial mounts sit atop a simple tripod system that comes with the mount itself as an integral part. The big differences in these mounts are in whether they track the sky automatically, how they move, and how they align.
The simplest type of mount is your pair of arms. Believe it or not, if you learn how to read a simple star chart (covered in another section of this exciting series) and learn how to “aim” your body using your shoulders, the human body makes a very good telescope pointing and aiming system. My favorite beginner star chart that always “gets you in the ballpark” is produced by Fourmilab and can be downloaded for free here.
Figure 1 -
The Alt-azimuth Big Box
The next simplest type of mount is the kind usually found by those who violate the Golden Rule of telescope purchase (see above) when they buy a scope at their local big box retailer that offers 600X Magnificent Magnification to enable viewing slimy green space aliens on Mars. Yes, this is the alt-azimuth telescope mount (Fig 1). BEWARE. This mount has two perpendicular axes of motion, vertical (altitude) and horizontal (azimuth). Standard camera tripods have alt-azimuth mounts. These mounts are usually mated to the optical tube, badly aligned, shaky and make tracking the sky almost impossible at all but the lowest magnification.
Figure 2 - The Excellent,
Now we’re going to see another type of Alt-azimuth mount, that really makes life easy for the new observer. One of my personal favorites is the Dobsonian or “Dob” mounted telescope. The Dobsonian telescope takes its name from the astronomer/philosopher John Dobson, who introduced the concept of inexpensive, large aperture telescopes to astronomy and founded the San Francisco Sidewalk Astronomers. Amateur astronomers at the time were so amazed that a telescope built from simple, inexpensive materials performed so well that they could hardly believe their eyes. As home-built Dobsonians started showing up at star parties and people saw what Donsonians could do, the Dobsonian revolution swept the world.
Figure 3 - EQ, or Equatorial
The Equatorial Mount
The equatorial mount has two perpendicular axes, but they are called right ascension (R.A. or polar) and declination (Dec). When the RA axis is aligned parallel with the Earth's rotational axis, objects can be easily "tracked" as they drift across the sky (due to Earth's rotation) by turning just one of the slow-motion controls (R.A.) instead of two, as is required with an alt-azimuth mount. A motor drive can be coupled to the R.A. axis for automatic sky tracking. Sounds complicated and sort of is complicated because in order to have any chance of an accurate alignment, one must accurately align the polar axis of the mount to what we call “Polar North”. In the United States this is pretty simple as long as you have a polar finder scope in the mount, know what time it is, and have a way to identify the astronomical or “Sidereal” time. I don’t recommend purchasing an equatorial mount on your first go unless you’re pretty space savvy. The image in Fig. 3 illustrates perhaps the most inexpensive type of equatorial mount, but don’t be deceived as these mounts really form the basis of the high-end of science being performed today by the top amateur astro-photographers and researchers.
Finally, we’ll look at one of the more popular telescope mounts, the Fork Mount. I have owned two systems with fork mounts and both provided hundreds of hours of excellent astronomical viewing and imaging, however I “dismounted” my observatory based telescope from its forks about a year ago in favor of an equatorial mount because it tracks and points at the sky more accurately.
Figure 4 - Fork Mounted 8" SCT
Several commercial manufacturers, namely Meade and Celestron, have made fork mounts popular. With most fork mounts comes a computerized capability right in your hands. Using a thing called a hand paddle, which is really a mini-computer processor with memory and everything, you simply tell the telescope where you are, what time it is and point it at a couple objects in the sky and you have an alignment. The downside to all this automation is that when you press “Go-To” an object, if the telescope doesn’t put the object in your eyepiece or finder scope, the mount is really not very user friendly in terms of searching the sky for the object. However, there are some simple fixes to better enable this search process. On my 10-inch Meade LX200 I removed the little finder scope so I could have a second imaging telescope and added a TELRAD zero-mag finder. One of the most appealing aspects of a fork-mounted telescope is its ability to accurately track objects in the sky. You might find that you like to take pictures of the night sky and this kind of mount will allow you to do just that for several seconds at a time. Unfortunately, when the mount is sitting “flat” as depicted in fig. 4, any images of longer duration than maybe 30-seconds will result in a thing called “star-trailing” that is most pronounced at the edges of the image. But wait, there’s a solution to this problem. You can make your alt-az fork mounted telescope into an equatorially based fork mount by placing it on a thing astronomers call a “wedge”. In fig. 5 you can see what my Meade used to look like in my observatory before I dismounted it in favor of a German equatorial mount.
Figure 5 - EQ Fork Mounted
10" Meade Telescope
Meade and Celestron both manufacture wedges but they are considered entry level. So by the time you decide you need a wedge to support what may become your new “addiction” to astrophotography, you may want to consider a high-end wedge such as a Milburn or Mitty product. The robust wedge in fig. 5 is an aerospace grade aluminum Milburn wedge.
Every good story needs a summary and this one is no different. I will use a simple table to summarize the telescope mount story.
Clear Skies and welcome to TAS!
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