Even the most expensive and carefully crafted telescope isn’t worth much if it’s not on a solid and stable mount that lets you accurately point it anywhere in the sky. A good telescope mount is as important as the optics of a telescope, and it must be sufficiently solid and stable such that if you give the telescope tube a good tap on the side, the mount should damp down vibrations in less than 5 seconds (max), and faster if you’re planning on astrophotography. Most telescopes, especially scopes aimed at beginners, include a mount when you buy them. Smaller telescopes, especially high-end refractors, may just have mounting rings or plates which allows them to be attached to a mount which you buy separately. And if you are contemplating astrophotography, a solid mount is a must-have.
All telescope mounts can be classified as one of two types: alt-azimuth or equatorial. Let’s have look at each.
Alt-az mounts are a good choice for observers who value simplicity, fast set-up, and who have no plans to do photography through the telescope. An alt-azimuth mount lets you move a telescope move up-down (altitude) and left-right (azimuth). With these two motions, you can point a telescope to any object in the sky. But an “alt-az”, as they are called, does not follow the natural motion of the sky. Stars and planets appear to move around the sky in circles centered about an imaginary line through the north and south celestial poles. They follow a path in the sky that’s a combination of altitude and azimuth. So to keep an alt-az-mounted scope centered on a celestial object, you’ll have to move the scope in both axes, which can be bothersome for visual observing and completely unacceptable for photography through a telescope.
Dobsonian reflectors are usually mounted on a fork-type alt-az mount called a “rocker box”. You give them a push in one or both axes to point the telescope. In many cases, the telescope is held in place by the mount’s friction. Other types of alt-azimuth mounts are simply like a camera tripod, with bearings and joints that enable a simple and intuitive motion of the telescope. Still others are drive by motors and computers to track objects across the sky and help you point to any of thousands of objects in a database in a handheld computer. But even with motorized tracking, alt-az mounts do not work for long-expsosure astrophotography because the field of view in the eyepiece or camera appear to rotate over time, an effect known as ‘field rotation’.
A more complex mount, one that’s designed to track the motion of the stars by turning on a single axis, is called an equatorial mount. When the “polar axis” of an equatorial mount is aligned to the celestial pole, objects can be tracked with the movement of only the polar axis. Because only one axis needs to be moved, equatorial mounts can be more easily motorized to track celestial objects and keep them in the field of view. And when the object is tracked, the field of view stays in a fixed orientation, which is essential for astrophotography. Of course, to get an object in the field of view in the first place, the telescope must still be moved in both axes.
There are many variations of equatorial mounts; they all tend to be larger and heavier than alt-az designs. The most common is the German equatorial mount, pictured above. The “GEM” holds the telescope in a saddle and balances the weight of the telescope with a set of counterweights. A fork mount, with the tines of the fork aligned with the pole, is another common type of equatorial mount.
An equatorial mount takes a little getting used to, but it’s a powerful tool for moving a telescope about the sky and it’s indispensable for astrophotography.
Go-To and Push-To Mounts
Some telescopes come with motorized alt-azimuth or equatorial mounts with computerized databases, hand-held computers, and motion sensors on each axis to let you automatically point the scope around the sky at the push of a button. You must enter the current date, time, and location, and align the mount to two or three bright stars (which you need to know how to find yourself). Then the computer and mount can point and track thousands of celestial objects. Some “go to” mounts include a GPS module so you don’t have to enter your time and location. A few even let you choose a guided tour of the best celestial sights, complete with a digital readout or audio track describing information about each celestial object. There is a learning curve with a go-to mount simply because you have to set it up and align it to the night sky. But it’s not too hard.
Go-to telescopes are a great convenience, and help you spend more time looking at objects and less time finding them. Go-to’s are a great help for beginners who are often frustrated by finding faint celestial objects. And they are a wonderful tool for city-based astronomers, even experienced astronomers, who struggle to find faint stars to guide them from object to object in murky urban skies.
Most major telescope brands have their own version of go-to mounts and controller/computers. Well-tested and reliable incarnations of go-to controller systems include NexStar (Celestron), SynScan (Skywatcher), Autostar (Meade), and Orion’s version which does not have a fancy name.
Some telescopes, especially Dobsonians, have altazimuth mounts equipped with sensors that detect the movement of each axis, and a computer to help make sense of the movement. But they do not have motors to move the telescope in either axis. The movement must be supplied by the observer, who merely pushes the telescope’s mount to aim towards a particular celestial object. These are called “push-to” or “shove-to” mounts. As with a go-to mount, the user must supply time and location data to a hand-held computer before the observing session, and aim at two or three bright stars. Once the computer knows where and when, it provides a digital readout to the observer to help find the way to a selected object. Because they don’t use motors, push-to mounts are low-cost alternative to go-to mounts for large telescopes like Dobsonians. Push-to telescopes include the Intelliscope series by Orion and other variations.
A word about the wise use of go-to and shove-to mounts…
Go-to and shove-to telescopes are no substitute for learning your own way around the sky. Beginning astronomers who use go-to telescopes to hop from object to object like they’re flipping channels on a television are bound to lose interest in astronomy in just a few nights. The pleasure of amateur astronomy comes not from mindless sightseeing, but from gaining your own understanding, using your imagination, and enjoying the occasional feeling of pride and accomplishment when you find and understand a new sight in the night sky.