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Telescope Mounts

by
Jim Harris
January 1, 2001
Abstract

A guide to telescope mounts from the beginners viewpoint.


This web page represents the research trail I followed while deciding what telescope to buy. I am new to astronomy and not an expert. Nor do my opinions represent the Memphis Astronomical Society. I just collected all the information I used in considering what kind of telescope mount I wanted and made a web page. I hope this information is useful to other new amateur astronomers. To make the page generally useful I made a list of links for quick reference.

When people think about purchasing a telescope they primary think about optical systems. A very important aspect to using a telescope is the mount. The mount refers to both the legs and the mechanism that holds and points the telescope. The mount is critical for how you use and enjoy your telescope. In the old days mounts were just mechnical devices, but in these modern computerized times, mounts are becoming more intelligent. Mounts have computer guidance systems with built-in astronomical databases. There are many ways to mount and use a telescope, and thus many things to consider when choosing a telescope mount.

Altazimuth Mounts

If all you want to do is look at the stars and planets then any mount that holds up your telescope and allows for easy aiming, steady viewing and comfortable use is a good mount. Dobsonian telescopes are a well known design with a mount configured for easy viewing. The Dobsonian is an example of the altazimuth mount combined with a Newtonian telescope optical tube assembly (OTA). An altazimuth mount is a design to hold the OTA and allow the user to guide the scope by using two motions. The up and down motion is call altitude, and the back and forth or round and round motion is call azimuth.

Between these two motions you can point a telescope to view any position above the horizon. It's a simple system and easy to use and lends itself well to casual stargazing.

Equatorial Mounts

Because the earth spins, the stars and planets move in the heavens above us. Tracking a star while viewing it requires constant repositioning of the telescope to compensate for the motion of the earth. A second type of mount called the equatorial mount was design to make tracking match the apparent motion of the stars. The gimmick of this mount is the telescope is aligned along an axis that runs parallel to the earth's spin.

To understand and use the equatorial mount requires a good deal more skill and conceptualization. This knowledge is not required to enjoy using a telescope, or even to use an equatorial telescope because such scopes can be used for casual viewing without mastering all the skills first. The technicque of properly setting up an equatorial mount is called polar alignment. Once configured the equatorial mounted telescope can be aimed with their setting circles using an astronomical coordinate system. Looking at the picture on the right try and imagine these concepts:

Motors along both right ascension and declination will allow a computer to position the scope and track a star. If the computer has a database of steller objects, you can request an object and the mount will look up the object's position and move the telescope to show that object. The newest GOTO telescopes can do all of this without equatorial mounts and polar alignment because the computer can be programmed to know the position of the scope by using guide stars.

If you like learning how things work then buying an equatorial scope and using it properly will force you to learn the concepts of celestrial coordinate systems. If you don't care, then consider buying a GOTO scope.

Equatorial telescopes come with setting circles which allow the user to manually position the telescope by using a coordinate system. It's possible to position the scope using the angular markings on the circles, and then track a target by manually turning the RA knob. Motors on one axis or both axis will allow you to observe without readjusting the scope. This is important because as you get into astronomy, the skill of just watching an object is critical to the skill of observing. Looking at one object for longer periods of time will help your eyes adjust and observe more detail. Constant readjusting the view hinders this process.

Accuracy of tracking will depend on the precision of the mount's mechanisms. With decent motors, good gear machining and precise polar alignment, the viewer can watch his target for several minutes or hours without making any adjustments. With fine precision a camera can be used with the telescope.

Which Kind of Mount to Choose?

Determining which kind of mount to start with is based on a number of factors. This can be confusing so don't let it keep you from enjoying astronomy. Use this information to help with buying a telescope. Also check out Yahoo groups, and if you know how to use Usenet newsgroups read sci.astro.amateur (SAA).

Aiming the Telescope

The first thing you need to decide is whether or not you want to aim the telescope yourself or do you want a computer to aim it for you? The GOTO Telescope is becoming more common, even for the lower priced telescopes. Locating faint objects is very difficult and failure to learn how to find these objects is a common reason why people quit the hobby. But learning star hopping is a major accomplishment and brings great satisfaction.

Most experienced amateur astronomers recommend that newbies start out with binoculars and learn the sky. That's good advice before buying a telescope and mount. The next piece of advice is to go to some observing events and try out various telescopes. Following these two pieces of advice will help in deciding whether you want to do the driving or leave it to a computer.

Comfort of Viewing

If you get into this hobby you might spend long periods of time staring through your telescope. If this requires bending, kneeling, standing on a ladder, sitting on the ground or in a chair, leaning right or left, viewing through a telescope can get hard on the body. Your telescope mount will determine how you position your body to view.

Refractors mounted on tripods can be comfortable to use while sitting in a chair if the object you are viewing is not near the horizon or zenith. If the star you want to watch is at zenith, you will have to get on your knees and even contort yourself to look up sideways. If the object is near the horizon, you will have to stand, bend, or adjust your tripod to sit.

Reflectors which have their eyepieces toward the front of the telescope make looking up easy, as long as the telescope is of similar length to the observer's height. If the scope is huge, an observer will have to use a stool or ladder to view. Smaller scopes require kneeling or bending over to view.

SCTs position the lens at the end of the scope, but because the tubes are short, the eyepieces are generally higher off the ground. Like refractors, the diagonals can be turned to help with viewing.

Stability of Viewing

One of the most important functions of the mount is to keep the image stabile while you view through eyepiece. The telescope will vibrate under weak mounts, especially during focusing, wind or when you touch the telescope. A stable mount will allow for easy focusing or repositioning of the telescope without making you wait for the vibrations to settle down. A bad mount will cause the the telescope to vibrate for many seconds. This is very annoying because you have to wait for the telescope to be still to view again.

Heavier telescopes stress the mount, and this is particularly true with equatorial mounts. A keen advantage of dobsonian mounts is their inherent stability.

Stability is of utmost importance when astrophotograhy is being attempted. If you do want to take pictures, either with cameras or CCDs and make long exposures, then prepare to spend a large amount of money for a sturdy stable mount.

Transportability

Another hugely important factor is how hard is it to take your telescope outside and set it up? Does it fit in a car? Can it be carried on a plane? If you have a 20" dobsonian you may not feel like bringing it out for a quick twenty minute gaze at Jupitor or Mars. Some people like small telescopes that they can keep near the door and just grab and run outside for awhile when the mood strikes them to stargaze.

Some people have a variety of scopes to fit different needs. But if you have just one telescope and you like to use it often, make sure the process of carrying it outside and setting it up won't hinder you from using the telescope.

Manual Target Finding & Tracking

Altazimuth telescopes are basically push and pull. Look in the sky and find what you want to view, then target it with your finder, such as a Telrad and get the target in position. Then view through the eyepiece. Then you nudge the scope along to track.

Equatorial telescopes can be crudely pushed and pulled into position, but if they are polar aligned you can use the manual setting circles to position your scope to get into the general area of where you are supposed to look. This takes skill, and it is more mathematical and scientific than just aiming and nudging. Once at object is found, you can use RA knobs to track your target.

You can add motors to both types of mounts and have your telescope automatically follow your target.

Digital setting circles (DSC) allow a computer to help you manually aim your scope. They appeal to the observer who likes to understand the concepts of astronomical coordinate systems, but have the computer do the actual math. Digital setting circles, once configured, give a RA/Dec readout as you move your telescope around. Thus you look up the coordinates of the object you want and push your telescope around till it reaches those coordinates.

Motorized Target Finding & Tracking

With the newer GOTO telescopes, finding a target is a matter of punching in a number and the telescope does the rest. A simple alignment procedure is required, but this is far easier than polar aligning an equatorial scope. The newest mounts are being aided by GPS navigation tracking.

Once a GOTO telescope finds your target, it will also track it while you are viewing. So if all you like about astronomy is just the viewing, this type of mount is worth considering.

For astrophotography a GOTO mount combined with an autoguiding CCD camera can be used to achieve extremely precise tracking.

Allignment

The goal of astrophotography is to follow a target precisely for long periods of time, up to hours. If the mount is not alligned perfectly, either by the user manually, or by its built in system, photographs of stars will not be pinpoints but streaks. Astronomers will pay great sums of money to get precision of allignment and suburb craftmanship.

Very few amateurs get into astrophotography. It's a very demanding and expensive enterprise. Don't expect your first telescope to need the precision required to take photographs.

Summary

If you are new to this hobby then you should consider something simple, like a dobsonian or a GOTO telescope. I bought one of the new cheap Chinese refractors on an equatorial mount. The mount is barely adequate so I need to decide if I want to spend $175 more on wooden legs for my tripod that will stabilize the mount or consider buying an even more expensive mount. And I quickly learned I don't like constantly repositioning my telescope to track the target, so I'll want to spend $120 for dual-axis motors to add to my mount to automatically track my target. But I knew that before I bought my scope because I had read many reviews about my mount.

After a year's experience I'm not sure if I would make the same purchasing decision. I think I can safely say that if you are new to astronomy be prepared for one of two things to happen:

  1. You will buy a telescope and you won't really use it, or
  2. you will get into the hobby and spend a lot of money.


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