Telescope Buyers' FAQ - Part Seven
What is the Best Mount?
Equatorials vs. Altazimuths: the Truth
The various telescope mounting systems available for use by amateur astronomers have been discussed at length on sci.astro.amateur. There has been a great deal of debate, a little ill-informed opinion, and some real misconceptions concerning each of the basic mounting schemes, so perhaps it is time to clear the air.
One basic and irrefutable fact must be stated up front: No mount system is perfect for all situations! Any attempt to champion one mount scheme over another without considering all the facts is doomed to failure. Below are the true advantages and difficulties of the two most popular mounting system.
(i.e.: Dobsonian, etc.). This mounting system has gained considerable popularity over the past 20 years, evolving from the old 'pillar and claw' system originally used only in inexpensive small telescopes, to a modern well-designed one which boasts of supporting some of the largest apertures in amateur astronomy today.
- Simple and stable mounting system requiring no axis counterweights or heavy off-centre concentrations of weight to induce vibration or mount flexure problems. Only two vertical and one horizontal bearings are needed.
- Easy and intuitive mount for beginners to learn on.
- More portable than many equatorial mounting schemes, especially for apertures over eight inches (often faster setup time).
- Easy to build, and often allows simpler mirror-support schemes to be used.
- Lower overall cost, especially for large apertures.
- Unable to track objects with single axis motor drive system. For long term tracking, an altazimuth must be computer dual-axis controlled or supported on an equatorial platform (only 1.25hrs maximum tracking time on such a platform). Cannot track objects directly through the zenith in dual axis driven mode ('Dobson's' hole).
- Lack of fixed field orientation makes star hopping the primary mode of faint object location in non-computerized altazimuth mounts (right-angle sweep and R.A./Dec. setting circles cannot be used). Objects not located in easy-to recognize starfields can be more difficult to find manually.
- Changing altitude and azimuth coordinates can make finding objects more difficult using only altitude and azimuth circles, often requiring computer readouts or nearly continuous manual calculations to keep track of pointing directions for locating objects.
- Field rotation limits photography to short exposures (unless expensive field de-rotators are used). Guiding long exposures can be very difficult, since corrections for drift are sometimes non-intuitive.
These mounts are aligned to the celestial coordinate system and have been the mainstay of serious amateur and professional astronomical telescopes for over a century. They come in a variety of designs: German Equatorial, English Yoke, English Cross-axis, Polar disk, Fork, Split Ring, etc.
- Can use one 'clock-drive' motor to drive the telescope in right ascension for long-term tracking of celestial objects.
- Most equatorial schemes (except for Yoke mount) can reach and track through all areas of the sky.
- No field rotation enables easier long-exposure photography with more intuitive guiding corrections. Also makes planetary observations a bit easier, since the object in the field does not rotate.
- Finding techniques such as the 'right-angle sweep' or star drift method can be used to make locating faint objects easier and faster, even with non-clock driven scopes (only one nearby visible reference star is needed).
- R.A./Dec. Setting circles (both digital and analog) can be used for locating non-visible objects. Digital circle design for equatorial scopes can be simpler, since no real-time guiding calculations need to be performed.
- Good equatorial mountings tend to be bulky and heavy, making them less portable than some altazimuth designs (often have to be broken down into many smaller components for transport).
- German Equatorial mount requires heavy counterweight on a long shaft to make the scope balance. This can be a problem in the dark with people running into them. German Equatorial mounts can also have problems with the scope running into the pedestal for some objects near the zenith, requiring a 'roll over' reversal for continued tracking.
- Moment arms of equatorial mounts tend to allow flexure and vibration to become problems unless the mount is heavily overbuilt. Fork mount tines tend to flex, making for mild tracking errors and periodic lower-frequency 'pogo' oscillation vibration problems with heavy scopes and longer tine length.
- Good Equatorial mounts usually have four bearings, and can often be more expensive than altazimuth mountings (they can still be home built, however).
- Proper polar alignment is necessary for accurate tracking.
- Less intuitive for beginning amateurs, although once the amateur gets used to them, the amateur can often find and track objects faster and more easily than with altazimuth mountings.
Note: none of these disadvantages will eliminate a mount design from use by the amateur. For strictly visual use (especially for the beginner), the altazimuth can easily be recommended, while for long-exposure photography, the equatorial is often the mount of choice. For very large apertures intended for easy portability, the altazimuth almost has to be used.
However, the compact split-ring equatorial design can also remain fairly portable even with telescopes as large as 18 inches. Computers and computerized driving systems have narrowed the choice between the two mounting systems (and driven up their prices), but their basic characteristics have not changed. In any case, both the altazimuth and the equatorial have a firm place in amateur astronomy.
This piece was written by David Knisely.
There have been some people who have built their own Binocular Telescope, this is two telescopes who are the same in every way that are mounted together and are used more or less like a pair of binoculars but are much more powerful. During the last few years a bino-viewer has been made that turns a normal scope into a bino-scope.
Now there is at least one company that is making and selling their own Binocular Telescope. While I have not been able to use such a scope (I don't have that kind of money!), I would think that the images from it would be awesome! They are SanJO Instruments.
What Accessories Will I Need?
In addition to a telescope, you absolutely must have a mounting and a tripod. You will also need a few eyepieces, a telescope with only one eyepiece is like a piano with one key.
These accessories don't come cheap, expect to pay as much for the mounting and tripod as you paid for the optical tube. For a first telescope, you probably will want to buy an entire system, it tends to be less expensive that way.
Which eyepieces should you start with? I'd suggest three or four, maybe a 30mm, 25mm, 20mm, 8mm and a 2x Barlow (which will give you coverage of 30,25, 20, 15, 12.5, 10, 8, and 4 mm). Buy eyepieces of like quality to your telescope. Putting a $300 Nagler eyepiece on a $150 telescope is pointless (it would also probably tip over the entire telescope).
Author: Dennis Bishop