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The Traditional Symmetrical Equatorial Mounting

At the beginning of the twentieth century, the American genius George Willis Ritchey electrified the world of astronomy by re-inventing the reflecting telescope. His first great masterpiece, the sixty-inch reflector at Mt. Wilson, was a form of symmetrical equatorial mounting today known as a "fork" mounting. After Ritchey demonstrated the usefulness and versatility of this instrument, no one ever again seriously contemplated the construction of very large refractors.

Other large, reasonably successful fork mountings for reflectors had been constructed before, notably that of A. A. Common, another reflector of sixty inches' aperture. Common's sixty inch reflector was a commendable pioneering effort, and he got everything "almost right" with his design; Ritchey, however, got everything exactly right with his design for the Mt. Wilson sixty inch reflector. With Ritchey's sixty inch reflector, the modern fork mounted reflecting telescope suddenly "came of age", so to speak, almost overnight.

Nowadays the computer driven altitude-azimuth mounting has replaced the equatorial mounting for very large and extremely large reflectors because of the enormous savings in cost of construction compared to a comparable large symmetrical equatorial mounting. For reflectors between about .5 and 1.5 metres' aperture, however, there are essentially no significant engineering and construction cost advantages for mounting the instrument as an altitude-azimuth unit. In fact, for the most part, there are only disadvantages for the altitude-azimuth mounting for instruments in this size range for the Cassegrain optical configuration. Dr. Frank Melsheimer of DFM Engineering (America's premier manufacturer of fork equatorially mounted Cassegrain reflectors in this size range) has duly noted this fact in detail in his company's advertising literature. Dr. Melsheimer has delivered fork mounted research instruments to observatories worldwide.

The modern altitude-azimuth mounting as constructed by today's amateur telescope makers has advantages that make it particularly attractive to those amateurs dedicated principally to visual observation of faint "deep sky" objects at relatively low magnifications under dark skies. This very popular mounting is usually referred to as a "Dobsonian" mounting, after John Dobson, who popularized its use among amateur telescope makers. The simple design of the Dobsonian mounting makes it both easy and inexpensive to construct. Large and very large Dobsonian mountings made to the time perfected formula as used by today's amateurs are also very transportable, being designed to break down into easily manageable components quickly and conveniently.

However, all of these advantages come at the expense of trading off certain advantages that the traditional, symmetrical equatorial mounting offers. As an old English aphorism has it, "You can't keep your cake and eat it, too." In my original draft of this article, I went into considerable detail explaining point by point the advantages and disadvantages of the altitude-azimuth versus the equatorial mounting. However, inasmuch as my intent is principally to develop an awareness of the wonderful utility of the symmetrical equatorial mounting as it has been established by a long history of usage, I will only make a quick, general summary of the disadvantages of the altitude-azimuth mounting. For those of you wanting a more exhaustive explanation of the advantages/disadvantages of the two types of mountings by comparison, I invite you to read Dr. Frank Melsheimer's articles on the subject. Dr. Frank Melsheimer of DFM Engineering is one of America's foremost authorities on the design and engineering architecture of large observatory telescopes. His firm has been designing, building, and delivering large reflectors to observatories worldwide for nearly two decades now. See his articles, "Alt-Az Myths", and "Equatorial Mount (compared to Alt-Az)" by clicking on the picture of his company's standard design for middle sized observatory Cassegrain reflectors here in this paragraph.


Here is a brief list of the major disadvantages of the altitude-azimuth (Dobsonian) mounted reflecting telescope:

(1): Solutions to enable alt-az tracking in Right Ascension are limited and imperfect.

(2): Accessing and tracking through near zenith regions is very difficult.

(3): For Alt-Az mounted Cassegrains, space at the tailpiece for mounting instrumentation is limited and access to those instruments is limited when the telescope is aimed at near zenith regions.

(4): Computer synthesized tracking in Right Ascension results in field of view rotation.


All of these difficulties are avoided with the true equatorial mounting. And a modern symmetrical equatorial mounting, particularly a fork or split ring mounting, can be made very nearly as transportable as the popular Dobsonian mount. The advantages of being able to track in right ascension for long periods of time without any field rotation are considerable.

For those of you who might be thinking of building a fork mounting for your large Newtonian (or Cassegrain) optics, we have an unusual offer of free assistance that will save you an enormous amount of time and work.

Our last project here at Great Plains Instruments, before we ceased to be a commercial enterprise, was the design and construction of the prototype of our intended kit-build large fork equatorial mounting for reflector optics up to twenty inches' aperture.

Anticipating the need to be able to cut out parts for this standard design in volume, we produced precision cut parts templates for each part of our new, large fork mounting. These templates are essentially patterns for quickly outlining the parts to be cut out for building the mounting. Each template is laid on top of a sheet of the chosen construction material and its outline is quickly traced off onto the material by drawing a pencil around its rim (or an inscribing awl, for metal work). Each template also has accurately located apertures for tracing off parts' joint intersection lines and screw hole locations.

These parts templates are made to a very high degree of accuracy so that the parts will fit together precisely and yield a finished instrument with high fidelity to its design geometry. The templates for this mounting required the better part of a thousand hours' worth of tedious, painstaking, perfection-seeking work to produce. The result is that for this design the tedious and very time intensive drudgery of laying out the parts accurately is now done once and for all instead of for each example to be produced: one may quickly trace off the outlines of all of the parts for each new instrument to be built.

Our prototype is very nearly complete and is turning out very well. This large mounting is not expensive or difficult to build, and has some pretty neat features:

(1): Unit is transportable by one able-bodied person.

(2): Unit is fully latitude adjustable- a handy feature for those who frequent star parties at widely differing locations.

(3): Unit is very easy to equip with one or the other of our two very easy to build Right Ascension drives. Each of these drives is both easy and inexpensive to build, and will perform as well as the costliest commercially made worm gear Right Ascension drives.

And so, then, we come to our unusual offer to any and all of you who might like to build this mounting for your optics between sixteen to twenty inches' aperture: we invite you to use our parts templates to lay out your parts for cutting out to build this mounting, at absolutely no charge to you whatsoever!

The mounting is imminently customizable at the stage of parts production. The unit you see pictured throughout our website is equipped with a square cross-section Warren truss tube, as it is intended to carry Cassegrain optics of up to eighteen inches' aperture. However, those of you who would be mounting Newtonian optics would probably prefer our traditional octagonal cross-section tube, which allows repositioning of the focuser to any one of the eight sides of the tube as needed for different aiming orientations of the instrument (Great Plains Instruments' standard solution for repositioning the eyepiece, in lieu of rotating the entire tube or an upper end cage). The white 12.5" Newtonian reflector you see pictured throughout our site in the various articles has our standard, octagonal style tube with repositionable focuser. We have very precisely made templates for octagonal tubes for 12.5", 13.1", 14.25", 16", 17.5", and 20" aperture optics.

I would be delighted to lend these templates to any one of my readers who would like to build this new, large mounting, and nothing would give me more pleasure than to spend time helping you lay out your parts for cutting out. I have, in addition, a large number of photographs documenting the assembly steps for building the mounting that would be available to you also. There is an excellent hotel almost directly across the street from my residence (its lights ruinous for deep sky observing, here!) and the hospitality of my home would naturally be available to you. A small rental trailer would suffice to transport your construction materials home (I recommend AC plywood in the three different thicknesses required for the mounting) after you've accurately traced off all parts outlines onto them. The #1638DC and #1621DC precision ground ball bearings are also available locally at reasonable prices, as well as other resources, including a vendor who is happy to provide the 4.5" diameter "black pipe" cut to length for machining into your polar axle.

Building my traditional design observatory telescope has been a wonderful adventure for me. Using it will be an even more wonderful adventure. If you'd like to have this kind of adventure, I stand ready to give you every kind of assistance (at no cost whatsoever) that you would require to accomplish it. Please correspond with me via email with any inquiries you may have about coming to our residence to launch your project to build an example of our new, large, traditional symmetrical equatorial mounting for your optics between sixteen to twenty inches' aperture. My time is mostly my own-I would be delighted to share it with you to help you with an exciting adventure in astronomy! Please contact me via email with your inquiries.


© 2001 David Anthony Harbour


Contact Dave
via E-mail

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