Background and History
The MI-500, MI-750, and MI-1000 family of telescope mounts has a long history dating back over 24 years. In 1978, the Thomas Mathis Company began manufacturing custom gear drives for commercial and custom telescope mounts. These worm gear drives were available in diameters of 7 to 24 inches, and they featured custom machined clutches, precision ground worms, matching ball bearing housings, and a synchronous AC motor.
In 1982, we designed and manufactured the Model 400 and Model 500 fork mounts. The Model 400 was a compact fork suitable for use with the Celestron-11 tube assembly. The Model 500 featured a larger fork, and was originally intended for those wishing to permanently mount their Celestron-14 telescope . However, these fork mounts soon became popular for use with a wide variety of optical tube assemblies.
The Model 500, predecessor of the MI-500, featured thick-walled aluminum castings, a massive conical polar base, precision worm gear drives, and rigid box fork arms. The Model 400 & 500 mounts were used with Celestron tube assemblies, 12 to 16 inch Cassegrain telescopes, and compact 10 to 14 inch Newtonian telescopes.
12 inch worm gear with differential worm assembly
The Model 500 featured a 12-inch diameter right ascension worm gear drive with an integrated gear differential. It used dual motors: a synchronous motor for tracking and a DC motor for slewing. Both axes of the telescope had slip clutches allowing one to manually move the telescope.
Alternately, one could use the variable speed DC motors on each axis to slowly slew the telescope across the sky. The Model 500 included precision engraved setting circles to indicate the telescope's celestial coordinates.
Those were the days when computer controlled telescopes were found only on mountain tops in the major observatories.
In 1986, the U.S. Navy needed a site-survey telescope with a computer controlled drive system . They ordered two Mathis Model 500 fork mounts modified to accommodate stepper motors.
Bill Smith, an electronic engineer and astronomy enthusiast, designed high torque motor drivers that were capable of slewing a telescope at over 12 degrees per second. These telescopes were nearly able to knock people over in the dark, and it became standard practice to shout "stand clear" before slewing to a new object.
These early computerized telescopes included a software interface with a graphical model of the sky. The Apple Macintosh was only a few years old at the time, and Windows 95 was years away. With a computer sky chart, one could click on a planet or a cluster on the monochrome Mac computer screen, and the telescope would slew to the object In 1986 people were amazed at this capability. The popular and affordable Meade LX200 would not make its debut for another 4 years.
Testing a computer-controlled Model 500
with stepper motor drive.
Fremont Peak, CA, Spring 1987
The software interface that controlled the Model 500 fork was called "ComTel". The program was written in 1987 using the Forth programming language, which has its origins in telescope control. For most programmers today, Forth is a strange and ancient tongue. The software was renamed Voyager I in 1988, and soon became a popular astronomy program. The descendent of "ComTel" is sold today by Carina Software as Voyager 4, Dynamic Sky Simulator.
In the early 1980's, Dennis Merrill, a pattern maker and amateur astronomer associated with Chabot Observatory, designed a large fork mount in the style of the Model 500 fork, but capable of handling telescopes of 24-inch aperture. The patterns for the first "Model 1000" mount were completed in 1985. We machined the components for this large mount and then designed a custom 20 inch diameter worm gear drive featuring a 7-inch diameter polar axis. The telescope mount was completed in 1986.
Unfortunately, several aspects of the design of the fork and polar base made the mount difficult to machine and assemble. This large mount never became a commercial product, though it was shown at several telescope conferences in the mid-1980's.
Dennis Merrill also scaled the popular Mathis Model 500 fork mount downward in size to create the "Model 250". This compact mount was intended for use with a Celestron 8 or Meade 8 tube assembly. It featured a rigid box arm fork and a precision 8 inch diameter worm drive. In 1990, we designed a German declination assembly for the Model 250 polar base, so the mount could be used as a German equatorial or as an equatorial fork.
Mountain Instruments licensed the original "Model 250". Larry Myers of Mountain Instruments customized several components of the original design, creating one of the finest portable mounts that is commercially available. Mountain Instruments now sells the popular MI-250 German equatorial with the Gemini Computer Control.
MI-500 Polar Cone Raw Casting
In 1994, we evaluated the various telescope mounts that had evolved over the previous 16 years. The original "Model 500" was redesigned with computer control in mind. With the growing popularity of apochromatic refractors, the new MI-500 would feature both a German equatorial and an equatorial fork configuration.
Unlike the hand-crafted patterns of the old design, the pattern components of the new mount were accurately machined on precision lathes and computer controlled milling machines. The resulting aluminum castings are accurate and uniform in the dimensions.
The new MI-500 German Equatorial (MI-500G) and equatorial fork (MI-500F) feature thick-walled castings with large metal cross sections, a 12-inch worm gear drive , and a 10-inch declination gear. The new MI-500 is a heavy mount (about 200 pounds) that we describe as "moveable", but not "portable". It is intended primarily for permanent installation.
MI-500G German Declination Raw Casting
Owners of original "Model 500" had for some years pushed the capabilities of the mount with heavy 16-inch and even 18-inch telescopes. For best performance, these telescopes should be used with a larger mount with a greater load capacity.
With this in mind, in 1997 we designed and built the MI-750, which is about 35% larger than the MI-500. Weighing nearly 300 pounds, it can handle an 8-inch to 10-inch refractor, or a 16 to 20-inch Cassegrain/Newtonian telescope. This size of this instrument obviously requires a fixed site..
The MI-750 is available in a German equatorial configuration (MI-750G) and an equatorial fork configuration (MI-750F). The MI-750 features a 15 inch RA drive gear and a 12 inch gear on the declination axis. Each axis features a slip clutch for easy setup and balance. The 2-inch diameter stainless steel counter-weight shaft (MI-750G) uses threaded counter weights.
The MI-750 is ideal for a small college observatory or a private observing site. The capacity of the mount is much greater than smaller, semi-portable, commercial mounts. The MI-750G readily handles large refractors, and the MI-750F is designed for 16-inch to 20-inch Cassegrain telescopes.
The New MI-750G German Equatorial
The new MI-1000 polar cone casting and base (unpainted)
shown with the MI-500 polar assembly
With the completion of the MI-750, in 2002 a new version of the MI-1000 mount was proposed for telescopes of 20 to 28-inch aperture. This would likely be the largest and final member of our family of equatorial mounts. Using the original "Model 1000" (circa 1984) as a starting point, new polar, fork, and declination assemblies were designed.
Over an 8 month period the enormous patterns and core boxes for the new mount were slowly fabricated. This work was completed in late 2003, and the major components of the MI-1000 were successfully cast.
Testing the MI-1000G German equatorial mount
The new MI-1000 weighs approximately 500 pounds, and it has about twice the load capacity of the MI-750. Like the smaller models, the MI-1000 is available in both a German equatorial (MI-1000G) and an equatorial fork (MI-1000F) configuration.
The MI-1000 is somewhat larger than the original Model 1000. With a 20-inch bronze RA drive gear and a 15-inch declination gear, the MI-1000 can easily handle a 350 pound (160 Kg.) payload. The German MI-1000G is capable of carrying as much as a 500 pound (225 Kg.) optical tube assembly.
This mount is at the practical upper limit of a telescope mount built using traditional patterns and aluminum castings. Larger custom mounts are usually constructed using fabricated components of steel or aluminum.
Last Updated: Jan.5, 2011
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