Mount Design

The exterior components of the MI-500, the MI-750, and MI-1000 telescope mounts are aluminum castings. A metal foundry creates complex metal shapes by pouring molten metal (steel, iron, brass, aluminum, etc.) into hollow molds. The molds are made by burying a wood, wax, or metal pattern into sand, plaster, or other suitable material. After the pattern is removed, molten metal is poured into the cavity. When the metal cools, the rigid casting assumes the shape of the original pattern.

Sand castings have been made for well over 6,000 years. The tricks and techniques of the foundry worker are ancient, yet today sand castings are found everywhere in modern life. The engine block in the car you drive is likely an aluminum or iron casting. The often heard complaint "They don't make things the way they used to" usually refers to the fact that many products, once made with strong metal castings, are today made with molded plastic.
The geometry of the MI-500, MI-750, and MI-1000
mounts combines style, strength and function.
 
The MI family of telescope mounts uses dozens of individual patterns to produce the necessary metal components. Each pattern is made using modern machine tools, resulting in castings that are accurate and uniform.

One approach in telescope design is to machine all the necessary parts from extruded tubes, plates, and bars of aluminum. The parts are then assembled with fasteners to create a complete telescope mount. Extruded aluminum has a tensile strength that is about 30% greater than heat treated cast aluminum. A fabricated mount can be strong and relatively lightweight. This type of portable mount, such as the Losmandy G-11 and the Paramount ME, has become popular in recent years.

However, telescope mounts fabricated entirely from extruded aluminum are limited in design flexibility. The mount must be some combination of cylinders, boxes, and plates. Just on aesthetic grounds, the capability of castings and molds to produce shapes of any complexity results in a more visually appealing design.

The MI-500 polar cone pattern
The MI-500 polar cone is made by pouring molten aluminum into a sand mold and then heat treated, producing a 356-T6 casting.
A properly designed pattern allows one to efficiently produce complex parts with the metal distributed in an optimum way. The MI-500 Polar Cone has a wall thickness of varying thickness with internal gussets. The structural areas have a very large cross section of metal. The small strength advantage of extruded metal can not match heat treated castings with critical cross sections that are 2 to 8 times thicker than equivalent fabricated metal parts. These thick cross sections produce rather heavy components, so the MI family of mounts is intended primarily for observatory installations.
The polar cone of each mount features a large north bearing that supports the declination assembly and telescope.
The conical polar and declination castings of the MI German mount feature a large upper bearing with a smaller lower guide bearing at the opposite end of each assembly. The 356-T6 aluminum alloy castings are heat treated and then machined on special mandrels that assure near perfect alignment of the opposing bearing housings. The telescope axes are machined perpendicular with an accuracy of 1 arc minute of angle.

The tapered axes of the telescope feature large slip clutches that support the right ascension and declination drive gears. For easy setup and for telescope balance, the clutches allow manual motion of the telescope. When controlled by computer, the clutches are locked. This clutch design also provides a margin of safety in the event the telescope strikes the pier or other fixed object.
Each axis features a precision worm gear drive with
matching worm running in class-7 ball bearings
Each axis of the telescope has a large, fine pitch, worm gear with matching precision worm. Since the right ascension axis generally experiences more gear wear than the declination axis, for the RA axis we offer a solid bronze worm gear with stainless steel worm. On the declination axis we normally use an aluminum worm gear with a bronze worm.

With over 28 years of experience in manufacturing telescope gears, our drive systems provide high precision tracking and exceptional pointing accuracy. Prior to shipment, the tracking accuracy of each mount is measured. This report is provided to the customer to verify that the periodic error is no more than 5 arc seconds peek to peek error.
Each fork arm is machine on a CNC machining center producing consistent and accurate parts.

The MI-500F, MI-750F, and MI-1000F equatorial fork mounts use the same polar drive assembly as the German configuration. The tapered fork arms feature a box design with thick walls and a large upper bearing housing. Each fork arm is machined on a computer controlled milling machine, assuring perpendicularity of the machined surfaces and uniformity in each fork arm.

Since the arms are detachable from the fork hub, the fork arm separation can be machined to the customer's required dimensions. The fork arms are bolted to the fork hub using stainless steel hardware. Mechanical contact on two surfaces assures accurate alignment of the arms.

A telescope plate in each arm is supported by a pre-loaded bearing assembly. The telescope tube attaches to these fork flange plates. Interfacing a particular telescope tube to the fork requires customer-supplied dimensions and in most cases some custom machining.

One fork arm holds the declination gear housing with a fine pitch worm gear, a worm bearing housing, and a DC servo motor. A slip clutch allows one to balance the tube assembly and make fine adjustment whenever auxiliary equipment is added to the telescope tube assembly.

The MI equatorial fork mounts are ideally suited for compact tube assemblies. The fork configuration offers a minimum "footprint" in the design of an observatory, with the added benefit of continuous tracking when passing through the meridian.
MI-750F Equatorial Fork Mount
 
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Last Updated: Jan 6, 2011

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