Mathis Instruments MI-1250 Equatorial Fork Mount
The equatorial fork mount is excellent at holding compact tube assemblies. Unlike the German mounting, a fork mount tracks objects continuously through the meridian, i.e. no German roll-over. These mounts are often used with Cassegrain optical systems. Generally, fork mounts provide easy access to the telescope focal plane, which is usually a short distance below the axis passing through the upper ends of the fork arms.
The Mathis Instruments MI-1250 Equatorial Fork Mount offers a minimum “footprint” in the design of an observatory, with the added benefit of continuous tracking across the sky. Since fork mounts do not need the large counter-weights of the German mount, fork mounted instruments are usually more compact. Forks do use a small counter-weight on one fork arm to balance the declination gear drive on the opposing arm.
However, fork mounts have a smaller load capacity than the equivalent German mount, since the telescope tube is suspended far from the center of the mount column or pier. Also, fork mounts are machined for a specific arm separation, making them less versatile at handling a variety of telescope equipment. The short length of the fork arms does not accommodate the long tubes of refractors and Newtonian telescopes.
The MI family of telescope mounts includes the following major features:
- Distinctive conical geometry combining style, strength, and function
- A minimum of machined parts with no protruding motors or brackets
- Large drive gears on both axes with spring loaded worm assemblies
- Minimal visible hardware (i.e. nuts and bolts)
- Homing disk on each axis for remote operation
- Large shielded ball bearings supporting the telescope axes
- Sliding rocker-style base for latitude adjustment
- Slip clutches on both axes for easy setup
- Stainless steel counter-weights with threaded counter-weight shaft (German).
- Cover plates that shield the drive gears and servo motors
- A choice of computer controls – Servo II, AP GTO3, Bisque TCS
Each mount includes a polar cone fitted with a large upper bearing and a lower guide bearing. These pre-loaded bearings support the polar axis. The polar cone is attached to the rocker-base, which in turn is attached to the black anodized base plate. The base plate is bolted to the top of the observatory pier or column.
The contact area between polar cone and the rocker-base is concave-convex surface whose center is near the top face of the north bearing. As one slides the polar cone across the rocker-base, nearly full contact with the rocker-base is maintained. The center of mass of the polar assembly remains near the center line of the pier.
For polar alignment, each equatorial mount has adjustments for accurately aligning the polar axis to the celestial pole. Along the lower edge of the rocker-base are three large recessed setscrews. One setscrew is located at the back surface of rocker-base. Turning this setscrew slides the cone on the concave rocker-base increasing or decreasing the altitude of the mount. With this one setscrew, one can adjust the altitude of the mount to the required latitude of the observatory.
A pair of stainless steel setscrews on the east and west sides of the polar assembly allows one to rotate the entire mount on the base plate. With this pair of push-pull setscrews, one can adjust the azimuth of the mount as needed.
The mount should be installed on the observatory pier or column within a few degrees of true north (or south). Using the azimuth and altitude adjustments, one can get within a few arc minutes of the pole on the first night. For a permanently mounted mount, very precise polar alignment is an iterative process extending over several observing sessions.
The polar cone encases the right ascension axis, worm drive gea, and slip clutch. The drive gear and clutch are located near the top of the upper bearing. The slip clutch allows manual motion of the telescope for initial setup and for adjusting the telescope balance. When controlled by computer, the clutch is locked. This clutch design also provides a margin of safety in the event the telescope strikes the pier or other fixed object.
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.
The RA worm gear is paired with a precision worm that runs in class 7 ball bearings. The spring loaded worm bearing assembly allows fine adjustment of the worm to worm gear tension to minimize backlash in the gear train. The worm assembly and servo motors are inside a protective casing . No motors or brackets protrude from the mount.
The equatorial fork configuration consists of a pair of tapered fork arms and a central hub. The thick-walled arms are cast of 356 heat trated aluminum using a hollow box design. Each fork arm is machined on computer controlled milling machines, assuring perpendicularity of the machined surfaces and uniformity in each set of fork arms. The declination axis of the assembled fork is perpendicular to the right ascension axis with an accuracy of 1 arc minute of angle or better.
Since the arms are detachable from the fork hub, the fork arm separation can be machined to the customer’s required dimensions by customizing the dimensions of the fork hub. The fork arms are bolted to the central hub using stainless steel hardware. Mechanical contact on the two bottom surfaces of each arm assures alignment and rigidity of the fork assembly. The central hub with the attached fork arms is bolted to the top face of the polar axis.
A pair of pre-loaded bearings in each arm supports each flange plate. The telescope tube attaches to the flange plate using dovetail plates. Interfacing a particular telescope tube to the fork requires customer-supplied dimensions, and in most case some custom machining will be required
One fork arm (usually the east arm) includes the declination gear housing with a fine pitch worm gear, a worm bearing housing, and a DC servomotor. A slip clutch allows one to balance the tube assembly and make fine adjustment whenever auxiliary equipment is added to the telescope tube assembly.
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.
Our fork mounts are available in several sizes. The compact MI-500F is suitable for payloads up to 120 pounds. This would includes telescopes such as Celestron 14 and Meade 14 Schmidet Cassedgrain telescopes, PlaneWave CDK12 and CDK14, and custom astrographs of 10 to 16 inch aperture.
The MI-750F is designed for payloads up to about 180 pounds. This would includes telescopes such as the PlaneWave CDK17 and CDK20, RC Optical 16 and 20 telescopes, Meade 16 Schmidt Cassedgrain telescopes, and custom imaging packages.
The MI-1000F is suitable for payloads up to about 300 pounds. This would includes telescopes such as the PlaneWave CDK24, and RC Optical 24 inch.
We also offer hybid fork model. This includes the MI500 polar base paired a MI-750 fork assembly. Using the larger fork assembly allows larger telescopes in the fork assembly, with restriction that the payload is 150 pound or less. A similar model is the MI-750 polar base with MI-1000 fork assembly.
Our largest fork mount is the MI-1000/1250. This features the MI-1000 polar base paired with our MI-1250 fork assembly. This mount is designed for telescopes such as the PlaneWave CDK24, and RC Optical 24-inch.
As an option, high resolution Renishaw encoders are now available for our fork mounts. Using the Servo II control, a precision encoder rings is installed inside the polar cone and on the fork arm opposite the declination drive. An readhead records the motion of the telescope with sub-arcsecond resolution. Independent of the drive gears and servo motor encoders, the tracking and pointing accuracy to the mount is enhanced using the Renishaw encoders. Tracking errors are reduced to a fraction of an arcsecond, and the pointing errors of the mount are measured in arc seconds over the entire sky.
The large 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 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 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.
A common 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 instrument. Extruded aluminum has a tensile strength that is about 20% greater than heat treated cast aluminum. Fabricated mounts are usually rigid and relatively lightweight.
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 small strength advantage of extruded metal can not match heat treated castings with critical cross sections that are 2 to 4 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.
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.
Newly cast parts are heat treated by placing them in an industrial oven at 400 degrees for about 12 hours. The parts are allowed to cool slowly after which they are more stable and markedly stronger. Our 356-T6 heat treated castings are then put in storage for three to six months where they age. This further increases the strength of the castings and improves the machining properties. This process, in particular, benefits our larger castings, including fork arms, declination assemblies, and polar cones.
Our various mounts are very simllar in appearance. This is because the patterns from which they are made were mostly scaled using the original MI-500 mount. A recent addition to our fork family is the MI-1250 fork, intended for use with 24-inch telescopes. This large fork arm was designed by scaling the MI-750 fork pattern by a factor of 1.5. Similarly, the MI-1000 declination assembly is a copy of the MI-500 scaled by 1.6.
These cast parts are the basic components of our telescope mounts. Each component must be accurately machined, finished, and painted before final assembly.