Frequently Asked Questions
What power supply is necessary for computer control?
We recommend a variable voltage DC power supply. The supply should provide a minimum of 5 amps output. The Servo II can be used with any voltage in the range of 12 to 24 volts. It works very well at 20 volts.
For the GTO3 control, use a 12-18 volts power supply. Higher voltage increases the motor torque, which is needed particularly for large mounts with a heavy payload. The TCS control includes it own 48 volt power supply.
How do I maintain a balance of the fork arm?
By their nature, fork mounts do not require the large counter-weights used on Newtonian telescope mounts. However, the large worm gear, gear casing, and servo motor mounted on one arm (usually the west arm) requires a counter weight on the opposite (east) arm. This weight is typically 20-30 pounds.
To balance the arms, we install a steel bar inside the fork arm opposite the declination drive. This provides about 70% of the weight needed to balance the arms. An additional weight is added on the outside near the top of the arm.
Adding small equipment to the tube assembly will not effect the balance significantly. The MI-750 fork is a 320+ pound mount, so a few pounds has little impact. However, if one is adding 10 pounds or more, you should use the fork dovetailplates or brackets to shift the telescope tube to re-balance.
I have C14 on a MI-500 fork mount. How do I polar align the mount?
The polar base of our equatorial mounts consist of a concave rocker-base and a convex rocker. The mount is supported across this broad surface. The rocker component can be slid by about 4 degrees to the north and to the south, while maintaining nearly full contact with the rocker-base.
There are no knobs or handles that you turn to change polar alignment. Rather, there are three stainless steel set screws recessed in the polar base. For azimuth adjustment, there are screws on the east and west side of the mount. For altitude adjustment there is a set screw on the south end (north end for southern hemisphere) of the base.
Using a 5/16 hex wrench, turn the azimuth or altitude set screws. One turn of altitude screw is 20 arc minutes. One turn of either of the azimuth screws is 50 arc minutes. It is easy to make a small fraction of a turn, so 10 to 20 arc seconds adjustments are possible. Remove the set screw wrench when done and put it away. As a fixed installation, you may occasionally need to make adjustments to the polar aligment.
It is probably reasonable to get within 30 arc seconds of the pole, but better than this is difficult. Your polar axis has class 5 bearings. There is some small runout in these bearings and some clearance in the diameter of the RA axis. This means that near the pole your mount generates an egg shaped oval on the sky, whose axes will vary by 10-15 arc seconds or more. Variation in barometric pressure and temperature will move the position of the celestial pole by another 10-20 arc seconds.
These factors and several others make mount modeling such a great solution. Get the polar alignment close, and then model the residual errors.
What is the payload capacity of your various mounts?
Our stated load capacities are meant as a general guideline of limiting the weight of the optical tube assembly for the various models of our mounts. The numbers we provide are conservative.
You could put 330 pounds, 150 kilograms, on an MI-500 German mount. Nothing will break, but there will be considerable load on the servo motors, the clutches will be stressed, and the overall rigidity of the mount will be compromised.
The exact point when the load on the mount is excessive is a gray line. If you keep the load well under the stated limit, the overall performance of the telescope and mount will be excellent. As the load increases past the recommended limits, the stability of the instrument declines until the performance becomes unacceptable.
In the case of fork mounts, another limiting factor is the fork arm separation. For example, the MI-500 fork can accommodate a 16-inch Cassegrain as long as the optical tube assembly is no more than about 20 to 21 inches in diameter. This requires that the dimensions of the optical tube assembly are no larger than the maximum arm separation. This is not an issue for German mounts.
An additional factor in matching an optical tube assembly and a telescope mount is the visual appearance. It is apparent when a telescope is too large for a specific mount - the optical tube assembly dwarfs the mount. As an example, look at any modest size German mount carrying a 20-inch telescope. Performance aside, the telescope overwhelms the mounting.
The telescope and the mount should be of the same proportions. Clearly this is a judgment call. But if a mount seems too small for a specific optical tube assembly, it probably is. As a general rule, we recommend that the telescope be lighter than the mount (German) that holds it. For a fork mount, the telescope should be no more than about 50% the weight of the mount. Also, an old telescope-making rule states thatthe polar drive gear should be approximately the same size as the aperture of the telescope. This still is a good guide.
How can I get my fork mount to be near the center of the observatory?
In the northern hemisphere, a fork mount leans northward, offsetting the telescope about a meter from the center of the pier. If you have limited space in the dome, a 12 to 24 inch offset of the mount to the south will put the telescope near the center of the dome. We can provide an offset base plate, customized to center the telescope over the pier No extra parts are required if the offset is machined as part of the base plate. This is a common request for those with fork mounts in a small dome.
I have a custom buit 18-inch Cassegrain telescope. Can it be mounted in your MI-750 fork ?
Each fork has a central hub. The fork arms are bolted to this hub. We need to know the diameter of your telescope and thickness of the plates used to secure the telescope to the fork arm. We will then machine the fork hub to match the dimension of your telescope. The MI-750 fork can accommodate telescopes of 16 to 20-inch aperture.
The MI-750 fork hub allows for an arm separation of about 28 inches, which is the upper limit for the MI-750 fork. Ideally, the OTA slides into the fork using dovetail plates.
What color are the MI mounts?
Prior to 2010, we used a light shade of gray. This had been in use since 1984. In 2010, we changed the mount color to an off-white with a subtle blue hue. Each mount is shipped with a small container of touch-up paint, which allows the owner to repair scuffs and dings in the painted mount.
What size pier do you recommend?
This will depend on the size and weight of your telescope. In general, we suggest a 8 to 16-inch (200 cm to 400 cm) steel column bolted to the floor of the observatory. A local fabrication shop can provide a welded steel column suitable for your observatory. Alternately, a concreted pier can be poured into the floor near the center of the building.
You should adjust the column so that the top surface is level within 1 degree or better. We provide an aluminum base plate that bolts to the top of your column using a compatible bolt circle. If you have exsiting holes in your pier, send us a diagram showing the hole positions, and we will drill matching holes in the base plate.
Can your fork mounts be used for portable use?
In general, it not practical to transport even our smaller mounts. The MI-500 weighs about 250 pound, and it is design for installation at a fixed location. Over the past three decades, we have had a few customers who transported their mounts to remote locations, often for site surverys. It seems likely the mount was transported by trailer.
What motors do you use for the different computer controls?
For the Servo II control, we use Pittman 9000 series motors. For the MI-1000 mount, we use the larger Pittman 14000 servo motor.
For the GTO3 control, we use the A-max 32, a 20 watt DC servo motors. This is the largest inthe A-max series, a family of motors manufactured by the Swiss company Maxon.
For the TCS control, we use brushless Pittman motors. The motors and control are provided as part of the TCS package.
How would you attach the MI-1000 mount to my observatory pier?
The MI-1000 mount has a 15 x 21 x 1.5 inch base plate. The base plate is attached to you observatory pier, and it has a bolt pattern consisting of six holes of 1/2 inch diameter. If the top of your pier has no bolt holes, add holes that match our base plate holes. If you have existing holes, we can match these holes in the base plate. Please send us a diagram of your pier, and we will recommend the best approach.
What is the accuracy of your gear drives?
We have found over the years that a major concern of those purchasing our mounts is “what is the tracking accuracy”. The gold standard is 5 arc second or less peak to peak error.
With these considerations, we spend much time on the worm assemblies, in particular the worm, spur gears and belt pulleys. If your goal is an error of 5 arc seconds or less, the worm can have run-out of no more than about ±.0001 inches/2.5 microns for a 12-inch gear. We polish the worm shaft to get the diameter within a few tenths (.0001) of the optimum diameter. We use class 7 shielded bearings in the worm journals. The precision gears and pulleys that connect the motor shaft to the worm shaft are machined using finest toolroom lathes.
Larger drive gears will generally produce better tracking, since linear worm errors are smaller angles on larger radius. Our 12-drives have an accuracy of 5 arc seconds peak to peak error. The 15-inch and 20-inch drives have errors general less than 5 arc seconds. Our spring loaded worm plate reduces the backlash to less than 10 arc seconds. With a heavier spring load, the backlash is near zero.
A secondary consideration is pointing accuracy. Most users spend a few night tweaking the polar alignment to within a few arc minutes of the celestial pole. At that point, the mount will put an object on the chip of their CD camera, and probably very near the center. They frame the object as needed and then image. Others will use software to model the mount errors and optimize the pointing.
From user reports, 3-4 arc minutes is easily achieved, and with very careful polar alignment and a well-made optical tube assembly, the mounts should achieve 1-2 arc pointing with no modeling.
We base this on the machining tolerances of the various mount components. For example, the fork arm castings and the declination castings are machined to few thousands of an inch over a length of about 25 inches. As measured with a height guage, the declination assembly (German or fork) is typically orthogonal to the RA axis with about 30 arc seconds, 1 arc minute worst case. Achieving pointing accuracy better than one arc minute requires modeling, which can will reduce pointing errors to 30 arc seconds or less.
What do you recommend for lubricating the worm gear?
The grease we use is Molybdenum Disulfide (MoS2), usually shortened to "moly". It a good general purpose grease. We send a small container of extra grease with each mount we ship. In general, most any common grease used in an automobile would works fine. The drive gears on a telescope mount rotate so slowly that there is no heat generated and very little friction.
To grease and clean the gear, you loosen the clutch and run the servo motor at a modest speed. This rotates the gear so that one can remove any grease from the gear teeth and apply new grease. As the gear rotates, use an old toothbrush to clean the teeth. Use another to apply new grease.
You should examine the gear teeth at least once a year. The grease tends to migrate to the lower and upper edges of the teeth. With an old toothbrush, you can brush the grease so that it is mostly on the gear teeth, not on the gear teeth edges.
If the grease on the gear is old (4-5 years) and seems somewhat hard, use a clean brush with a small amount of solvent to clean the teeth removing most of the old grease. Apply new grease to each tooth as the gear rotates.
What is the capacity of the of 500 mount?
The MI-500F is our smallest equatorial fork mount with a variable arm separation of 14-21 inches and a load capacity of about 150 pounds. The MI-500 fork mount will accommodate instruments of 12 to 16 inch aperture. The "swing through" of the fork is about 17-inches. There is a central pocket in the fork hub that adds about 1.5-inches to the "swing through".
You need to measure the central balance point of the intended OTA to determine the distance from this balance point to the end of the mirror cell. The remaining space would be used by your equipment.
There is the option of adding some weight to the back of the OTA, to shift the balance point and thus add additional space for equipment. For even more "swing through" use the MI-500 polar base with the larger MI-750 fork. This is our MI-500/750 fork mount.
How do the TCS & GTO3 controls compare?
For use with the observer near the telescope, the Astro-Physic GTO3 control is a good choice. The GTO3 includes a very capable hand pad, which gives immediate access to planets and Messier objects. This control uses Swiss made Maxon servo motors and includes quality bayonet-style cables. The GTO3 can be used with MI-500 and MI-750 mounts.
The Bisque TCS control is best suited for robotic control of the the telescope from a remote location. It features brushless servo motors, built in homing sensors, and periodic error correction. The TCS package includes a complete set of software for imaging, control, and mount modeling. There is usually some delay in our obtaining the TCS control, so please allow an extra 6-8 weeks of time for delivery of the mount with a TCS control.
The prices of the GTO3 ($1500) and TCS ($5000) are significant. Four years ago we started looking for a control with the some of best features of the GTO3 and the TCS, and ready to use out of the box.
We have been using the Servo II as our standard control for about 3 years. It is designed as a stand alone product, suitable for use with a wide range of telescope mounts. It supports 4 amps per axis and works well with our MI-500, MI-750, and MI-1000 mounts. It has a functional setup utility to configure any parameter of the control. Support for high resolution mount encoders, home sensors, PEC sensor, and limit switches are all built into the control electronics. With this range of features, nothing we have seen compares.
With the Servo II, we provide very high quality Turck cables and connectors. To suppress signal noise, we use separate cables for the motors and encoders/sensors on each axis. This is common on any high performance servo system.
The Servo II can use any 12-24 volt power supply with about 5 amps output. A variable voltage supply is recommended, since you can tune the motor speeds for best result; we have found 18-20 volts work very well.
The Servo II is ASCOM compatible so it can be used readily with the The Sky and other PC software. The only significant downside to Servo II is the hand control; it is a rather simple plastic box with NSEW buttons – nothing fancy.
What are the advantages of an altazimuth MI-750 or MI-1000 mount?
Our MI-750 and MI-1000 equatorial fork mounts are designed for payloads up to about 200 and 300 pounds respectively. In an alt-azimuth configuration, with the fork and optical system directly over the supporting pier, these mounts can handle 50% to 100% larger payloads. These fork mounts are usually paired with RC Optical 16-inch, 20-inch or 24-inch telescopes and the PlaneWave CDK17, CDK20, and CDK24 inch astro-graphs.
We have built several altazimuth fork mounts over the past 36-months. The altazimuth fork assembly is identical to the equatorial fork. The polar cone is replaced with a horizontal bearing assembly. We use the same RA drive gear and gear casing on the azimuth axis.
The major benefit of the altazimuth fork mount is that the fork remains vertical. In an equatorial configuration, the entire fork assembly is tilted to match the latitude of the observer. However, for astronomical imaging, the altazimuth fork has an image plane that rotates as you track; this requires a third axis at the eyepiece to compensate for this rotation.
How can I attach a Meade 14 or Celestron 14 to your MI-500 fork?
The easiest way to mount a Meade 14 or Celestron 14 telescope is to use Losmandy style dovetail plates that attach to the sides of the OTA. We can provide matching female dovetails on the fork arms, and we will machine the fork hub to the dimensions of your dovetail-equipped telescope. The optical assembly will then slide into the fork.
An alternate approach is to use tube rings. Mount your Meade or Celestron telescope in the tube rings. Tube rings have plates on opposite sides of the telescope tube. The plates can then be bolted to the fork arms. The dovetail approach is the easier solution.
What size fork is suitable for the PlaneWave CDK astrographs?
The MI-750 fork mount is used with the PlaneWave CDK20 and CDK17 telescopes. For the CDK17, which weighs about 100 pounds, one can use the smaller MI-500 polar base with the larger MI-750 fork. For the 160 pound PlaneWave CDK20, the MI-750 polar base and fork are needed.
The MI-750 mount has 70% more capacity than the MI-500, and the larger fork arms give more “swing through”. This provides room for a wider range of equipment to be attached near the focal plane. You need to determine the balance point of the your OTA to determine the distance from the balance point to the rear of the telescope tube. The remaining space is available for your equipment.
What dovetail plates do offer?
The PlaneWave telescopes have dovetail plates attached the CDK tube. The plates are unique to the CDK telescopes. We can provide a pair of matching female dovetails. With this setup, your slide the CDK telescope into the fork and adjust the position to achieve a proper balance. Our female dovetail plates can be used with CDK17, CDK20, and CDK24 telescopes.
We also supply Losmandy-style dovetails for telescope such as the Meade 14, Meade 16, the Celestron 14, and the new CDK14 telescope.
Can Renishaw encoders be added to an existing MI-750 mount?
Renishaw encoders provide .10 arc second resolution on each axis, but they must be integrated into the mount during construction. The components that support the encoder rings are made of thick stainless steel disks, which provide a thermal barrier between the encoder ring and the aluminum components in the mount. The Servo II is the only current control that supports this feature.
Since the encoder rings and read-heads are built into each axis of the mount, to install these on an existing mount, the mount must be returned to our shop. The right ascension and declination axes can be machined to accommodate encoder rings, and the read-head can be added to each axis.