Thoughts About Telescope Mounts - Doc G - November 1997
Ut date to the New Mount project as of April 1999, goto New Mount Progress
I have had very good viewing time with both of my LX200 telescopes. The 12" f10 has an excellent optical system and the 10" f 6.3 has what can fairly be called a superior optical system. From the beginning, using the GOTO feature of the telescope has been a joy. For viewing I still use it and have generally been pleased with its uncanny accuracy. I have had to repair both of the declination drives but after the repair found the drives to work well. I have not been as happy with them when using them for imaging. They have tended to be mechanically fragile and have needed constant adjustment to keep them working well enough for imaging. I have felt that the strength and rigidity of the mounts prevents high accuracy imaging because of mechanical wobbles and oscillations. This is because of basic design problems. The mounts and bearings are light weight and do not have high quality, accurate gear reducers in their drives. The strength of the drive motors is also marginal in my opinion. It is possible to do good imaging with these telescopes, as many have shown, but it is a constant trial to do so. I have owned three sizes of LX200 and still own a 12" and a 10". I found that the 8" and even the 10" had mounts that worked quite well. The drives, gears and bearings for the three telescopes are essentially the same. Only the frames are of differing size and weight. While these elements are adequate for the smaller telescopes, the size, weight and focal length of the 12" simply overpowers the mount and the ability of the mechanisms to guide the scope accurately or to hold the scope steady. Now, understand that I am being very critical about the terms "to guide and hold the scope steady." How steady is adequate?
This clearly depends upon the angular accuracy that the telescope must maintain to give an image that is not degraded by the telescope motion. Other limits are, the resolution of the telescope optics, the atmospheric seeing and the resolution of the imaging medium. I believe that the telescope pointing should be no greater than the best of those other limits. This will give a mount excessively stable for a telephoto lens with film. For this case, a guiding accuracy of 10 arc seconds is often good enough. However, for a 3000 mm telescope with film or with a CCD imager, 1 arc second pointing accuracy is not excessively great. At 3000 mm one pixel is about 1 arc second. Thus I have specified that the mount is good enough to give a pointing accuracy of 1 arc second and should be designed for this accuracy if possible.
If 1 arc second pointing accuracy can be attained, the telescope will generate images at the limit of atmospheric seeing. This is the seeing that I find typical when there is any seeing at all in Wisconsin. I have initiated a plan to build a mount that will satisfy the stated criterion. The idea, described in some detail below, is to make a mount that will hold a 100 pound load and point it with an accuracy of 1 arc second. This would be called a small but high precision mount. It would hold a SCT of 12 to 16 inches, a 6 to 8 inch refractor, a standard camera with a large lens, a Schmidt camera or any other instrument desired. The only requirement would be that the instrument be within the weight limit, could be attached to the mount and could be reasonably well balanced.
The plan I have initiated is to have an RA drive and wedge constructed which is up to the task of holding and guiding an optical instrument to the specified 1 arc second accuracy. The design is described in some detail below. The wedge and RA part of the drive are under construction at this time. I plan to use at least two different mounts with the RA drive. One will be a fork type mount to hold an SCT. This mount will have its own declination drive. The second will be a platform designed to hold any type of optical instrument such as a camera of some sort or other instrument. It will also have its own declination drive.
The new mount will not be principally a GOTO type in its first stage but it is designed to have GOTO capability added at a later time. I have found that GOTO is wonderful for viewing, where one dashes from object to object in a nights viewing. This feature is much less important for imaging, where one spends considerable time on one object. I usually try to image only 1 or 2 objects in any given night. Once the image is in view, it is the guiding accuracy and the stability of the mount that are of primary importance. Even with a CCD imager exposures of 3 to 10 minutes are usually required. If color imaging is required, that time can extend easily to an hour or more for each image. For film imaging, the sky glow limits exposures to about 40 minutes to an hour or so. Thus while GOTO is not a top priority, precision guiding to an hour or so is. Never-the-less the current design calls for computer control to find objects to within 1 arc minute, frame them and then guide on them with an accuracy of 1 arc second. The first and primary emphasis of the design is guiding accurately and holding the optical instrument very steady. Establishing solidness, rigidity, lack of vibration, freedom from oscillation or whatever it is to be called, is not an easy task.
The RA drive axis is being build with the wedge as an integral part. The wedge will be cut for my latitude of 43 degrees. It can be adjusted by several degrees in two planes and rotationally for accurate polar alignment on top of a permanent pier. This design insures absolute rigidity of the RA drive shaft and accurate polar alignment. A key to the RA drive design is that it has a 2" RA shaft with a bearing spacing of 16". The bearings are loaded ball bearings mounted in large pillow blocks. The blocks are mounted directly on the wedge which is made of 0.625 steel plate. The base of the wedge is 11"" wide by 16" long and mounts directly on the pier top plate which is 16" in diameter. Adjustments are made by means of a number of 1/2 inch bolts, washers and nuts. This RA drive will be exceedingly solid. I emphasize the rigidity of the mount and the long baseline of the bearings because the strength and rigidity of the RA shaft is highly dependent on these factors. A principal design flaw of too many fork mounts is the close spacing between the RA shaft bearings. In some designs it is only 3 or 4". The short lever arm thus provided has very great bending forces on it and it will give to these forces causing springiness in the fork mount. This problem is avoided with a greater bearing spacing.
The RA shaft will be fitted with a custom cut 9" main worm gear driven by a precision worm. The shaft will rotate once per pointing degree and be connected to a precision reduction gear box with a 60 to 1 ratio. This shaft, which will turn one revolution per arc minute movement of the pointing tube, is in turn driven by a motor through a 10 to 1 gear reducer. With this design, shafts are available for connection of a direct encoder, an arc minutes encoder and an arc seconds encoder. With this set of encoders, it will be possible to move the right ascension shaft absolutely and/or incrementally with appropriate electronic controls. The encoder on the main shaft will allow for an absolute accuracy of 1 arc minute. The motor shaft will turn at 2.5 RPS which is a good speed for precision motor control. The motor will be 1/30 HP. The centering of the object and guiding upon it will be through a CCD guider/imager via computer control. The guider CCD will provide the final critical guiding of the RA drive to, I hope and expect, 1 arc second or better.
The 9" main worm gear is adequate, by normal design guidelines, for a mirror of up to 18". This is not over design. Comparable gear sizes are not often used in amateur telescope mounts. The Losmandy GM-8 uses a 2.8" gear, the G-11 a 5.625" gear, the GM200 a 9" gear, the LX200 a 5.75" (for the 16", an 11" gear). I emphasize gear size since the required mechanical tolerances required in the drive are easier to effect with a larger gear.
The drive motor will be under the control of computer generated timing signals. The encoders will generate position information at intervals of 0.1 arc second. The frequency of the control pulses at sidereal motion rate is then 150 pulses per second. This rate is fast enough to guarantee that the telescope will not shake do to the pulsing of the RA motor drive.
The system is identified by a rather unusual design feature. The RA drive shaft is terminated in a plate, 6" diameter and 2" thick which serves as the connector between the RA drive and the accessories that will do the imaging. This plate is drilled and tapped to accept mating mounting plates on the accessories. A fork mounted SCT with a 10" or 12" mirror will be one such accessory. The telescope will simply be mounted on the RA shaft when it is to be used. The fork mount will a standard fork designed to hold the telescope tube and its accessories. It will have its own custom designed and built declination drive. I am currently working on this equipment.
Another accessory will be a platform which will hold various optical instruments. I am designing, and building, this general purpose platform which will hold anything. That is, a camera and lens or several cameras with different lenses, a guider telescope and CCD guider or whatever else might be required for a particular imaging mission. This general purpose platform will be in a custom mount which I am calling a "tine" mount. That is, a fork with one tine. The tine mount will be easily interchangeable with the fork mounted telescope by means of the universal plate on the RA drive shaft. The platform will be about 18" by 24" in size and have its own computer controlled declination drive. Anything, within reason, that fits on the platform and can be fastened to it can be guided accurately. I might point a fast Schmidt camera, a refractor, a large telephoto camera lens or whatever. I joke that the platform will even point a giant bratwurst wherever required with an accuracy of 1 arc second. :-)
Thus this is a rather modern, unusual and versatile mount in concept. While this is a major undertaking, I have become determined to bring it off. I have worked very hard on the concept, details, drawings and am now starting construction of the several parts of the system. It might take another year to finish, but I believe I have the technical details well in hand and the resources required to finish the job. So this summarizes my concept of a high quality telescope mount. It is to be modest in size, but of great precision and strength. A precision I feel necessary to match the very fine optics that are available from several sources. In my opinion more than enough effort has been devoted to the optics and insufficient effort to the mechanics of pointing the optics in a precise way.