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Progress on the Big Mount: Latest Progress on the Declination Drive (August 1999)
Photographs of the final design are shown below. The new worm wheel is 8" diameter and driven by a 1" diameter worm. The worm is mounted on a ball bearing shaft 1/2" diameter. The this secondary shaft is driven by another worm wheel 3 1/4" diameter which is driven by a second worm connected to the drive motor. The structure is nearly complete. All of the parts are heavy in cross section and short to assure good rigidity. This drive is very strong. In this design weight is not a primary consideration since the drive will be permanently mounted. The declination mounting plate fits on the main, 1" shaft. It is coupled to the shaft and also directly to the worm wheel. The design has a two stage motion control. The main declination gear moves the declination platform to the approximate declination position and the fine control moves it to its final position. The fine control covers a range of one degree. The entire drive is placed under the platform and tightly fastened to it to insure rigidity and reduce any possible flexing of the structure.
Notice that the declination drive is a half gear which is sufficient to cover horizon to horizon. The top portion of the gear would get in the way of the platform in this design and so is simply eliminated. No provision is made for clutching the declination drive since the coarse (high speed) slewing action is very fast. The desired position can be reached under full control of the electronics in a shorter time than would be taken by manual operation of the declination position through some sort of clutch mechanism.. The motor used for the course position is a DC motor with tachometer feedback to control rampping the speed of the slew and slewing to final position electronically.
The small motor shown is for the fine adjustment. It works through a lever arrangement that pushes the worm shaft a small amount axially to attain final position. The motion of the course drive and the position of the fine adjustment are coordinated electronically. Essentially, this design makes use of what is normally a defect in declination drives. That is the end play in the worm shaft. It is the adjustment of the end play that is used to set the final position. The fine adjustment will be electronically controlled through a CCD guider which will be part of the final mount, drive system. The CCD guider will be on a permanently mounted guiding scope. The plan is to lock the platform to a position in the celestial sphere and mount all imaging equipment on the locked platform.
Since the entire drive is equatorially (polar) mounted, the declination adjustments will (or should be) very be small. The entire declination drive train is under a fixed tension so that there should be no lash in the mechanical system. (I hope) This has all yet to be tested. The motors are quite strong compared to what is usually used. The course adjustment motor is a 24 volt DC type just short of 0.1 horsepower. The fine adjustment motor is much smaller but is geared to a lever with significant mechanical advantage and needs to only move the worm shaft about 0.07 inch for its entire range of adjustment.
To get some idea of the scale of this declination drive it is shown with a Losmandy dual mounting plate on the top in the photograph below. The Losmandy plate which can be added or removed with a few retaining screws has dual universal mounting rails on which any number of pieces of equipment can be mounted. Or, it can be replaced with any other desired OTA and mount, an imager, a camera with lens or essentially anything that one wants to fasten to the plate and point.
The declination drive will be mounted on the massive RA assembly shown elsewhere. The declination drive weighs about 50 pounds while the main RA drive assembly weighs nearly 150 pounds. These weights will go up considerably when the counter weights are added. But every moving shaft is on heavy duty ball bearings and the gears and motors are quite capable of moving assemblies much heavier. There are 6 ball bearings and 9 thrust bearings in the mount.
The declination drive will have encoders mounted on the shaft which extends to the left in most of the photographs. These will provide feedback to the computer/controller for setting the course position. The course position will be to either 2.5 or 5 minutes of arc depending on the encoders which are available when the design of the control section is completed. The control will not be a full GOTO design but will point to a location which is transferred to the control box from a planetarium program like SKY with T-point or a similar program. This aspect of the design is not complete at the moment. It is planned to run the observatory from two and possible three computers to avoid the software problems associated with some multitasking setups. The imager will certainly be on a separate computer.
After a few final touches on the declination drive, I will need to finish the RA drive. The design is complete, the motors, shafts, bearings and gears in hand and the main parts are cut and cleaned. All that is necessary is to assemble, drill, tap and bolt the thing together. But there is no rush since the new building will not be finished until Spring 1999 in any case. The following photographs show the declination drive in its final form. I had to rebuild the main worm structure to make it very strong and rigid and to improve a slight binding in the ball bearing mechanism. It is now extremely smooth and seems almost frictionless even under load.
The new main worm wheel is shown in place on the left and the fine control
is on the right. In the second photograph the main declination gear
is in place.
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Above are two views of the drive entirely assembled but for the top
plate. As can be seen, the drive is quite complex since it has two
motors to drive the declination platform. The main motor controls
the gross position of the declination platform while the small motor on
the right controls the fine motion of the platform. The design centers
is to have the main motor get the declination to within a few arc minutes
and the fine control to do both the final centering of the image and the
guiding motion when imaging. The motors will be under a combination
of analog feedback and digital control.
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Above is an older photograph showing the declination platform with the Losmandy dual track mount. This is shown to establish the scale of the platform. The platform is large enough to carry a fairlylarge optical tube or a combination of cameras, lenses, imagers and a guider simultaneously.