DFM Engineering, Inc.
1035 Delaware Ave. Unit D
Longmont, CO 80501
Phone: 303-678-8143
Fax: 303-772-9411

 
 
The Boyden 1.52M Rockefeller Telescope Upgrade
 
 

In September 2001, DFM Engineering personnel completed a major upgrade of the 1.52M telescope at the Boyden Observatory near Bloemfontein, South Africa.

 

history

upgrade action

 

previous upgrades

final testing

Telescope History:

The telescope has had an interesting history dating back to the 1880's when it was funded by a wealthy Boston mechanical engineer named Uriah Atherton Boyden. Mr. Boyden willed $238,000 to Harvard College for the purposes of constructing an observatory. The observatory was founded in 1891 in Arequipa, Peru, as a southern observing station of Harvard College. In 1926 the observatory was moved to Bloemfontien, South Africa, where better weather conditions exist.

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Previous Upgrades:

Several upgrades have been performed during the history of the telescope: for example, the majority of the telescope mounting was replaced in 1933. A new mirror cell was designed and built in Hamburg, Germany, in the early 1960's. New Cervit optics were installed in the late 1960's.

More recently the control system and motors were replaced with a single micro stepped motor per axis using commercial motor controls commanded by a home built controller. This system was unreliable, unwieldy, difficult to use, and provided no pointing information.

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Upgrade Action:

DFM Engineering was contracted to install our well proven Telescope Control System (TCS) to the telescope and to provide engineering and technical services needed to get the telescope functioning in a research manner.

Based upon a previous visit by Dr. Frank Melsheimer, new secondary drives were designed and fabricated in

the DFM Engineering shops for the Right Ascension and Declination motions and for the focus drive. A quality focus position encoder was also added to allow precise and repeatable focusing of the telescope.

Installing the new Right Ascension secondary drives was quickly accomplished.

The Declination drive was another matter.

A cutout approximately 8 inches by 12 inches was needed in the Declination counterweight (housing the Declination

worm gear set) to allow access for the new secondary gearing.

The wall thickness of the cast iron housing was 1.5 inches (38 mm). This cutout and another smaller one required 3 days of drilling a series of holes then sawing through the remaining metal.

The sides of the new cutout were then carefully cleaned up with a grinder and files.

The Declination drive worm and wheel were cleaned and re-lubricated with a special grease that we have found to provide the best lubrication for telescope worm gears. Lubrication tubes were provided to allow lubricating the Declination worm and wheel from outside of the housing.

We replaced the existing single speed focus motor with a D.C. servo motor. The new motor provides 2 speed operation from the hand paddle and a "GO TO FOCUS" position capability from the TCS. The faster speed is significantly faster than the original speed while the slow speed allows setting the focus very accurately.

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An absolute encoder was added to provide focus position with a resolution of 12 microns at the secondary mirror. One resolution element produces a change in the image diameter due to focus of less than 0.1 arc seconds.

We also reworked the attachment of the secondary mirror cell to the focus ram as there was some lost motion between these components affecting the collimation and preventing satisfactory Declination position repeatability. Pointing measurements show we were successful as the Declination pointing is now better than 10 arc seconds RMS.

The dome was encoded to provide azimuth information for the automatic dome control feature of the TCS. The TCS was interfaced to the existing dome motors so the dome may be controlled from the hand paddle or automatically. In the process of installing the dome encoder and controls, recommendations were made for the maintenance of the dome drive gear reducers - these gear reducers have been in service for decades without any service.

The primary mirror cell was also reworked.

The existing radial supports were found to be frozen from corrosion.

The repair required critical machining of some of the components to increase the clearance.

The cell design also suffered from the lack of tip/tilt collimation screws.


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Although there were 36 counterweighted lever supports, there were no hard points to define the mirror axial position. We removed 3 of the counterweight assemblies and replaced them with suitable adjusting screws. Now the primary mirror is adjustable in tip/tilt and centering which allows the optics to be collimated.

With the worm gears un-meshed, we were able to determine that the bearing friction was quite low in Right Ascension and Declination and the difference in the bearing torque required to initiate movement and to continue movement (the "stiction") was also very low. These bearing friction values are key to providing a telescope that responds well to motion commands and points well.

The mesh of the R.A. and Declination worm gears was then set for minimum backlash and still operate over the entire motion range of the telescope. After extensive balancing of the telescope in 4-axes, the telescope slews and tracks satisfactorily.

Extensive pointing measurements were made with the telescope on both sides of the pier. The typical pointing is now better than 25-arc seconds RMS. If preload motors were added to the drives to remove the backlash, the pointing could be improved to about 15 arc seconds RMS.

A new primary light shield was designed to provide an 80 mm diameter field to support the new CCD camera system that will be used on the telescope.

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Final Testing:

The excellent weather conditions allowed extensive testing of the telescope and training of the observatory personnel in the use and maintenance of the telescope and control system. This experience will allow the observatory personnel to maintain the system in-house.

With the completion of the control system upgrade and the other necessary improvements, the telescope

is now ready to begin another productive life serving the astronomical community.

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