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