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In October 2000, DFM Engineering, Inc. completed a major
upgrade of the McMath-Pierce East Auxiliary Heliostat. The
upgrade included retrofitting a special version of our Telescope
Control System (TCS) and major rework of the heliostat itself.
Original Design
Features:
The McMath-Pierce Solar telescope on Kitt Peak, Arizona,
was originally designed and built by Charles W. Jones Engineering
in 1961 and was completed a few years later. The system includes
three heliostats to drive sunlight downward into the fixed
telescope optical tube assembly. The central heliostat has
a 60-inch aperture and the East and West auxiliary heliostats
carry 36-inch diameter flat mirrors.
The East Auxiliary Heliostat was reworked about 15 years
ago, but modern observing programs place some new requirements
on the control system. It was also time to refurbish the R.A.
and Declination drives. DFM Engineering was contracted to
perform the rebuilding of the drives, polar axle and bearings,
and install our well proven TCS.
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Upgrade Evaluation:
The heliostat minus the mirror and its cell were removed
from the "Super Carriage" that carries all three heliostats
and was trucked to DFM Engineering's facilities in Colorado.
The 5 ton heliostat was removed from the truck with a crane
and attached to a handling cart and rolled into our shop.
The heliostat was cleaned, inspected, and using our inside
crane, disassembled.
Much of the engineering work was performed using data from
the existing drawings before we received the heliostat. However,
telescopes from this era tend to have inaccurate drawings,
and there are usually changes made later on that have not
been documented. Any design changes we made needed to be verified
against the actual parts.
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Upgrade Action:
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We designed and fabricated the new polar axle and reworked
the plate that mounts the RA worm housing. We replaced
the secondary gearing and motors on both axes. The original
Declination worm housing mounting scheme was very awkward.
We designed and installed a new machined mounting plate
to allow the Declination worm housing to be installed
with only the worm to wheel mesh needing to be setup.
This feature will simplify future maintenance.
The original polar axis bearing system consisted of
a single four point contact ball bearing that was custom
made. The ball separators were found to be inadequate
on the previous rebuild, and rather than rework the
bearing again,
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it was decided to replace the single bearing with a
pair of ball bearings separated by a new polar axle.
Fortunately, C. W. Jones Engineering had anticipated
that the single bearing might cause problems and had
designed and machined the polar housing so it was relatively
easy to design and fabricate a new polar axle with two
bearings. Unfortunately, bearings of the desired size
are not commercially manufactured and had to be custom
made-this time with proper ball separators.
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The new custom made polar axle bearings had to be designed
and ordered months ahead of taking the heliostat out of service
due to the long lead time involved. We designed a spacer into
the south bearing mount which allowed us to setup the preload
between the two bearings at shop assembly. The spacer was
initially made thicker than the old drawings indicated so
we could have one simple part that could be machined to make
up for poorly known dimensions of the existing parts and manufacturing
tolerances on the new parts.
The extensive changes to the RA and Declination secondary
drives required fabricating new drive covers. The amount of
effort involved to produce drive covers and moisture seals
is large and not well appreciated until you have done it a
few times. We also reworked the number 2 mirror mounts. The
#2 mirror brings the sun light to a focus and acts as the
primary mirror of a telescope. We added motorized and encoded
azimuth and elevation motions so the mirror can now be driven
from the control room to place the solar image where it is
needed. Also, the existing focus motion (all 50 feet of travel!)
of the #2 mirror was encoded and is now controlled by the
Telescope Control System.
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Final Testing
and Installation:
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The heliostat was assembled in our shop and the new
control system was installed and tested. A single D.C.
servo motor per axis is used to run the heliostat through
its entire speed range. DFM personnel assisted
the Kitt Peak crew install the heliostat. Then we polar
aligned the heliostat, tuned up the TCS, and determined
the pointing model coefficients. The refurbished heliostat
points twice as good as required and the tracking is
superb.
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