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, |
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. |
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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