The UCLA Astronomy Department operates a 24" Cassegrain telescope on the roof of the Math Sciences Annex building, on the UCLA campus.
In late 1981, the 24" was moved to the UCLA campus. A crane raised it
onto the roof of the Math Sciences Annex building and into an existing dome that had been
modified to receive the telescope. Plans had been made to bring the
telescope back into operation, but instead it languished in its cramped
dome for six years.
In 1987, Nick Magnone, Chris Shelton and I (Frank Henriquez) were asked to repair and prepare the telescope for a graduate Observational Astronomy class.
We got the telescope to work well enough for the graduate class, but only after extraordinary effort. It didn't take us long to realize that while the telescope's mirrors were in good shape, the drive electronics and large AC motors were no longer adequate for the task of slewing and tracking the telescope.
Between 1987 and 1993, Nick and I removed all of the old electronics and AC motors (more than 400 lbs. worth), recast new counterweights from lead, refurbished the mechanical parts of the telescope and dome and realuminized the mirrors.
The telescope's original drive electronics were a complicated and dangerous mess of circuit boards, wires and motors. The boards, cables, AC motors and planetary gear reducers on the RA and Dec axis were stripped out and replaced by a far more compact design. With the help of Brant Nelson, who at the time was an Astronomy department graduate student, new instruments and control software were added and the telescope (in its new configuration) saw first light in July, 1993. It was used extensively until January 17, 1994.
During our tests of the telescope between June and December of 1993,
Brant and I noticed that images taken to the extreme East or West
(around ± 3.5 hours in RA from the Meridian) were often
in both RA and Dec... as if the telescope had been jarred during the
exposure. We were puzzled by this, and could only guess that the image
distortion was caused by an improperly balanced telescope. The
would cause it to seize briefly at the extreme RA positions. We had
other pressing challenges, so we shrugged this minor problem off as
something to deal with at a later date.
On the morning of January 17, 1994 a very strong earthquake shook Los Angeles and many buildings on the UCLA campus (including ours) were extensively damaged. The earthquake caused the telescope domes (which include the walls and doors) to jump off their tracks. The motion was severe enough to pop the rivets holding down the dome's exterior sheet metal panels.
We were expecting to see a disaster inside the dome, but were surprised to find that everything was still in place. One monitor fell to the floor, but was undamaged.
But not all was as it seemed.
Days later, after Nick put the telescope's dome back into operation, Brant and I discovered that the telescope could no longer track properly. Obviously, the earthquake had undone our polar axis alignment, so we set out to realign it. To our dismay, we discovered that the telescope's pedestal adjustments no longer had the range necessary to realign the polar axis; either the telescope or the building had shifted in the earthquake.
With Nick's help, we removed the floor tiles and aluminum floor panels and examined the telescope's supports. The 24" is bolted to a polygonal steel riser that connects the telescope pedestal to a massive vibration-isolated concrete block that serves as a telescope pier.
According to the architectural plans of the 1981 move from Ojai, the riser had been bolted to the large concrete platform that covered the pier.
We were all surprised to see how easy it was for Nick to push the telescope around with a steel pipe and some leverage. In horror, we slowly realized that the telescope was not bolted to the pier!
The riser was indeed firmly bolted to the concrete platform...but the "concrete platform" itself was nothing more than a very large tin "pie pan" filled with cement. This pie pan appears in the 1950's vintage building plans as an extension of the concrete pier...but in reality was nothing more than a weather flashing to protect the pier itself from the elements during the building's construction. The concrete platform was not bolted down or held in place by anything other than its weight...and the weight of the telescope bolted to it. It had been like this for 13 years! The force of the earthquake had shattered the concrete, jarred the pie pan loose and twisted the telescope so that the polar axis was now pointing slightly to the East and South.
The earthquake had at least solved one puzzle - we finally had an explanation for the jarred images that Brant and I had seen earlier...the telescope, at its extreme RA positions had been causing the pie pan to tip and tilt slightly (by a few arc seconds!)
It took us a while to convince the campus engineers that their building drawings did not conform to reality, and a while longer to formulate a repair plan.
The campus engineers decided to remove the crushed weather flashing and to bolt the telescope riser directly to the concrete block - as it should have been done back in 1981. Of course, this meant lifting the telescope! We had to remove the mirrors and drive electronics, leaving only the tube, fork mount and gears, pedestal and riser. After we had removed all the delicate bits, the construction company built a massive steel framework inside the dome. Using chains, they hoisted the telescope into the air.
After several months of work, they had placed massive steel bolts deep into the concrete block, and Nick and I carefully aligned the riser to the meridian. The polar axis adjustment on the 24" has a very narrow range, so it was critical to get the riser in just right.
I had used the long hiatus to clean the telescope optics, so by June of 1994, we were able to put the telescope back into operation. After spending several nights aligning the telescope to the Polar Axis, we noticed that stars still left a small trail in Right Ascension, regardless of the integration time. This pointed to a problem with the telescope drive system. After some detective work, I discovered that the telescope was vibrating at a very low frequency. Just our luck! The 24" had a resonance near the undivided RA tracking rate (41.8 steps/second). We fixed this problem by placing the RA stepper motor on rubber vibration isolators and adding a flexible coupler between the stepper motor and the gear reducer.
A final legacy of the '94 quake was a strange tracking error right at the meridian. On inspection, we noticed that the large brass RA gear had been damaged during the earthquake. The intense shaking had caused the gear to vibrate rapidly against the steel worm, badly gouging two teeth on the gear. The solution was to rotate the gear 90 degrees so that the damaged teeth were beyond the normal range of telescope motion. Nick also added a thick steel bar that locks the telescope into place when not in use, but swings out of the way during normal telescope operation. This bar will eliminate any damage to the gear during future earthquakes.
Brant Nelson used the UCLA 24" telescope to gather most of the data for his Ph.D, thus making him the first graduate student to do so since the telescope was moved to UCLA.
The 24" is currently being used by several graduate students and groups in support of their research projects.
The 24" is a Cassegrain type all reflective
telescope, with a 24" clear aperture f3.5 paraboloid primary and a 6"
diameter hyperboloid secondary. The effective focal length is 384.1"
(f/16.0). The telescope carries two permanent instruments:
A CCD camera (not shown) equipped with B V R I and clear filters at the Cassegrain focus.
Kcam-III - A 2.2u Infrared Camera is mounted on the East Dec axis port. Kcam uses a cryogenically cooled 256 x 256 pixel Rockwell NICMOS III HgCdTe detector. A 2.2µ filter inside the dewar limits Kcam to the K band and gives it its name. A dichroic mirror in the return optical path from the Cassegrain secondary reflects light above 0.9 microns to Kcam while allowing most of the light below 0.9 microns to pass through to the Cass focus and the CCD. On the 24", Kcam has a plate scale of about 0.85"/pixel.
KCam-III has been modified and a motorized filter wheel and new optics have been added by James Larkin, Ph.D. The modified KCam has been used at Keck. When away, KCam's place is taken up by a counterweight and RockCam...a cardboard box full of pebbles that approximates the weight of KCam and its electronics.
Cleaning the optics of this telescope presented some challenges. While the 6" secondary - protected
by its long baffle tube - was very clean, the primary was a different
story. Even though the telescope tube has a dust cover, the earthquake
and the extensive campus repairs and construction that followed had
coated the primary with a thick layer of dust. One of my major concerns
had been the sandblasting work done on Powell Library; I could see
clouds of fine dust drifting towards our building, and on close
inspection, I could see that some of it had managed to land on the
Cleaning the primary by washing it was clearly out; not only would handling a wet, 300lb Pyrex mirror present some interesting challenges, I could just picture those sharp edged sand grains leaving long scratches in the aluminum coating as they washed off.
I contacted Thomas Waineo, a retired master optician (now deceased) on CompuServe, and he was kind enough to send me a small jar of 3M Strippable Coating. The 3M liquid is no longer available, but it's essentially Collodion. I taped a strip of paper around the outer edge of the primary and another along the edge of the central perforation, and slowly poured the 3M Strippable coating evenly over the surface. I used a very clean camel hair brush to distribute the viscous liquid, taking care not to touch the surface of the mirror with the brush hairs. The only difficulty I encountered was the smell of the solvent as it evaporated from the coating. This is a job that must be done in a well ventilated room, away from open flames, sparks and other localized heat sources!
By the next day, the 3M Strippable Coating had dried into a clear flexible skin. I used some masking tape to peel the skin off of the mirror. The results were amazing; I ended up with a translucent donut of dried coating and a very clean mirror. The coating felt like 400 - 600 grade sandpaper, and you could see all the grit and dust trapped in the coating.
The strippable coating didn't get rid of all the dirt and oils, but what was left was easily taken care of by rinsing the mirror with isopropyl alcohol followed by several rinses with distilled water and blotting dry with very clean, lint free cotton rags. I let the water drain through the central hole and into a bucket under the mirror cell. The mirror was left dust and oil free, with no new scratches or damage to the coating.
This Google search for collodion mirror cleaning should provide enough links on how to use collodion to clean mirrors, and where to buy it.
The AC motors and drive electronics were replaced by two small stepper
motors and 5:1 gear reducers. Each stepper motor was originally driven
by a 10 microstep/step driver from Oregon Microsystems (OMS). Before
leaving Astronomy, I replaced the original drivers with IMS
(Intelligent Motion Systems) 804
microsteppingdrivers. The new drivers are smaller, more efficient and
allow greater microstepping ratios, down to 256 microsteps per step.
new drivers will allow us to use a higher microstepping ratio during
tracking, which should eliminate all the vibrations caused by the RA
stepper motor. Since the microstepping ratio can be changed on the fly,
it can be increased during slewing. The microstepping drivers are in
turn controlled by two Cybernetic
Micro Systems CY545 stepper
motor controller ICs. The CY545s
acceleration/deceleration ramps and an Intel 8254 Programmable Timer
handles the RA tracking rate. The CY545s, Motorola MC 6821 parallel
port, Intel I8254 programmable timer, control PAL (Programmable Array Logic) , address decoding and
bus buffering ICs are built on a small 8 bit PC expansion card and
placed in an old Leading Edge PC. The schematics and parts list are
available on request.
Since the acceleration ramps required for slewing are handled by the CY545s, the telescope control software (written in Turbo Pascal) is greatly simplified. Tracking is done by the Intel 8254 timer, which acts as a free running oscillator at the selected tracking rate. A PAL is used to select between tracking and slewing, directing the appropriate step pulses to the RA driver.
Most of the software, which has grown and been modified as we gain experience with the telescope, is concerned with keeping track of time and the telescope's position. I wrote the CY545 control code and the rudimentary beginnings of the telescope control and pointing program, but much of the remaining code was written by and with the help of Brant Nelson, who received his Ph.D in 1997.