Saeko S. Hayashi, Tomio Kurakami, Tomio Kanzawa, and Coating Test Team
Subaru Telescope, National Astronomical Observatory of Japan
Presented at the C&C Conference at Palomar Observatory, April 2001
Section 1. Introduction (History)
Section 2. Coating facility
Primary mirror washing station
Large vacuum coating chamber at the summit
Section 3. CO2 cleaning of the primary mirror
Section 4. Monitor data and problems
Section 5. Other special "surfaces" at Subaru
Appendix. References of Coating and Cleaning of Subaru Telescope Optics
I would like to borrow a popular nursery song to describe how we maintain the surface of Subaru telescope’s 8.3 m primary mirror. Sweep, that is the in-situ cleaning of the primary mirror, is done by CO2 broom. Wash for realuminization is done by a giant dish washer with water purifier. Baking, namely recoating, is done in a vacuum coating chamber. The only feasible way to aluminize the mirror without causing dripping is evaporation from pre-wetted filaments. We are in the process of making such filaments for our upcoming run later this year. By the way, there is one verse that is relevant to our mirror (but not for this conference): Mending of the mirror is with special adhesives.
I must admit that because of a big jump from other Japanese telescopes which are considerably smaller than Subaru, we had difficulties in making realistic procedures for the primary mirror. Long cycle period of the realuminizaion (which will be no shorter than 2 year, I suppose) means slow "learning curve" for the personnel involved. Anyway, the brief history of the coating/cleaning system of Subaru telescope is the following.
(Other facilities are included in general contract with Mitsubishi Electric. Another major manufacturer is Hitachi Zosen.)
History of other coating experiment/campaign
There are three secondary mirrors and two tertiary mirrors for Subaru Telescope.
Conservative approach behind these developments comes from the very fragile structure of the primary mirror. It is a thin meniscus with 261 actuator holes in the back (plus three hard points). O. D. 8.3 m, with thickness of 20 cm, or 4 cm where the holes are. In order to avoid any extra stress for the mirror, it has to stay looking upward, not vertical, not looking down. Also no rotation mechanism to avoid shocks to the mirror. So we ended up with conventional filaments, not boats or crucibles, for evaporation. After extensive tests with various shapes of filaments, we adopted pre-wetting scheme. At least this part is not a traditional style. Current configuration is 16-coil filament, and 14 aluminum clips each. Can you imagine loading u-shaped clips at 13796 feet altitude? Not feasible at all. So we prepare the filaments at Hilo base facility. For 3 operations of the evaporation, we need almost 1000 filaments, that is 288 filaments x 3 (for rehearsal for system check, real run to coat the mirror, another set for just incase).
The evaporation rate is controlled by the power supply to the filaments. The film thickness/uniformity relies on the aluminum clad of filaments. Therefore the weight control of the filament wetting is critical.
The specification of the primary mirror coating was given as following:
91% reflectance at 500 nm,
film thickness 70 - 130 nm, better than +/- 5 % irregularity.
Summit
3 trolleys
detach/attach mirror with mirror cell from the telescope
receive mirror with mirror cell at ground level
lower part of the vacuum chamber, also to serve as washing bowl
5 stations at ESB (enclosure support building, ground floor of the enclosure)
trolley storage
washing station (see Section 2A),
with emergency drench and eye wash
10-m hatch and 10-m roll-up door, loading/unloading area
mirror lifting station, with another trolley storage
chamber station (see Section 2B)
2 stations at observation/telescope floor (~20 m above ground)
below Cassegrain focus and at 10-m hatch
waste pit 23 ton (6000 gallon) capacity
mirror handling fixture, 80-ton crane, rigging device
Hilo Base (Laboratory area)
small evaporation chamber (50 cm diameter, 10 filaments)
fume hood for cleaning materials (filaments, aluminum clips)
spectrophotometers, surface profiler
2 pairs of arms, rotate horizontally above the mirror, 0.25, 0.5, 1.0 rpm
1 pair is for the chemical (with trough)
the other pair provides water (tap water for pre-wash,
purified water for serious wash, warm purified water for final wash)
no dryer device (instead, manual flushers of nitrogen gas)
water purifier (activated charcoal, DI, RO, 0.2 micron membrane filter)
tap water 3000 liter
purified water 1800+200(warm) liter
chemical tank 100 liter
production rate: 500 liters/hour (130 gallons/hr)
water nozzle pressure 4.0 kg/cm2 (8.83 pounds / .155 sqinch = 57 psi)
manual scrubbing, if necessary
[figure 1. MicroScan data of the primary mirror during the pre-wash in Nov. 1998 run]
O.D. 8.7 m
Height 7 m (with support; bottom of lower chamber to the top of upper chamber when closed is 5.8 m)
Weight: 50 ton (upper chamber)+ 23 ton (lower chamber)
Vacuum level 10^-6 torr
Lower chamber stroke 700 mm (27.6 inches)
glow discharge rod 796.29 cm (313.5")
Trolley speed: slow 0.3m/min, fast 3m/min; lifting 2min
1.6m chamber and small jar (I.D. 38 cm=15”) at ATC, NAOJ-Mitaka, Hilo 50 cm(19.7"), IBS at ATC for experiments
Ar gas backfill to 0.1 torr for glow discharge (just once)
Filament array in 7 rings
ring 1 = 24, ring 2 & 3 = 24, ring 4 & 5 = 36, ring 6 & 7 = 72; total 288
Zoning for firing: Zone 1 (rings 1, 4, 5), Zone 2 (rings 3, 7), Zone 3(rings 2, 6)
Mirror support in 3 rings
ring 1 = 12, ring 2 = 30, ring 3 = 48, total 90
Vacuum pumps
rotary pump for atmospheric pressure - 20 torr
LEYBOLD SOGEVAC Rotary Vane Pump SV1200 (SOGEVAC Rotary Vane Pump SV300 for Mitaka)
mechanical booster pumps for 20 - 10^-2 torr LEYBOLD RUVAC RA Roots Pump 3001, LEYBOLD RUVAC RA Roots Pump 7001 (LEYBOLD RUVAC Roots Pump WAU1001 for Mitaka)
cryo-pumps for 10^-2 ― 1 0^-6 torr
CVI's Torr Master 1200, 48" x 3, Mitaka 35" x 1
vacuum gauges Convectron? 470 - 1e-3 Torr, Cold Cathode Vacuum Gauge 1e-3 Torr ~
thickness monitor: quartz oscillator
maximum voltage and current, 16.2 volt?, 60 amps?
max rate ~40 angstrom/sec?, film thickness 1600 - 1700 angstrom?
[figure 2. Reflectivity data of witness slides coated in Nov. 1998 and Aug. 1999]
[figure 3. Photo of primary mirror right after the first coating]
The cleaning machine has 4 wiper arms built into the telescope frame. They travel along the tertiary mirror spider. Each arm has 11 pairs of nozzles carefully tuned to produce sufficient CO2 snow. The pressure controlled by the various stage valves was determined from past experiments.
The cleaning period is currently 3 weeks. This is a compromise of the frequency and other telescope works. During the cleaning, the telescope points toward the 15 degrees of elevation. We carried out 24 runs so far after the last realuminizaion of the mirror. Each run requires 4 cylinders of liquid CO2. Like every other important works, it takes about an hour to set up and another hour to finish the system, the actual cleaning procedure is only 10 minutes. The measurement of the resultant reflectivity and scattering needs another hour or so, with a tall ladder or a cherry picker.
The samples to evaluate the coating performance are the microscope slides. We take them down to Hilo laboratory and measure their reflectances. The primary mirror itself is measured by microScan to keep track of the CO2 cleaning effect. We have two sensors of microScan, a portable scatterometer, that measure reflectance and scatter. One sensor is for the visible wavelength at 670 nm and the other is for the near infrared at 1300 nm. Visible sensor is sensitive to the degradation of the mirror surface, but we refer to 1300 nm as well to double check the variation. During the wet summer of the year 2000, the mirror surface seem to be getting worse even though we have done CO2 cleanings. (Or the cleaning in wet condition made it worse? The high humidity would encourage the cinder particle to stick to the surface.) Relatively drier winter seems to be helping keep the surface condition at this moment. There is not much degradation in the reflectance nor the surface roughness during this past winter. The cleaning interval at this moment, therefore, seems to be adequate to counteract the accumulation of dust onto the primary mirror.
[figure 4. diagram showing the reflectivity and scattering at 670 nm (with telescope ambient temperature and relative humidity data), reflectivity data not compensated for the temperature variation.]
The big gap of monitor data in last spring is the time when the telescope was immobilized after one of the three hard points of the primary mirror broke. (Here comes the “mending” part of the mirror.)
Problems
Fine cinder particles accumulates. Unable to remove with CO2 cleaning. The microScan measurement leaves footprints on the mirror. Will it degrade the surface? Hopefully not a significant amount.
Strange behavior of the cold surface right after CO2 cleaning. Scatter scatters. According to the environmental monitor, the rear/side temperature of the primary mirror returns to the normal temperature in about 2 hours.
Greedy coating system: Meisner coil consumes 700 gallons of liquid Nitrogen per run. The electric power supply became available not long before the construction, and without that commercially available/supported supply, it would have been difficult to repeat the test runs. For the cleaning procedure, the procurement office makes sure that there is sufficient water supply and calls dredging company at appropriate times. The 23 tons capacity of the underground waste pit is large, but not infinite (especially after an exceptional deluge in the island).
We are still struggling to find an efficient and yet thorough method of cleaning the mirror prior to realuminization. If only we had a dish-rotator (like Gemini's), it would have been much easier to go for varieties of chemicals and hence solutions. So we are left with additional manual work to scrub the mirror (by hands for the first aluminization, and with various mops for the realuminization).
infrared secondary mirror (silver coated, in house)
Cr/Ag experiments at Mauna Kea chamber (Dec. 99, Jan. 00; monitor from Feb. 00)
optical-black, infrared-shiny (less emissive) surfaces: spider surfaces, telescope paint, dome paint
We would like to give special acknowledgement to our counterpart in Mitaka, Tokyo, Mr. Goro Sasaki, Mr. Masami Yutani, Mr. Norio Ohshima, and Mr. Takeshi Noguchi. Contributions from Ms. Yukiko Kamata, Mr. Yasuo Torii, Mr. Masao Nakagiri, Mr. Masahiro Toda are greatly appreciated. Mr. Barney Magrath who was with CFHT gave crucial advice for our facility. The summit facilities referred above were constructed by Process Systems International, Mitsubishi Electric Company, and Hitachi Zosen.
# T. Noguchi, T. Kanzawa, T. Kurakami, S. S. Hayashi, M. Yutani, N. Ohshima, G. Sasaki, and Y. Kamata, 2000,"Silver Coating of Subaru Telescope IR-Secondary Mirror", Proc. of SPIE 4231,"Advanced Optical Manufacturing and Testing Technology 2000", pp. 32-35.
# T. Noguchi, T. Kanzawa, T. Kurakami, S. S. Hayashi, M. Yutani, N. Oshima, M. Nakagiri, K. Okita, K. Imi, R. Potter, G. Sasaki, Y. Kamata, T. Ishikawa, 2000, "Coating and Cleaning of Subaru Telescope Mirrors ", Proc. SPIE Vol. 4003, Optical Design, Materials, Fabrication, and Maintenance, [4003-40]
# G. Sasaki, T. Kanzawa, N. Ohshima, Y. Torii, M. Yutani, Y. Kamata, S. S. Hayashi, and T. Noguchi, 1999, "Aluminum Coating Experiments by Using Pre-Wetting Method", Report of the National Astronomical Observatory of Japan, Vol. 4, No. 3, pp. 121-128.
# T. Noguchi, T. Kanzawa, M. Yutani, T. Kurakami, N. Ohshima, M. Nakagiri, Y. Torii, G. Sasaki, Y. Kamata, S. S. Hayashi, K. Okita, K. Omata, K. Imi, R. Potter, and T. Ishikawa, 1999, "Coating of the 8.3 m Primary Mirror of the Subaru Telescope", Report of the National Astronomical Observatory of Japan, Vol. 4, No.3, pp. 129-137.
# T. Kanzawa, G. Sasaki, M. Yutani, Y. Torii, N. Ohshima, Y. Kamata, S. S. Hayashi, M. Nakagiri, K. Imi, and T. Noguchi, 1999, "Silver Coating of the 1.3 m Infrared Secondary Mirror of Subaru", Report of the National Astronomical Observatory of Japan, Vol. 4, No.3, pp. 139-144.
# S. S. Hayashi, Y. Kamata, T. Kanzawa, A. Miyashita, M. Nakagiri, T. Nishimura, T. Noguchi, K. Okita, N. Oshima, G. Sasaki, Y. Torii, and M. Yutani, 1998, "Status of the Coating Facility of Subaru Telescope", Proc. SPIE 3352 Advanced Technology Optical/IR Telescopes VI, 3352-33, pp.454-462
# Y. Kamata, S. S. Hayashi, T. Noguchi, T. Kanzawa, G. Sasaki, Y. Torii, M. Yutani, and T. Ishikawa, 1998, "Coating Experiment with 1.6m Vacuum Evaporation Chamber", Proc. SPIE 3352 Advanced Technology Optical/IR Telescopes VI, 3352-69, pp. 526-536
# Y. Torii, S. S. Hayashi, and M. Toda, 1998, "In-situ cleaning of the primary mirror of Subaru telescope", Proc. SPIE 3352 Advanced Technology Optical/IR Telescopes VI, 3352-79, pp. 808-818
# G. Sasaki, Y. Kamata, T. Kanzawa, T. Ishikawa, Y. Torii, S. S. Hayashi, K. Okita, M. Yutani, K. Imi, K. Nakamura, M. Narita, H. Koyano, E. Watanabe, T. Kurakami, N. Oshima, W. Tanaka, M. Tanaka, and T. Noguchi, 1997, "The 1.6m Vacuum Evaporation Coating Plant at the Advanced Technology Center" (abstract in English, main text in Japanese), Report of the National Astronomical Observatory of Japan, Vol. 3, pp. 35-43.
# Y. Kamata, S.S. Hayashi, T. Noguchi, T. Kanzawa, G. Sasaki, Y. Torii, M. Yutani, and T. Ishikawa, 1997, "Coating Experiment with 1.6m Vacuum Evaporation Chamber" (abstract in English, main text in Japanese), Report of the National Astronomical Observatory of Japan, Vol. 3, pp. 45-55.
# Y. Torii, S. S. Hayashi. and M. Toda, 1995, "Experiments on In-situ CO2 Snow Cleaning of Telescope Optics", Report of the National Astronomical Observatory of Japan (main text in Japanese), Vol. 2, No. 3, pp. 601-622.
# E. Watanabe, M. Yutani, T. Kurakami, K. Okita, and Y. Torii, 1995, "Cleaning and Aluminium Coating Test of Mirrors at the Okayama Astrophysical Observatory" (abstract in English, main text in Japanese), Report of the National Astronomical Observatory of Japan, Vol. 2, pp. 523-535.
# M. Nakagiri, 1986, "A Study of Aluminizing Method for the Main Mirror of Large Reflecting Telescope" (text in Japanese), Proceedings of the Symposium on Techniques in Astronomy 1986, pp. 136-144.