Facts about coating and cleaning
Maxime Boccas
Mirror cleaning and coating conference
Palomar Observatory
25-27 April 2001
1. Coating
1.1.- Coating plants for Aluminium evaporation - 4m chamber (used only for M1 of 4m tel.)
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6-day process, 4-6 people
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104 filaments on 4 rings (original NOAO plant): thickness uniformity +/-25%
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Glow discharge (3000V, 3A, 20min)
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6.10-6 Torrs with 2 diffusion pumps
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600-1000 Å deposited, 43Å/s max
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Total time of power in filaments: 4’ ; total aluminizing time: 45”
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Sigma SQM160 thickness monitor
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Total time of coating process: about 6 hours
1. Coating
1.1.- Coating plants for Aluminium evaporation - 1.5m chamber (all other mirrors)
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1 day process, 2-3 people depending on mirror size
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24 filaments on 1 ring
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Glow discharge (700V, 0.5A, 20min)
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4.10-5 Torrs (at least) with 1 diffusion pump
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950 Å deposited, 17Å/s max
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Total time of power in filaments: 5’ ; total aluminizing time: 90”
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Sigma SQM160 thickness monitor
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Total time of coating process: about 5 hours
Typical fresh Al reflectivity (%)
measured with IRIS 908RS
|
470nm |
530nm |
650nm |
880nm |
4m plant |
92.6 |
93.0 |
89.3 |
88.0 |
1.5m plant |
92.4 |
92.7 |
89.1 |
89.0 |
Quoted by G. Hass |
92.1 |
91.6 |
90.5 |
88.0 |
|
Scatter (both plants) |
|
0.4% typical |
|
Micro-roughnes |
|
30 Å typical |
|
TO DO:
4m chamber
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Fix leaks of the 4m chamber for higher vacuum and even better purity of coating BUT reflectivity excellent
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Modify filament configuration for better thickness uniformity (KPNO reached ±10%) BUT 400Å is acceptable (~l/10)
1.5m chamber
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Last coating done under 4.10-5 T was about 2% below ideal: gas contamination due to poor vacuum? Do leak testing.
OBSERVATIONS:
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45-90s aluminizing time doesn’t seem to show any difference in coating micro-roughness
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6.10-6 Torrs is low but enough for high purity and reflectivity
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Substrate free of dust when chamber is sealed to avoid pinholes in coating
2. CLEANING
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Cleaning before aluminizing
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Regular in-situ washing: every 6 months for M1s
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CO2 snow cleaning : every 15 days
CLEANING BEFORE ALUMINIZING
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Rinse with tap water
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Soap + contact with natural sponges. Rinse
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HCl (50%) + CuS04 (1%) + H2O + contact with Kimwipes. Rinse
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KOH (5%) + H2O + contact with Kimwipes. Rinse
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Rinse with plenty of medical grade bi-distilled water (80 liters at 4m M1)
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Dry with nitrogen gas guns (2-4 people depending on mirror size)
USUALLY NO WATER MARKS LEFT, EVEN IF WATER DRIES ON ITS OWN!
2. CLEANING - REGULAR IN-SITU WASHING
(since March 00)
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OPEN TUBES (4m, 1.5m tel.): M1 washed in-situ with inflatable seals on inner/outer diameters and water exhaust tube. Telescope at ZD 80°
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CLOSED TUBE (0.9m tel.): M1 cell removed from telescope (but mirror stays in cell). No collimation required.
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How long does it take? 4m: 3h ; 1.5m: 1.5h ; 0.9m: 4h !
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No need for dedicated engineering: day-time activity
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think safety: operator and mirror!
'RECIPE':
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Use garden hoses for all rinsing. Plenty of working light.
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Filtered water rinse (5’)
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Warm soapy water highly concentrated rinse. (5’)
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Soap + natural sponge contact. (10’)
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Filtered water rinse. (5’)
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Bi-distilled water rinse. (5+’)
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Dry with nitrogen gas guns (5+’)
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NO alcohol used (bad experience)
In-situ washing can be done anytime after ‘dust event’ or ‘rain leak accident’, when contamination is still fresh…
2. CLEANING - IN-SITU M1 WASHING
RESULTS
0.9m tel.
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No C02 cleaning, semi-annual wash
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22 months, 3 washs
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From fresh Al: only 1%reflection loss and 0.2% scatter loss!
1.5m tel.
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Monthly C02 cleaning (for 19 months), semi-annual wash
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25 months, 3 washs
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From fresh Al: only 1%reflection loss and 0% scatter loss!
4m tel.
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Bi-weekly C02 cleaning, semi-annual wash
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6 months, 1 wash
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From fresh Al: 0%reflection loss and 0.1% scatter loss!
CONCLUSION: 0.5% refl. loss per year
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Regular CO2 cleaning doesn’t seem to be relevant to help recovering reflectivity by washing, and only very slightly for scattering.
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M1 in closed (with extraction fans) and open tubes seem to behave equally.
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We now intend to wait 4 years (or more) between recoatings and maintain reflectivity above 90% at 550nm\
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3-4 extra nights for science per year (compared to our old bi-annual Al schedule)
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Purchase of 2 water distillers to produce our own bi/tri-distilled water. Investment paid in 1 year (we need about 600l/year at $3.4/l)
OTHER OPTICS IN-SITU WASHING
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Secondary mirrors: telescope looking at horizon, mirror vertical, sealing skirt easy to put around mirror, contact wash. Monthly loss rate is typically R-0.15% and SC+0.10%
=> do not disregard them. Semi-annual monitoring and annual washing?
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Upward looking field corrector: our Ø0.5m CFADC cell is sealed. Telescope looking at horizon, liquid collection bag attached below cell. Rinse (no contact because sol-gel AR coated) with 100ml bi-distilled water and 50ml isopropyl alcohol in squeeze bottle.
=> it is usually not much extra work, in the design phase of ANY large optical component exposed to ambient air, to think about seals to allow easy and safe in-situ washing. Seal retrofit usually possible.
CO2 SNOW CLEANING
Gain per cleaning session in pre-washing era (August 98- March00):
|
R400nm |
R700nm |
SC400nm |
SC700nm |
4m tel. |
+0.35% |
+0.38% |
-0.24% |
-0.32% |
1.5m tel. |
+0.70% |
+0.71% |
-0.20% |
-0.41% |
We do bi-weekly sessions at 4m and monthly sessions at 1.5m tel:
=> Refl. can wait one month to be recovered but scat. can not.
=> CO2 helps maintaining mirrors cleaner between washings.
Seasonal variations:
due mostly to ambient RH?
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Wet season, Oct-Apr, 32%<RH<55% : R loss is 0.63% per month
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Dry season, May-Sep, 12%<RH<32% : R loss is 0.05% per month
MONTHLY VARIATIONS in %
(average at 550nm)
|
R0.9m |
R1.5m |
R4m |
SC0.9m |
SC1.5m |
SC4m |
Sep98 - Mar00 |
-0.70 |
-0.20 |
-0.21 |
+0.44 |
+0.17 |
+0.17 |
Mar00 - Nov00 |
-0.66 |
-1.30 |
-0.71 |
+0.32 |
+0.55 |
+0.31 |
Nov00 - Apr01 |
-2.2 |
-1.0 |
-1.3 |
+1.2 |
+0.7 |
+0.71 |
CONCLUSION:
stronger-than-usual variations since roughly Nov 00 !
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No correlation with wind (‘summer’ is quiet, 1.8m/s average wind)
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Dust events?
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Pollens? (intense ‘desierto florido’ last spring in area starting 50km NW of CTIO, prevailing wind is NE...)
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Need for permanent site monitoring with a particle counter?
4m tel. M1 coating grounded in August 00 in order to minimize dust retained by electrostatic forces : no obvious effect...
CLEANLINESS OF SITE:
basic dust collection experiment in 4m and 1.5m tel. domes with horizontal microscope slides left exposed to ambient air for 21 months (June99-Apr01)
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Both indicate av. scatter increase of 1.11% (+/-0.02) per month.
Note: For comparison, Schmidt telescope M1 (tube closed with corrector): reflectivity and scatter worsen by 1% per year (average over 10 years)
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About 60 particles with 5 < size < 10 mm per mm2
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About 8 particles with size > 10 mm per mm2
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Purchase of a Metone particle counter. Will be used to detect ‘dust event’ and for GSMT site-testing in Atacama
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Replace the now-dusty summit soil cover with new gravel-type material (test Dimm at ground level was found covered with dust)
All this information
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