Facts about coating and cleaning at CTIO

 

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.)

• 6-day process, 4-6 people
• 104 filaments on 4 rings (original NOAO plant): thickness uniformity +/-25%
• Glow discharge (3000V, 3A, 20min)
• 6.10^-6 Torrs with 2 diffusion pumps
• 600-1000 Å deposited, 43Å/s max
• Total time of power in filaments: 4’ ; total aluminizing time: 45”
• Sigma SQM160 thickness monitor
• Total time of coating process: about 6 hours

 

1. Coating

1.1.- Coating plants for Aluminium evaporation - 1.5m chamber (all other mirrors)

• 1 day process, 2-3 people depending on mirror size
• 24 filaments on 1 ring
• Glow discharge (700V, 0.5A, 20min)
• 4.10^-5 Torrs (at least) with 1 diffusion pump
• 950 Å deposited, 17Å/s max
• Total time of power in filaments: 5’ ; total aluminizing time: 90”
• Sigma SQM160 thickness monitor
• 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

• Fix leaks of the 4m chamber for higher vacuum and even better purity of coating BUT reflectivity excellent
• Modify filament configuration for better thickness uniformity (KPNO reached ±10%) BUT 400Å is acceptable (~l/10)

1.5m chamber

• Last coating done under 4.10^-5 T was about 2% below ideal: gas contamination due to poor vacuum? Do leak testing.

 

OBSERVATIONS:

• 45-90s aluminizing time doesn’t seem to show any difference in coating micro-roughness
• 6.10^-6 Torrs is low but enough for high purity and reflectivity
• Substrate free of dust when chamber is sealed to avoid pinholes in coating

 

2. CLEANING

• Cleaning before aluminizing
• Regular in-situ washing: every 6 months for M1s
• CO₂ snow cleaning : every 15 days

CLEANING BEFORE ALUMINIZING

• Rinse with tap water
• Soap + contact with natural sponges. Rinse
• HCl (50%) + CuS0₄ (1%) + H₂O + contact with Kimwipes. Rinse
• KOH (5%) + H₂O + contact with Kimwipes. Rinse
• Rinse with plenty of medical grade bi-distilled water (80 liters at 4m M1)
• 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)

• 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°
• CLOSED TUBE (0.9m tel.): M1 cell removed from telescope (but mirror stays in cell). No collimation required.
• How long does it take? 4m: 3h ; 1.5m: 1.5h ; 0.9m: 4h !
• No need for dedicated engineering: day-time activity
• think safety: operator and mirror!

'RECIPE':

• Use garden hoses for all rinsing. Plenty of working light.
• Filtered water rinse (5’)
• Warm soapy water highly concentrated rinse. (5’)
• Soap + natural sponge contact. (10’)
• Filtered water rinse. (5’)
• Bi-distilled water rinse. (5+’)
• Dry with nitrogen gas guns (5+’)
• 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.

• No C0₂ cleaning, semi-annual wash
• 22 months, 3 washs
• From fresh Al: only 1%reflection loss and 0.2% scatter loss!

 

1.5m tel.

• Monthly C02 cleaning (for 19 months), semi-annual wash
• 25 months, 3 washs
• From fresh Al: only 1%reflection loss and 0% scatter loss!

 

4m tel.

• Bi-weekly C02 cleaning, semi-annual wash
• 6 months, 1 wash
• From fresh Al: 0%reflection loss and 0.1% scatter loss!

 

CONCLUSION: 0.5% refl. loss per year

• Regular CO2 cleaning doesn’t seem to be relevant to help recovering reflectivity by washing, and only very slightly for scattering.
• M1 in closed (with extraction fans) and open tubes seem to behave equally.
• We now intend to wait 4 years (or more) between recoatings and maintain reflectivity above 90% at 550nm\
• 3-4 extra nights for science per year (compared to our old bi-annual Al schedule)
• 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

• 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?
• 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?

• Wet season, Oct-Apr, 32%<RH<55% : R loss is 0.63% per month
• 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 !

• No correlation with wind (‘summer’ is quiet, 1.8m/s average wind)
• Dust events?
• Pollens? (intense ‘desierto florido’ last spring in area starting 50km NW of CTIO, prevailing wind is NE...)
• 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)

• 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)
• About 60 particles with 5 < size < 10 mm per mm2
• About 8 particles with size > 10 mm per mm2
• Purchase of a Metone particle counter. Will be used to detect ‘dust event’ and for GSMT site-testing in Atacama
• 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

 

 

is under ‘Telescopes’ item of CTIO web site
http:// www.ctio.noao.edu/telescopes/opteng/optics.html [1]

 

 

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