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

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

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)

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

• Note: This page does not contain images, so the slides from the graphic version are not contained here.
• Table of contents

back to top