UV signal lossesand straylight by chemicalcontamination ...

UV signal losses and straylightby chemical contamination

Pierre ETCHETO, Delphine FAYE, CNES

Xueyan ZHANG, IAS

COMET RTS contamination, Toulouse Dec. 11th 2018

Contamination issues for Space UV optics

UV losses and straylight by chemical contamination Dec. 11th 2018

� Molecular contamination very critical for UV instrumen ts

� Loss of reflection / transmission + spectral shift

� Straylight by scattering (drops, droplets..,)

� Ageing

� Condensation on optical components

� On ground : manufacturing, AIT, transport, storage, on launch pad. UV optics cannot be cleaned!

� During launch : outgassing + re-condensation on (cold) optics and detectors

� In flight : ageing (under Sun UV, radiations, atox, thermal cycling… ).

Few data available to build contamination requirements + cleanliness plans

⇒ CNES + IAS R&T (2014-2017) for Solar Orbiter / EUI :

Try to assess effects of molecular contamination on scatter and transmission in the EUV (17-30 nm) and FUV (100-200 nm)

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Direction des Systèmes Orbitaux – sous-direction des Systèmes Instrumentaux

Outline of experiment(1) spectral losses a Lyman alpha(2) scatter in EUV

Samples tested : o (Lyman α) filters (MgF2 substrates) o (EUV) Mirrors (Al/Mo/Sic coating on SiO2 substrates)o (EUV) Filters (Al sheet on hexagonal grid)

Contaminants tested : outgassing products from typical Space contaminantso Scotchweld EC2216 epoxy resino RTV-566 Silicone elastomero M55J/RS-3C Carbon fiber-polycyanate resin composite honeycomb

Sequence :

o Measure all clean samples (transmittance and BRDF) + setup signature

o Contaminate samples with 2-3 levels of the 3 contaminants

o Measure samples again and check clean references

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Contamination process

� Samples exposed in CNES oven

o 4 days at 10-4 mbar / 100°C

o Several quantities of chosen contaminants => outgassing + condensation

o Quantity assessment by micro-weighting before and after

� Contamination levels obtained

o Composite honeycomb : 10 and 20 µg /cm2 (mirrors).

o EC2216 : Below measurement error (disturbed by water adsorption / desorption)

o RTV Silicone : 20-120 µg /cm2

Contamination level control : major issue . All we have today are orders of magnitude

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Molecular contamination aspect

Condensation of contaminants depends on :

o Sample surface state (Wettability)

o Sample temperature

o Sample exposure to evaporate contaminants…

⇒ Drops, droplets, RARELY uniform film

⇒ Strong effect on absorption (in drops) and on scatter

EC2216 contamination : Droplets and Drops =>

Silicone contamination : Droplets and drops =>

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Lyman αααα Contaminated samples

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Lyman αααα transmittance facility

SOLEIL synchrotron, APEX line :

• energy range : 1 - 20 eV (60 - 300 nm)

• spectral resolution : 0.1 nm

• beam size : about 4 x 4 mm² (split in 2 spots)

• detector : IRD AXUV100 photodiode +transimpedance amplifier

• MgF2 window : to cut spectral harmonics below 115 nm

• Measurements at 2 locations on sample : � centre + 2 mm off centre (to check non-

uniform cibtmination)

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Exposure of contamination to flux

• 13 -14 h (EC2216 and composite) : below

• 2h30 (Silicone)

⇒ Notable change in aspect and drop of transmittance

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Effects of EC2216 at Lyman αααα

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Effects of honeycomb resin at Lyman αααα

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Effects of silicone at Lyman αααα

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EUV : Scatter vs peak transmission

Contaminated BRDF vs. Clean BRDF and signature

Scatter depends on both surface state an specular T/R :

o Direct comparison with signature shows component T/ R and actual scatter

o Comparison between contaminated and clean shows T/R loss and actual scatterchange

o Comparison between normalised clean and contaminated shows scatter to specular ratio, ie. straylight ratio

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EUV Scatter measurement facility� Soleil synchrotron « Metrology » XUV beamline : both EUV spectrum and BRDF

Angle-resolved scatter (BSDF) configuration

o Wavelength 17.4 nm (72 eV)

o Beam 0.1 x 0.2 mm, divergence negligible

o Detector : Si photdiode + 1.5 mm pinhle, on 287.5mm long arm

o Incidence 0° (for transmitting filters) / 6° (for mirrors)

o Measuring range : +/- 3°

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Vacuum chamber

EUV References and signature

Checks :

o Check beam centering on samples : scan => uniform area (inside a stitch for grid filters)

o Measure and substract dark signal

o Signature (direct beam over +/-4°)

o Measure clean samples + compare with signature

Results :

o Signature : specular peak, width : +/-0.5° at 10-4, +/-0.7° at 10-5. Peaks 10-4 high at +/-3.5°, due to reflections on slitchamfers. Not critical : falls outside the samples => absorbed by sample holder

o Clean samples : specular peak + scatter. On mirrors, strong dispersion depending on sample roughness

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Signature and clean mirror samples


EUV : Results on EC2216-contaminated mirrors

MA-1 (strong level, unknown) :

Scatter x10 scatter at all angles

MA-2 (slightly higher level, unknown) :

Contaminated scatter slightly higher than MA-1. Clean scatter very high

=> Significant scatter increase (even whenmasked by strong clean scatter)

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EUV : Results on composite-contaminated mirrors

MC-1 (10 µg/cm2) :

x10 scatter near specular, then drops to clean scatter (medium level)

MC-2 (20 µg/cm2) :

x20 scatter close to specular,then slightlyabove clean scatter (similar to MC-1)

⇒Significant scatter increase.

⇒Linear relationship with contam. Level?

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EUV : Results on EC2216-contaminated filters

=> Those levels of EC2216 contamination cause little scatter.

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FA-1 (7,35 µg/cm²) :

Scatter similar to clean scatter : few, small droplets

FA-2 (60 µg/cm²) :

Scatter similar to FA-1, but clean scatter much lower


EUV : Results on composite-contaminated filters

=> No significant effect of contamination

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FC-1 (58 µg/cm²) :

Scatter similar to clean scatter (rather high)

FC-2 (52 µg/cm²) :

Scatter similar to clean scatter (medium level)


EUV : Results on Silicone-contaminated filters

FS-1 (50 µg/cm²) :

Scatter not increased, but specular peaknarrowed

FS-2 (69 µg/cm²) :

Scatter slightly increased + specular peakmore narrowed

=> Non-scatter effect : focusing by drops?

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EUV : Results on heavily Silicone-contaminated filters

FS-3 ( 117 µg/cm 2) :

Measurements on droplets alone areas and aiming at large drop

Droplets :

• Slightly reduce transmission

• Specular shape unchanged

• Increased scatter

Drops :

• drastically reduce signal (high local contaminant thickness)

• Specular beam strongly narrowed (lenseffect ?)

• Scatter +/- similar to droplets

⇒Drops combined with droplets?

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ConclusionsFirst exploration of yet poorly known effects : It confirms how critical molecular contamination is fo r UV instruments.

Signal loss at Lyman α α α α (peak) :

o 40% (EC2216) => 60% (13h under flux)o 25-80% (composite, depending on droplet size) => 85% (14h under flux)o 75-97% (Silicone, depending on droplets) => opaque (2.5h under flux)

Signal loss at 17.4nm (peak) :

o 10-25% (EC2216 and composite)o 25-50% (Silicone) especially through large drops

Noticeable scatter increase:

o Especially on mirrors (EC2216 and composite), o Much less on filters (except heavy Silicone contamination)o Can be blurred by strong scatter of clean component (manufacturing dispersion)o Non-scatter effect : narrowing of specular beam by drops (lens effect ?)

Trouble in assessing contamination levels (EC2216) : o Water adsorption / desorption disturbs micro-weigh measurementso Non-reproducible behaviour between sampleso Comparison with Alu samplesmay not be relevant

⇒ More effort needed in assessing contamination levels a nd process

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UV signal lossesand straylight by chemicalcontamination ...

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