OPTICS and COATINGS for -80°C - News — English · Tertiary mirror for the WFCAM experiment of the UK- Infrared Telescope ... (= mounting/adjustment ... for an ESO experiment *

23/03/2007 1ISO 9001CERTIFIED

OPTICS and COATINGS for -80°C(ARENA workshop, Puerto Santiago 26-29 march 2007)

Presentation by : Denis FAPPANI – Commercial Engineer of SESO

1) SESO experience in OPTICS and COATINGS for -80°CA few typical examples of past realisations of : ⇒ Mirrors (and reflective coatings) ⇒ Lenses/objectives (and antireflection coatings)⇒ Specific assemblies (optical contacting, …)

2) Guidelines for specifications and design of OPTICS and COATINGS for -80°C and Conclusions



Tertiary mirror for the WFCAM experiment of the UK- Infrared Telescope (Hawaï)(with the courtesy of the UK-ATC Edinburgh)

SESO did manufacture in 2003 for the WFCAM experiment several mirrors and optical componentsincluding a« tertiary mirror assembly » (i.e. mirror + attachment flexures) installed inside

a cryostat operating at low temperature (150°K, so -123°C)Features :*Mirror material : ZERODUR*Mirror size : Φ820mm*Mirror shape : aspherical (elliptical) CV*Mirror geometry : double-arch lightening concept*Material of the Cryostat housing : aluminium*Material of the (3) flexures :INVAR*Mounting concept :

•Flexures onto cryostat housing => with screws•Flexures onto mirrors => with structural glue

⇒From RT to cold temperature diametralrelative dilatation is quite 2mm !!



Tertiary mirror for the WFCAM experiment of of the UK- Infrared Telescope (Hawaï)(with the courtesy of the UK-ATC Edinburgh)

Thermal test was performed by customer to check the stability of the mirror WFE from RT to cold temperature

Test was done by interferometry at conjugates using a interferometerand converging beam (outside the cryostat) and a (small)

CV retro-reflecting mirror (located inside the cryostat)

Recording of the double path WFE vs. temperaturehas been done from 289°K (+16°C), which is roughly

the temperature of manufacturing/mounting,down to operating temperature of 148°K (-125°C)




289°K (+16°C) 188°K (-85°C)215°K (-58°C)

161°K (-112°C) 148°K (-125°C)179°K (-94°C)




Reflective coating of the WFCAM mirror was here

« protected aluminium »for use in the spectral band 1µm – 2,5µm

which withstands the cold temperature without any problem(protected silver or protected glod should have been also OK)



MIPASMIPAS telescopetelescope forfor thethe ENVISAT satelliteENVISAT satellite(MIPAS = Space Programm for ESA, for earth observation in the infrared, forecast purpose)

SESO delivered this complete Telescope about 10 years ago (including complete mechanical structure)

Operation at 180°K so about -100°C

Features :* Afocal and anamorphic Telescope* Aluminium sandwich structure * Traditionnaly polished nickel-plated(on both sides, front and back) aluminiummirrors (2 spherical, 1 toroid and 1 flat)* Dimensions ≈ 750*175*173 mm3* Operating temperature range : 180K-300 K* Spectral range band : from 4 to 15 µm* Wavefront error (4 mirrors) : 1 µm RMS

View of the mirrors only(i.e. real telescope structure not represented)



MIPASMIPAS telescopetelescope forfor thethe ENVISAT satelliteENVISAT satellite

Reflective coating of the mirrors was here« protected gold (called Hard Gold) »

for use in the spectral band 4µm-15µm (useful MIPAS band) but limited to (i.e. also available from Near IR to Far IR

This coating withstands the cold temperatureand passed successfully a complete space qualification



Mirrors for the MIRI instrument of the James Web Space Telescope

Operation temperature is here down to 20°K or -250°C)For MIRI, SESO was in charge (several moke-ups delivered in 2006) and is stil in charge

(about 120 different components to deliver in 2007) of the reflective coatings of the flight models of the MIRI mirrors (from diamond-turned mirrors supplied by customer)

Reflective coating is similar to the one of MIPAS (i.e. protected gold) andhas been qualified down to 20°. Useful spectrum is here from 5µm to 28µm

Views of different kinds of MIRI mirrors (flat, double toroids, slicers mirrors, …..)



SESO is currently manufacturing the optics for the spectrograph EMIR of the IAC(camera assembly+collimator assembly+large diameter filed lens Φ500mm)

EMIR is an infrared spectrograph (0,95µm-2,5µm) for the GRANTECAN telescope,

operating at 77°K (= -196°C)

View of the camera barrelLens diameter 160-180mm

(glasses are BaF2, IRG2, Silica and ZnSe)

View of the collimator barrelLens diameter 160-180mm

(glasses are BaF2, IRG2 and Silica)



Major problem to solve in the spectrograph EMIR is to allow diametric dilation of lensesfrom RT (= mounting/adjustment temperature) down to 77°K (= operating temperature) :

1) Without risk of thermal breakage because barrel is made out of aluminium,so with a CTE different than the ones of the glasses.

* Silica INFRASIL (CO2+CA3) = 0,5 e-7* BaF2 (CO3+CA1+CA4) = 14,7 e-6* IRG2 (CO4+CA2+CA5) = 8,8 e-6 All CTEs are are averaged ones (RT=>77°K)* ZnSe (CA6) = 5,5 e-6* Aluminium (Barrel) = 21 e-6

2) But keeping centerings of the lenses (within spec., here all about less than 25µm)in order to get a good image quality and good stability of image plane

To solve that double requirement, SESO solution is to install lens inside cells which areflexible annular rings manufactured with appropriate CTEs (something inbetween CTEof the glass and CTE of aluminium). This make also easier the alignment and factory testof the complete integrated objective because it can be performed at RT

(cheaper test that performing it at cold temperature)



Analysis of worst case N°1 of EMIR : CO2,because made out Silica (« stong » material

but CTE far from aluminium )

Modelisation and FEM analysis gives a Max tensile strength in the lens of 1,2MPa

Analysis of worst case N°2 of EMIR : CO3, because made out BaF2 (CTE not so far

from aluminium but very fragile material)

Modelisation and FEM analysis gives a Max tensile strength in the lens of 0,45 MPa

Notes : Full scale demonstration by test of cold temperature behaviour ofComplete integrated objectives are stil to be done but representative

full scale moke-ups have been realized and tested successfully



AR coatings of EMIR spectrograph (infrared spectral band 1µm – 2,5µm) :

Activity still to be done (mid-2007) but the SESO AR coatings available for thatare already existing and already qualified for cold temperature.⇒ Avg. Refl. < 1,5% for « low index » glasses (e.g. Silica and BaF2)⇒ Avg. Refl. < 2% for « high index » materials (e.g. ZnSe)

Such cryogenic AR coating has already been realized in the past also on several projects operating at cold temperature :* ZnSe prism (150mm*150mm) for the CRIRES experiment of ESO* Slit viewer objective (Silica + BaF2, Φ50mm) for an ESO experiment* Fringe Sensor Unit (FSU) cold optics (mainly Silica + ZnSe, Φ20mm) for thePRIMA metrology experiment of the VLT* …………….



Specific assemblies available for cold temperature

MIRI etalons (OGSE for MIRI-JWST)SESO has produced in 2006, a ZnSe Fabry-Perot

with specific Rmax and AR Coatings tofilter the band 5µ-7,7µ (Channel 1A of MIRI)Mounting by molecular adherence (= optical

Contacting) as well as coatings, have been tested

successfully down to 20°K (or -250°C)(3 other units for channels 2A, 1B and 2B

are under construction in 2007)

ALADIN programm (space borne LIDAR) SESO has produced in 2003 a (100%) Silica

“Double Channel” Fabry-Perot assembly qualified for Space Application.

Operating temperature is +20°C stabilised but qualification has been

achieved from +60°C down to -40°C



Guidelines for specifications and design of OPTICS and COATINGS for -80°C

Designing (large) mirrorsPrevious SESO experience in cryogenic mirrors mainly based on mirrors usedin vacuum (cryogenic chamber or space application), so with « stabilized »environmental conditions⇒ Spatial thermal gradients (YES during cool-down but NO in operation)⇒ Temporal thermal gradients (YES during cool-down but NO in operation)⇒ Humididy (YES during transport/storage, NO in operation)

Conditions at Dôme C will (or could) be different (TBC)⇒ Level of spatial thermal gradients in operation ?⇒ Level of temporal thermal gradients in operation ?⇒ Level of humidity/icing in operation ?

Next is a trade-off between 3 different typical materials as possible candidates : ALUMINIUM, ZERODUR & SiC




Designing (large) mirrorsAluminium :Aluminium :•High CTE (23.10-6/°K) so very sensitive to all spatial gradients in operation(including unavoidable spatial gradients met during the mirror manufacturing so difficulty to polish anyway a large and accurate mirror because of distorsionsvs. time)•Good thermal conductivity (220W/m.K) so less sensitive to temporal gradients(assuming all telescope made out of aluminium too)•Complete telescope made out of aluminium not sensitive to homogeneous temperature changes (so, except gradients) thanks to good thermal conductivity properties everywhere•Quite « well known » material properties at low temperature •Stiffness (Young modulus = 70GPa) and density (2,7) close to ZERODUR•Lowest price among all (if mirror is feasible within specifications). •Aluminium can be diamond-turned (up to certain limit of size) but with limitedmicro-roughness and high frequency ripples. Post-polishing is possible but addinga layer of Nickel so inducing more or less parasitic bi-metallic effect




Designing (large) mirrors

ZERODUR :ZERODUR :•Very low CTE ( down to a few +/- 10-8/°K) so not sensitive to all spatial gradients in operation (assuming all telescope made out low expansion material too, such as Carbon/Invar)•Bad thermal conductivity (2 W/m.K) so could be sensitive to « fast » temporal gradients.• Complete telescope made out of ZERODUR+ Carbon/Invar not sensitive tohomogeneous temperature changes if they are « slow » ones• « well known » material properties at low temperature •Stiffness (Young modulus = 90GPa) and density (2,5) close to ALUMINIUM•Intermediate price among all (including also price of telescope structure)




Designing (large) mirrorsSiC :SiC :• Intermediate CTE ( about 4. 10-6/°K) so a bit sensitive to all spatial gradients in operation (assuming all telescope structure made out of SiC too)• Good thermal conductivity (180 W/m.K) so less sensitive to temporal gradients• Complete telescope made out of SiC not sensitive to homogeneous temperaturechanges •Not very « well known » material properties at low temperature (e.g. CTE value ?)•Not « well known » uniformity of properties over large blanks (e.g. unif. of CTE)•Stiffness (Young modulus = 420GPa) very high and density (3,2) not so far fromALUMINIUM and ZERODUR•Requires an (expensive) overcoating of e.g. CVD to get good roughness+S/D quality•Highest price among all (including also price of telescope structure made out of SiC)



Guidelines for specifications and design of OPTICS and COATINGS for -80°CDesigning (large) mirrors

SiC :Very promissing material because of its high stiffness + high conductivity but majorproblems are the (very) high price and the fact that it is a not «well known » materialat cold temperature and concerning uniformity of its properties for large blanks

ZERODUR :Major problem is the bad thermal conductivity (but exact temperature conditions atDôme C, for spatial and temporal thermal gradients, must be well known to state the compliance) and major advantage is the fact that it is a well known material with a very low CTE anyway and leads to global intermediate price

ALUMINIUM :For simultaneously, accurate and large diameter optics, major problem is the high CTEvalue, so sensitive to all spatial gradients and major advantage is the price.Also, feasibility of simultaneously, accurate and large diameter optics is not reallypossible due to limitations during themanufacturing and control of the mirror




Designing objectives with (large) lenses

As for mirrors, previous SESO experience in cryogenic lenses/objectivesare mainly based on systems used in vacuum (inside cryogenic chamberor for space application), so with « stabilized » environmental conditions

Conditions at Dôme C will (or could) be different (TBC)

Depending on exact conditions at Dôme C, we recommend to avoid useof temperature sensitive materials such as CaF2/BaF2 This is in case there are important (spatial) and/or fast (temporal) temperature changes




Designing coatings of (large) optics

Reflective coatings for mirrors :•Protected aluminium will be the best for environmental conditions•Protected Silver/Gold can be also available (can withstand cold temperature) butresistance to other environmental conditions (humidity/icing, if any…) are less•One must take care of Rmax coatings (i.e. mulitidielctric coatings with a lot of layers)onto large & accurate mirrors because of risk of thermal distorsion (like bi-metalliceffect) from RT to cold temperature.

Anti-Reflective coatings for lenses :• Risks are less everywhere than reflective coatings•A large range of antireflection coatings that can withstand cryogenic temperatures•are already existing



END OF PRESENTATION

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OPTICS and COATINGS for -80°C - News — English · Tertiary mirror for the WFCAM experiment of the UK- Infrared Telescope ... (= mounting/adjustment ... for an ESO experiment *