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Performance of the VLT Mirror Coating UnitE. ETTLINGER (LINDE), P. GIORDANO and M. SCHNEERMANN (ESO)

In August 1995 ESO signed a contractwith LINDE AG to supply the coating unitfor the mirrors of the Very LargeTelescope. The coating unit was firsterected and tested in Germany and thendisassembled, packed and shipped toChile. After integration into the MainMaintenance Building on Paranal, the firstmirror was coated on May 20, 1998("First Light" on May 25–26, 1998). Thefinal runs for provisional acceptance wereperformed in January 1999.

The Coating Unit

The Coating Unit encloses the follow-ing main components (see Fig. 1):

• the vacuum chamber with a diame-ter of more than 9 m and an inner volumeof 122 m3

• the roughing pump system and thehigh vacuum pumping system, consistingof 8 cryo pumps and a Meissner trap

• the mirror support system, a whiffletree structure with lateral and axial padsto support the glass mirror during coat-ing

• the mirror rotary device including a fer-rofluidic vacuum feedthrough, to rotate themirror underneath the magnetron duringcoating

• the coating cart, enclosing an aircushion system to drive the lower cham-ber section and a lifting device, to close

the vacuum chamber after mirror load-ing

• the magnetron sputter source with awater-cooled shutter system and cryo-genic shields

• the glow discharge cleaning device,to heat up and clean the mirror surfaceprior to coating.

Thin Film Deposition Equipment

The main component for the coatingprocess is the thin film deposition equip-ment of the Coating Unit comprising thefollowing components:

• sputter source for aluminium includ-ing power supply

• shields to trim the aluminium coatingdeposited

• shutter panels • cryo panels attached to the shutters• Glow Discharge Cleaning Device

(GDCD) including power suppliesThe DC Planar Magnetron Source

consists of the target 99.995% pure alu-minium cathode bonded to a water-cooledbacking plate to reduce the heat radiat-ed to the mirror. The discharge is pro-duced by the use of an inert gas (Argon)to support the flow of current betweencathode and anode. The planar mag-netron source uses magnetic fields to fo-cus electrons in the region of the sput-tering target.

Stainless steel trim shields, which areplaced below the cathode to trim the de-position of the sputtered aluminium, en-sure that the mirror is evenly coated witha uniform thickness of aluminium.

The shutter panels are stainless steelsheet box constructions, which are cooledby water, fed through the panels underpressure. There are two shutters, one toform the leading edge of the coating, andone to form the trailing edge. Both shut-ters are pivoted about the centre of themirror, and at the beginning of a coatingrun both shutters are closed togetheralong the line of the joint band.

Below the shutter panels, copper cryopanels are suspended, filled with liquid ni-trogen.

Their purpose is to provide an area ofhigh purity and homogeneity between theshutter opening below the magnetron andhence aid and improve the quality of thereflectance of the aluminium depositedonto the mirror.

The GDCD consists of two aluminiumelectrodes, shaped to give the requiredprofile. The glow discharge electrodesare water-cooled and are suspendedfrom a dark space shield, which is alsomade of aluminium. The purpose ofthe GDCD is to reduce adsorbed watermolecules from the mirror, the inner sur-faces of the chamber and all compo-nents mounted inside the chamber. The

YEPUN (UT4)

The fourth Unit Telescope (YEPUN) isnow in the final phase of mechanical as-sembly. Acceptance tests with the sup-plier will start soon, after which the con-struction of the four telescope structureswill have been finished. The fourth M1Cell arrived at Paranal in late August(Fig. 5). "First Light" for YEPUN is sched-uled for July 2000.

VLTI

In Europe, major achievements oc-curred in the area of the VLT Inter-ferometer (VLTI) construction. The twosiderostats are being tested on the cloudyEuropean sky (see article by F. Derie etal. in this issue). ESO concluded a con-tract with REOSC (France) for delivery ofall mirrors to equip the four VLT UnitTelescopes with coudé foci optics. An im-portant milestone was reached with thecompletion of the polishing of the opticsfor the first delay line for the VLTI atREOSC. The integration of the cat's eyewill be done at TNO (Netherlands) andthis part will then be integrated into thedelay line at Fokker Aerospace (The

Netherlands). Shipment to Chile will takeplace immediately afterwards. In addition,the 1.8-m primary mirror for the first VLTIAuxiliary Telescope has reached the fi-nal stage of the light-weighting processand the optical polishing will soon beginat AMOS (Belgium). The already manu-factured optics (null correctors, M2 ma-trix) have shown excellent optical quali-ty results. A contract for the purchasingof the Auxiliary Telescope number 3 wasconcluded with AMOS.

The “Residencia”

It has always been ESO’s intention toprovide better quarters and living facilitiesat Paranal. Plans for the design and con-struction of the “Facilities for Offices,Board and Lodging”, also known as theParanal Residencia and Offices, weretherefore begun some years ago. Theproject was developed by Auer andWeber Freie Architekten from Munich(Germany). Their design was selectedamong eight proposals from Europeanand Chilean architects.

The unusual conceptual design isstructured as an L-shaped building, lo-cated downhill from the Paranal access

road on the southwest side of the presentBase Camp. The building is fully inte-grated into the landscape, essentially un-derground with its southern and westernfacades emerging from the ground, pro-viding a view from the bedrooms, officesand restaurant towards the Pacific Ocean.The common facilities, restaurant, offices,library, reception and meeting rooms arearticulated around the corner of the build-ing, while the hotel rooms are distributedalong the wings of the L-shape. A circu-lar hall, 35 m wide and four floors deep– covered with a dome emerging atground level – is the building's focalpoint with natural light. At the floor of thehall there is an oasis of vegetation witha swimming pool. Special protectionagainst light pollution is foreseen. To alarge extent, the ventilation is with naturalairflow through remotely controlled air inoutlets.

The Chilean firm Vial y Vives won theconstruction tender. Work on the first partof the building began on July 1, 1999, andis scheduled to be completed within twoyears. The excavation work for theParanal Residencia is proceeding ac-cording to schedule, with the first concreteto be cast shortly.

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glow discharge also heats the mirrorsurface, which improves film adhesionand film purity.

Process Description

The method selected to deposit a thinaluminium film on the mirror is the sput-tering process.

The sputter coating process is done byinserting a mirror into a vacuum chamber,which contains a specially designed cath-

ode and an inert process gas. A negativevoltage is applied to the cathode (target)and a glow discharge (plasma) igniteswithin the vacuum chamber when the ap-propriate environment (or conditions) areachieved.

At this point, positively charged atomsof the gas (ions) are attracted to thesurface of the target which is nega-tively charged. The positive atoms strikethe negatively charged target with suchforce that atoms of the target are eject-ed and deposited on the mirror surface,

building up a thin layer atom by atom.What differentiates a magnetron cath-

ode (used here) from a conventionaldiode cathode is the presence of a mag-netic field which confines electrons in theregion of the sputtering target. A magneticfield is supplied from below the cathodetarget, which is arranged so that free elec-trons are captured in the crossed mag-netic and electric fields in the region. Thecaptured electrons enhance the ionisationin the inert gas atmosphere. Overall, theeffect is to lower the operating voltage re-quired, from several kilovolts to less than1000 volts. The most striking gain of themagnetron design, however, is an order-of-magnitude increase in coating depo-sition rates per unit area of target.

Coating Procedure

The mirror is loaded into the lower halfof the coating unit. The two halves of thevacuum chamber are brought together,sealing the coating unit. Then the cham-ber is pumped down to achieve the re-quired vacuum level.

The mirror is rotated beneath the sput-ter source and GDCD so that it can firstreceive a surface treatment and then itscoating of Aluminium.

After the surface treatment via theGDCD has been completed the mag-netron is powered up to pre-sputter untilthe cathode’s surface is clean.

The mirror is rotated at a pre-deter-mined speed calculated to give the in-tended coating thickness at the intended

Figure 1: Main components of the VLT Coating Unit. Cart chassis and drive (1), Lateral pads (2), Mirror rotary device (3), Mirror support system(4), Axial pads (5), Supporting frame (6), Magnetron, shield and shutter (7), M1 mirror (8), Vacuum vessel (9).

Figure 2: The lower part of the coating unit vacuum chamber. For loading and unloading of themirrors, the lower half of the chamber is moved on 8 air-cushions from the coating unit locationto the mirror handling tool and vice versa.

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sputtering rate. The leading edge shutteris opened as the intended position of thejoint band passes underneath. Thereby,the rotational speed of the shutter exactlymatches that of the mirror.

After the mirror has completed one rev-olution, the trailing edge shutter is closedby rotating it at the same rotational speedas the mirror, as the film joint passes un-derneath. This produces sharply steppededges to the coating, the trailing edge ly-ing on top of the leading edge. Shadowingof the coating by the shutter edges pre-vents them from being perfect steps.

Because both shutters are pivotedabout the centre of the mirror, all pointsalong the joint band take the same timeto cross the mirror opening.

Figure 4 shows an overview of thepressure conditions inside the coatingchamber during the glow discharge clean-ing treatment of the mirror and the sput-ter process. Prior to glow dischargecleaning, the chamber is pumped downto less than 0.001 Pa. Then air is fed inand at a pressure of 2 Pa (cryo pumpsthrottled) the GDC process is started.After the mirror has completed one rev-olution, the glow discharge cleaning is fin-ished and the chamber is pumped downto less than 0.0002 Pa within less than10 minutes, which demonstrates the highpumping speed of the high-vacuumpumping system. Finally, argon is fed inand sputtering is started at an argon pres-sure of 0.08 Pa.

Test Procedure

During the tests of the coating unit theM1 mirror was replaced by the dummysubstrate, a test platform which repre-sents the surface of the M1 mirror. It al-lowed the mounting of glass slides (50 ×

50 mm, 1 mm thick) onto which the alu-minium coating was deposited.

Up to 60 glass slides were mountedonto the dummy substrate at intervalsalong the M1 radius. The position of theslides was recorded on the back of theslides, together with the coating run num-ber.

During a long experimental phase,which covered more than 30 test runs, thefollowing parameters were optimised:

• pressure of the argon process gas• clearance between aluminium target

and mirror surface• adjustment of shutter for an ex-

tremely small joint line• rotation speed of mirror rotary device.The influence of the glow discharge

cleaning device was investigated with re-

spect to the film adhesion and reflectivi-ty of the samples. In addition, the tem-perature raise on the sample surface wasmeasured during glow discharge clean-ing treatment and coating.

The optimisation goal was:• a film thickness of 80 nm with a thick-

ness uniformity of 5%• a sputter rate of 5 nm/s at 80% of the

mirror surface• the best possible reflectance of the

aluminium film in the wavelength rangebetween 300 and 3000 nm.

Aluminium Film Reflectance

After coating in the VLT CoatingChamber, the reflectance of the sampleswas investigated in a reflectometer withan accuracy of about 0.1%. The re-flectance measurement was absolute, notrelative to a standard material whose re-flectance must be known.

The reflectance data of the sampleswere compared (see Fig. 5) with two re-flectance standards:

• The reflectance standard used in theUSA: “Standard Reference Material 2003,Series E”, issued by the National Instituteof Standards and Technology (NIST).

• “Infrared Reflectance of AluminiumEvaporated in Ultra-High Vacuum”, pub-lished by Bennet et al. in J. Opt. Soc. Am.53, 1089 (1963).

Figure 5 shows the absolute re-flectance versus the wavelength of thecoated samples (solid line) in the ultra-violet, visible and infrared region. The cir-cles (filled and not filled) represent the ex-perimental points measured by Bennet(fresh and aged samples), while the as-terisks represent the reflectance accord-ing to the US NIST standard.

Over the whole range from 300 to2500 nm the results obtained in the VLTCoating Chamber are between the agedand not-aged Bennet data and are clear-ly above the NIST data.

Figure 3: A look into the coating unit vacuum chamber. The upper vacuum chamber part car-ries the DC planar magnetron sputter source. In the lower part the rotatable whiffle tree for sup-port of the primary mirror is visible.

Figure 4: Chamber pressure during GDC- and coating process. After the GDC treatment, thehigh vacuum pump system pumps down the pressure inside the coating chamber (volume 122m3) from 2 Pa to 0.00015 Pa in less than 10 minutes.

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Considering the size of the coating unit(chamber diameter more than 9 m, innervolume about 122 m3) and the size ofthe mirror surface (more than 50 m2) tobe coated, the reflectance data measuredare excellent.

Bennet’s “ideal” data were obtainedin a bakeable, grease-free, all-glass,small laboratory system. To reduce thepossibility of oxidation of the aluminiumduring deposition, the evaporation wascarried out in an ultra-high vacuum ofabout 10–8 Pa (vacuum inside VLT coat-ing chamber 10–4 Pa prior to coating).During the measurement of the re-flectance inside a reflectometer, theBennet samples were flushed with drynitrogen to minimise the rate ofchemisorption of oxygen by the alu-minium.

Due to these optimum test condi-tions Bennet’s results are in very goodagreement with the theoretically predict-ed reflectance data for highly pure(99.999%) aluminium and can be con-sidered as the best possible experimen-tal results.

Aluminium Film Thickness

Beside the reflectance, the film thick-ness of the sputtered aluminium was in-vestigated.

With a special tool a line of coating wasremoved from the sample. The thicknessof the step at the edge of this line wasmeasured in at least three places with aprofilometer.

Figure 6 shows the film thickness uni-formity along the radius of the mirror(measured at single glass samples). Ata mean film thickness of 82.48 nm thestandard deviation was 4.16% (3.43 nm).This high film thickness uniformity over aradius of more than 4 metre shows theperfect adjustment of the trim shields andthe shutter.

Depending on the magnetron dis-charge power, a dynamic deposition rateof more than 5 nm per second could bereached.

This rate is the film thickness dividedby the time taken for a point on the mir-ror to cross the shutter opening. If themeasured film thickness is δ nm and themirror rotational speed is ω degreesper second, and the shutter opening is θ,then

Rate = (δ*ω)/θ

Film Joint Zone

Special attention was given to thearea where the film coverage was fin-ished. The azimuth position of the joint lineand the inclination angle with respect tothe radius vector were exactly adjusted.The defined shutter control and the shad-owing of the coating by the shutter edgesresulted in a joint line zone, which couldno longer be detected by variations in thefilm thickness uniformity. The only indi-cation was a slight decrease in the re-flectance (see Fig. 7) in the ultraviolet andvisible region which could be measuredin a band of less than 20 mm width. Thereflectance in the infrared spectrum wasnot noticeably influenced.

Temperature Increase During Coating

In order to measure the temperatureraise of the mirror during coating, ther-mocouples were glued to the bottom

Figure 5: Measured reflectance values of coated samples (5 × 5 cm) as function of the wave-length in the range from 300 to 2500 nm (ultraviolet, visible, infrared). The results of the VLTCoating Unit are compared with the US standard NIST data and best possible measurementsin a small laboratory system (Bennet et al. in J. Opt. Soc. Am. 53, 1089 (1963)).

Figure 6: Aluminium film thickness, measured on glass samples along the radius of the mirror.Film thickness mean value 82.48 nm, standard deviation 4.16 % (3.43 nm).

Figure 7: Decrease of reflectance in the ultraviolet and visible region of wavelength inside thejoint line of the mirror at a radius of 2.1 m. The dotted lines show the beginning and the centreof the joint line zone, which had a width of less than 20 mm.

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side of glass samples. Then they were po-sitioned at 7 different radii and fixedthermally insulated onto the platform ofthe dummy substrate.

During the passages underneath themagnetron, the temperature increase onthe samples could be determined at thesame time when the aluminium film grewup.

The result was a temperature raiseof less than 10°C on the surface. Thesamples at the outer radii warmed upmore due to their larger clearance tothe liquid nitrogen cooled shields under-neath the shutter and the larger shutteropening.

Summary

The sputtered aluminium film on the 8-metre-class mirrors produced in the VLTCoating Unit is very uniform along thewhole radius (standard deviation 4.16%).The reflectance in the ultraviolet, visibleand infrared regions is near to that whichideally can be expected. Furthermore, thedynamic deposition rate, defined as thefilm thickness cumulated in a single passof the mirror past the source opening, is

higher than 5 nm per second. The shut-ter system produces sharply steppededges at the start- and end line of the

coating and minimises the joint line to awidth of less than 20 mm. All results arewithin or above ESO specification.

Figure 8: Temperature increase of the mirror surface during coating, measured by thermocou-ples positioned at 7 different radii and rotated underneath the magnetron. The magnetron dis-charge power was 102 kW, the rotation speed 20 degree per minute.

Climate Variability and Ground-Based Astronomy:The VLT Site Fights Against La NiñaM. SARAZIN and J. NAVARRETE, ESO

1. Introduction

Hopes were raised two years ago(The Messenger 89, September 1997) tobe soon able to forecast observing con-ditions and thus efficiently adapt thescheduling of the observing blocks ac-cordingly. Important steps toward this goalwere achieved recently with the start ofan operational service forecasting precip-itable water vapour and cirrus (high alti-tude) cloud cover for both La Silla andParanal observatories1. This was madepossible thanks to the collaboration of theExecutive Board of the European Centrefor Medium Range Weather Forecasts(ECMWF) who accepted to distributetwice a day to ESO the Northern Chileoutput of the global model. The ground-level conditions2 are also extracted fromthe ECMWF datasets with immediatebenefits for observatory operation: badweather warnings could be issued 72hours in advance during the July snow-falls at La Silla. Once embedded into theObservatory control system, the tempera-ture forecast is due to feed the control loopof the VLT enclosure air conditioning.

These forecast systems still have to beoptimised and equipped with a properuser interface tailored to astronomer’s ex-pectancies. They might then join estab-lished celebrities like the DMD DatabaseAmbient Server 3, paving the way tomodernity for ground-based astronomi-cal observing.

2. Unexpected, Improbableand Yet, Real

In this brave new world, however, noteverything is perfect: as if the task of build-ing a detailed knowledge of our siteswas not hard enough, climatic eventssuch as the El Niño–La Niña Oscillation(The Messenger No. 90, December 1997)can turn decade long databases into poor-ly representative samples. As was re-ported with UT1 Science Verification(The Messenger No. 93, September1998), the VLT site started to behaveanomalously in August 1998. A re-analysis of the long-term meteorolog-ical database pointed out an ever-in-creasing frequency of occurrence ofbad seeing associated to a formerlyquasi-inexistent NE wind direction. This

contrasting behaviour is illustrated inthe examples (Figs. 1 to 4) as displayedby the Database Ambient Server. Whilegood seeing occurs under undisturbedNNW flow from above the Pacific, thisNE wind is coming from land, it israpid (Fig. 5), turbulent, and up to 2degrees warmer than ambient. Findingits exact origin became a challengingtask.

3. Searching for Clues

It is commonly stated that obser-vatories are worse when operationstarts than they appeared during the pro-ceeding site survey. This is the well-known 3σ effect, the most outstand-ing site being probably at the top ofits climatic cycle when it is chosen. Thistheory did not apply to the VLT site forwhich we had records since 1983 show-ing an impressive climatic stability.Therefore, before accusing The Weather,a complete investigation was undertak-en, starting with man-made causes. Inparticular the chronology of the stress-es imposed to the site since the con-struction of the VLT had started (e.g.,water sewage, power generation, heat ex-changing) were reviewed in collabo-

1http://www.eso.org/gen-fac/pubs/astclim/fore-cast/meteo/ERASMUS/

2http://www.eso.org/gen-fac/pubs/astclim/fore-cast/meteo/verification/ 3http://archive.eso.org/asm/ambient-server/