1. Introduction

In 2004, OmegaCAM will start op-erations on Paranal as the sole in-strument on the 2.6-m VLT SurveyTelescope. OmegaCAM is a huge opti-cal CCD imaging camera: its 16k × 16kCCD pixels cover the square degreefield of view of the VST almost entirely.The primary function of the VST and itsinstrument is to provide surveys in sup-port of VLT science, be it in the form oflarge homogeneous multi-colour imagingsurveys which form the basis for large-scale spectroscopic follow-up work, orin its ability to find rare or extreme as-tronomical objects for further study.

The designs of both VST andOmegaCAM try to take full advantageof natural good seeing, so it should alsobe a superb instrument for weak gravi-tational lensing surveys, or for monitor-ing projects designed to detect micro-

lensing or supernovae. In fact, applica-tions are manifold: one has only to lookat the exciting science that is now com-ing out of the Sloan Digitized SkySurvey to realize the potential of VST/OmegaCAM, which has a comparablefield of view to the SDSS camera, butwill operate continuously, with betterimage quality and higher throughput.

The scale of the instrument meansthat once operations start the challengeis not at all over: OmegaCAM will gen-erate of order 50 GByte of raw data pernight, year after year, and such a vol-ume of data can only be digested bymeans of a strict observing protocol(encoded in the Observation Blocks)combined with highly automated pro-cessing of the data. Exciting and chal-lenging times are ahead!

In this article, the OmegaCAM con-sortium presents the basic features anddesign of the instrument.

2. The VLT Survey Telescope

The VST (Arnaboldi et al. 1998), nowunder construction in Naples, is a 2.6-m modified Ritchey-Crétien telescopewhich will stand next to the four UT’s onParanal. It is specifically designed forwide-field imaging, and has been opti-mized for excellent image quality in nat-ural seeing. Thus, it will have active pri-mary and secondary mirrors, a re-tractable atmospheric dispersion cor-rector, a constant focal plane scale of0.21arcsec per 15 µm pixel over a 1.4degree diameter field, and a theoreticalPSF with 80% of its energy in a 2 × 2pixel area over the whole field.OmegaCAM will be the sole instrumenton the telescope, and will be mountedat the Cassegrain focus.

3. Overview of the Instrument

3.1 Detector system

The heart of OmegaCAM is the CCDmosaic (Fig. 1), being built at ESOheadquarters in Garching. It consists ofa ‘science array’ of 32 thinned, low-noise (5e–) 3-edge buttable 2 × 4kMarconi (now E2V) 44-82 devices, for atotal area of 16384 × 16384 15 µm pix-els (26 × 26 cm!). The science array fitssnugly into the fully corrected field ofview in the focal plane of the VST, andcovers an area of 1 × 1 degree at 0.21arcsec/pixel. Around this science arraylie four ‘auxiliary CCDs’, of the sameformat. Two of these are used for auto-guiding (on opposite sides of the field:the field is so large that also field rota-tion will be auto-guided), and the othertwo for on-line image analysis. For thispurpose the latter CCDs are deliberate-ly mounted out of focus (one 2 mm infront, one 2 mm behind the focal plane),and the resulting defocused imagescan be analysed on-line and used toinfer aberration coefficients such asdefocus, coma, or astigmatism every

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OmegaCAM: the 16k ××16k CCD Camera for the VLT Survey Telescope K. KUIJKEN1,2, R. BENDER 3, E. CAPPELLARO 4, B. MUSCHIELOK 3, A. BARUFFOLO 5,E. CASCONE4, O. IWERT6, W. MITSCH 3, H. NICKLAS7, E.A. VALENTIJN 2, D. BAADE 6,K.G. BEGEMAN 2, A. BORTOLUSSI5, D. BOXHOORN 2, F. CHRISTEN 2,6, E.R. DEUL1,C. GEIMER6, L. GREGGIO5, R. HARKE7, R. HÄFNER3, G. HESS6, H.-J. HESS3, U. HOPP3,I. ILIJEVSKI3, G. KLINK8, H. KRAVCAR3, J. L. LIZON6, C. E. MAGAGNA5, PH. MÜLLER9,R. NIEMECZEK6, L. DE PIZZOL5, H. POSCHMANN8, K. REIF8, R. RENGELINK1, J. REYES 6,A. SILBER6, W. WELLEM7

1Leiden Observatory; 2NOVA/Kapteyn Astronomical Institute, Groningen; 3Universitäts-Sternwarte München; 4INAF - Osservatorio Astronomico di Capodimonte, Napoli; 5INAF – Osservatorio Astronomico di Padova; 6ESO, Garching; 7Universitäts-Sternwarte Göttingen; 8Sternwarte Bonn; 9Radioastronomisches Institut Bonn

Figure 1: Layout of CCDs in the focal plane. This arrangement minimizes the amount of deadspace between devices, given the constraints imposed by connecting the read-out ports. Theglobular cluster ω Cen is superimposed on the field, which covers a 1 × 1 degree area. Theauxiliary CCDs, shown in green and purple, are used for autoguiding and for online wavefrontanalysis.

Figure 2: General drawing of the OmegaCAM instrument. The view is dominated by the cryo-genic cooling system (with pre-amplifiers, vacuum equipment, etc. attached on the outside)and the detector head on top of it containing the CCD mosaic. Above the CCDs, obscuringthe curved dewar window, is the filter exchange system (shown in blue), which moves filtersbetween the two magazines and the beam. The exposure shutter is mounted at the top of theinstrument, just below the telescope flange.

minute. The whole detector system ismounted behind a large, curved dewarwindow (the final optical element in theVST design) and is cooled using a 40-lNitrogen cryostat. Readout of the fullmosaic takes 45 s, and is accomplishedby two FIERA controllers (a thirdFIERA takes care of the four guidingand image analysis CCDs).

The OmegaCAM detector team atESO is led by O. Iwert.

3.2 HardwareIn front of the dewar window is the

mechanical part of OmegaCAM: clos-est to the CCD window sits the filter ex-change mechanism, and above that theshutter. Both components have to fitinto a design space of a mere 16 cmbetween the dewar window and theVST’s Shack-Hartmann unit. The hous-ing provides the mechanical link be-tween the telescope flange and the de-tector/cryostat system.

Figure 2 gives a section view of thefinal design that foresees a cylindricalhousing with a spoke-like rib structureto support the axisymmetrical loads atthe Cassegrain focus. The housing canbe seen in Figure 3.

The filters are stored in two maga-zines which can move up and down,either side of the focal plane, throughlarge shafts in the housing. A linearstage slides filters into the beam, wherethey are locked into place by means ofmovable notches. High-precision filterpositioning ensures that intensity varia-tions in the flat fields due to optical im-perfections in the filters (dust grains,etc.) are less than 0.1%. The filter ex-change unit is built in such a way that itallows one filter to be pulled into thebeam while the previous one is pushed

the opening edge of the one blade andthe closing edge of the other blade withan identical time difference, even if theblades are still accelerating – this pro-vides an impact-free, high-accuracyphotometric shutter. Tests of the shutterconfirm that it meets the key technicalspecification: for an exposure time asshort as 1 second, deviations from ahomogeneous exposure are well below±0.2% over the whole field of view.

The OmegaCAM control electronicsare based on VME-based local ControlUnits (LCUs), with a higher level ofUnix-based workstations to managethe user interface, coordination, testingand maintenance. The LCU is a stand-alone VME crate equipped with aMotorola CPU board, an Ethernetboard, the real time operating systemVxWorks, as well as specialized con-trol and interface boards. All the con-trolled functions are standardized asmuch as possible, and the modular de-sign facilitates the maintenance andshould ensure efficient and reliable op-erations.

The mechanics are designed andbuilt by the German consortium part-ners in Göttingen (H. Nicklas – housing,filter exchanger) and Bonn (K. Reif –shutter). More details can be found inNicklas et al. (2002) and Reif et al.(2002). The control electronics weredesigned at INAF-Naples by E.Cascone, and are being assembled atMunich University Observatory underW. Mitsch.

3.3 Optics

The VST telescope will work in twoconfigurations, which can be selectedremotely. In the standard configuration,foreseen for work at small zenith dis-tances, a two-lens field corrector is

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Figure 3: The1.5-m diameterhousing struc-ture during atest assembly ofthe main units:the two storagemagazines (tobe inserted intothe big shaft atleft and hiddenright), the filterexchange unitwith theexchange car-riage and a 420× 320 mm2

opaque,aluminium ‘filter’(at the bottomleft).

out, allowing efficient observing in spiteof the rather large distance the filtershave to travel.

The filters are large (in the languageof our latest ESO member state: asquare foot) and heavy: when fully load-ed with 12 filters, the instrument will con-tain 40 kg (90 lbs.) of filter glass alone!

The exposure shutter (Reif et al.2002) is one of the key units ofOmegaCAM (Fig. 4). It consists of twocarbon fibre blades which open andclose the light path. They are driven bymicro-stepper motors and movesmoothly on linear motion guides.These movements are controlled suchthat each individual CCD pixel ‘sees’

used. The second configuration re-places this corrector with one includingan Atmospheric Dispersion Corrector(ADC), consisting of one lens and twocounter-rotating prism pairs. The oper-ating wavelength ranges are 320–1014nm and 365–1014 nm for the two-lenscorrector and corrector + ADC respec-tively.

The only optical parts located in theinstrument are the filters, and the en-trance window to the cryostat, whichdoubles as a field lens.

The primary filter set of OmegaCAMwill be a set of Sloan u′, g′, r′, i′ and z′filters. In addition, there will be JohnsonB and V filters for stellar work and forcross-calibrating the photometric sys-tems, a Strömgren v filter, an Hα filterconsisting of 4 segments with redshiftsof up to 10,000 km/sec, and a seg-mented ugri filter for efficient photomet-ric monitoring of the sky.

The procurement of large format fil-ters of the required size turned into achallenging task. Only one manufactur-er (the French company SAGEM, for-merly REOSC, who figured the VLT pri-mary mirrors) could make an offer forproducing the primary set of filters with-out resorting to a segmented design,which would have created vignettingshadows on the detector array. Ratherthan using coloured glass – barelyavailable in the required size – the filterpassband is generated by means ofmultiple layer coating of up to 5 sur-faces in a sandwich of three plates. Theexpected throughputs of the Sloan fil-ters are very high (Fig. 5).

Filter procurement is coordinated byU. Hopp and B. Muschielok.

3.4 Control Software

All instrument functions (filter ex-change, shutter, detector readout, aswell as monitoring the instrument state)are controlled in software. The pro-gramming environment is defined andprovided by ESO through the releasesof the VLT Common Software whichhas to be used as the basis for designand development. The partitioning ofthe OmegaCAM Instrument Software(OmegaCAM INS) into software sub-systems also follows the VLT stan-dards. Nevertheless there were severalchallenges peculiar to OmegaCAM.

The Autoguiding Software and ImageAnalysis modules normally belong tothe Telescope Control Software. In thecase of OmegaCAM it was necessaryto move these functionalities to the INSbecause during normal operations theVST guiding arm will not be used, as itslightly vignets the science array. A newsoftware algorithm was developed toextract optical aberration coefficientsfrom the out-of-focus images recordedon the Image Analysis CCDs. On thedetector software side, particular atten-tion had to be paid to the coordination

of the readouts by the differentFIERA’s, and to the efficient storage ofthe data on disk.

The Instrument Software is beingproduced by the Italian part of the con-sortium, headed by A. Baruffolo (INAF-Padua), and is described in more detailin Baruffolo et al. (2002).

4. Calibration and DataReduction Software

The amount of data produced byOmegaCAM will be truly huge. We es-timate that there will be over 15 Tera-byte of raw data per year. This raw datavolume contains roughly 5 Terabyte ofcalibration data and 10 Terabyte of rawscience data. Data processing will thenproduce another 10 Terabyte of re-duced science data and may create,with about 100,000 astronomical ob-jects per OmegaCAM field, enormouscatalogues. To efficiently handle thisdata volume the data acquisition, cali-brations and the pipeline reductions arestrictly procedurized, a key aim being to

maintain the instrument, not individualdata sets, calibrated at all times. ESOwill operate the instrument in servicemode, optimizing the observing pro-gramme to ambient conditions, androutinely taking calibration data. Thuseach night the instrument’s overall re-sponsivity and also the transmission ofthe atmosphere will be monitored in theu′, g′, r′ and i′ bands irrespective of theschedule of science observations. Datareduction recipes, run in ESO’s DFS,will provide a continuous characteriza-tion of the behaviour of the instrumentin these key bands. When other filtersare used, the calibration plan foreseesa cross calibration of these filters ver-sus these key bands.

The basic technique to overcomeany gaps or artefacts in the CCD pixelsis to take more exposures of the samefield with slightly shifted field centre andto co-add the images off-line. We dis-tinguish the following observing modes:

• Dither has offsets matching themaximum gap between CCDs, ∼ 400pixels (5.6 mm). It will be operated with

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Figure 4: The OmegaCAM exposure shutter. The aperture size is 370 × 292 mm, the short-est possible exposure time is smaller than 1 msec, the deviations of the effective exposuretime from pixel to pixel (homogeneity) are smaller than ±0.2% for a 100-msec exposure, theexposure time accuracy is about 0.3 msec. (The laptop computer gives an idea of thescale.)

Figure 5:Theoreticalthroughputcurves fromSAGEM for theSDSS filter set,and measuredquantumefficiency of oneof theOmegaCAMCCDs.

N (with 5 as the default value) pointingson the sky. Although this will nearly cov-er all the gaps in the focal plane andmaximizes the sky coverage, the con-text map of such data is complex. Anadvantage is that it will be relativelyeasy to couple the photometry amongthe individual CCDs.

• Jitter has offsets matching thesmallest gaps in CCDs ∼ 5 pixels. Thismode optimizes the homogeneity of thecontext map and will be used during ob-servations for which the wide gaps arenot critical, but which, for instance, re-quire a well-mapped smoothly varyingPSF.

• Stare allows re-observing one fixedpointing position multiple times. It is themain workhorse for monitoring the in-strument and allows detection of opticaltransients.

• SSO is the mode for observingSolar System Objects, which requiresnon-sidereal tracking.

For all these modes dedicated ob-serving templates are being developed.

An observing strategy employs oneor a combination of the basic observingmodes. It also defines a number of ad-ditional instructions for scheduling ofthe observations. We distinguish thefollowing strategies:

• Standard which consists of a singleobservation (observation block)

• Deep which does deep integrations,possibly taken at selected atmosphericconditions over several nights

• Freq which frequently visits (moni-tors) the same field on time scalesranging from minutes to months andhas overriding priority on the telescopeschedule

• Mosaic maps areas of the sky larg-er than 1 degree, which is essentiallyan item for the scheduling, as thepipeline has to produce uniform qualitydata anyway. The combination of vari-ous field centres into one image is notconsidered a standard pipeline task.

The observing modes and strategiesare fully integrated with the data reduc-tion software being developed by theOmegaCAM consortium. We distin-guish between a calibration pipelineproducing and qualifying calibrationfiles, and an image pipeline that appliesthe calibration files to raw data andtransforms them into astrometricallyand photometrically calibrated images.ESO users will be provided with theoutput of the image pipeline, run inGarching, on the data contained in asingle OB. The nominal photometricaccuracy of this pipeline will be ± 0.05mag, exceptionally ± 0.01 mag. Thenominal accuracy for the astrometry is± 0.1 arcsec rms over the entire field ofview.

As part of the contract, theOmegaCAM consortium will deliversoftware modules that ESO will inte-grate into the image pipeline. In addi-tion, a project has been set up amongEuropean wide-field imaging groups toprovide a ‘wide-field imaging surveysystem’ that will combine pipeline pro-cessing of image data with archivingand data mining tools. Further detailscan be found on http://www.astro-wise.org, and in Valentijn & Kuijken(2002).

The development of the analysissoftware is being done by a team basedin Groningen and Leiden, led by E.Valentijn.

5. Current Status

The OmegaCAM project is now wellinto in the manufacturing phase. Mostof the CCDs have been delivered andtested; most of the mechanics exist andare ready to be integrated; instrumentcontrol and data analysis software isbeing coded. Extensive tests in Europeare foreseen for the second half of2003, and the camera should see firstlight early in 2004. Exciting times!

Acknowledgements

The consortium was formed in 1998in response to an announcement of op-portunity from ESO, and comprises in-stitutes in the Netherlands (NOVA, inparticular the Kapteyn Institute Gronin-gen and Leiden Observatory), Ger-many (in particular University Observa-tories of Munich, Göttingen and Bonn)and Italy (INAF, in particular Padua andNaples observatories). The ESO Op-tical Detector Team provides the detec-tor system at cost to the consortium.OmegaCAM is headed by PI K. Kuijken(Groningen and Leiden University) andco-PI’s R. Bender (Munich USM/MPE)and Cappellaro (INAF Naples/Padua),and project management is done by B.Muschielok and R. Häfner (USM).

OmegaCAM is funded by grants fromthe Dutch Organization for Research inAstronomy (NOVA), the German Fed-eral Ministry of Education, Science, Re-search and Technology (grants 05AV9MG1/7, AV9WM2/5, 05 AV2MGA/6and 05 AV2WM1/2), and the ItalianConsorzio Nazionale per l’Astronomiae l’Astrofisica (CNAA) and Istituto Na-zionale di Astrofisica (INAF), in additionto manpower and materials provided bythe partner institutes.

ReferencesArnaboldi, M., Capaccioli, M., Mancini, D.,

Rafanelli, P., Scaranella, R., Sedmak, G.and Vettolani, G. P., 1998. The Mes-senger 93, 30.

Baruffolo, A., Bortolussi, A., De Pizzol, L.2002, Proc. SPIE 4848, in press.

Nicklas, H., Harke, R., Wellem, W., Reif, K.2002, Proc. SPIE 4836-34, in press.

Reif, K., Klink, G., Müller, Ph. and Posch-mann, H. 2002 in Scientific Detectors forAstronomy, Beletic, Amico eds., Astro-physics and Space Sciences Library(Kluwer: Dordrecht), in press.

Valentijn, E.A. & Kuijken, K. 2002 in Towardan International Virtual Observatory,Quinn, P., ed., ESO Astrophysics Sym-posia Series (Springer-Verlag), in press.

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The VLTI – 20 Months after First Fringes A. GLINDEMANN, ESO

1. Introduction

In 2002, the second year of fringes atParanal, the VLTI has made substantialprogress. The highlight was the comple-tion of the combination in pairs of all fourUnit Telescopes on September 15/16and 16/17 using a total of five differentbaselines. Only the combination MELI-PAL – YEPUN could not be provideddue to the current configuration of delaylines in the interferometric tunnel.

Of equal importance was the start ofa total of 150 hours shared risk scienceoperations with the VLTI in October.

Forty proposals from the communitywere received representing about 10%of all proposals submitted to ESO forthe VLT observatory. A summary of thefirst semester with VLTI science opera-tions will be given at the end of this se-mester. For Period 71, the shared riskscience operations became a part ofthe ESO Call for Proposals with 25 pro-posals submitted for the VLTI. A num-ber of observation preparation tools havebeen developed in collaboration withthe Jean-Marie Mariotti Centre for Inter-ferometry (JMMC) in Grenoble. Two ofthem are now available on the web

(http://www.eso.org/observing/etc/ pre-view.html). In the course of the year, allscience data between First Fringes inMarch 2001 and September 2002 havebeen released through the archive re-sulting in first scientific results whichare described in [1]–[4]. A summary ofthe first results is given in [5]. In thecontext of science operations, the re-sults of the on-going observations ofcalibrator stars are reported in [6], incollaboration with the NOVA ESO VLTIExpertise Centre (NEVEC) in Leiden.

Amongst the runners-up for achieve-ments are the integrated optics beam