OmegaCAM:Wide-fieldimagingwithfine spatialresolution

OmegaCAM and the Wide-field Imaging of the Sky OmegaCAM and the VST will act as a pure survey instrument. During its 10 year mission OmegaCAM will map a large fraction (about 30,000 square de- grees) of the sky visible in Chile. To achieve this extraordinary area, OmegaCAM combines 32 CCD detectors to create a 16,384 x 16,384 pixel array. This implies an unprecedented field-of-view of one square degree (equivalent to 4 full moons), at a scale of 0.21 arcsec per pixel. Expressed in stan- dard computer monitor resolution, a single Omega- CAM exposure would result in a picture measur- ing more than 20 square meters, whereas the size of the total proposed surveys would be almost one square kilometer. OmegaCAM will be instrumental in providing statis- tics on large samples of objects, as well as finding unusual rare objects. Aside from its role in provid- ing large area public surveys, a large number of in- dividual science programmes have been proposed. These include searches for supernovae, Kuiper belt and Oort cloud objects, gamma ray bursts, and high proper motion objects, as well as mapping large scale structures, Galactic halo populations, and high redshift galaxies and quasars. Clearly, the VST and OmegaCAM will provide a veritable flood of objects for follow-up observations with the VLT telescopes. Currently, the final instrument housing has been ma- chined and delivered to the Universitaets- etiendesignspecificallydesignedforwide-fieldimaging,and hasbeenoptimizedforexcellentimagequalityinnaturalseeing. Thus,itwillhaveactiveprimaryandsecondarymirrors,are- tractableatmosphericdispersioncorrector,aconstantfocalplane scaleof0.21arcsecper15µpixelovera1.4degreediameterfield, andatheoreticalPSFwith80%ofitsenergyina2◊2pixelarea overthewholefield.OmegaCAMisthewide-fieldopticalcamera designedandconstructedforthistelescopebyaconsortiumcon- sistingoftheLeidenObservatory,theKapteynAstronomicalInsti- tute,theUniversityObservatoriesofMunich,G¨ ottingenandBonn, theAstronomicalObservatoriesofPadovaandCapodimonte,and ESO.Itwillbethesoleinstrumentonthetelescope,andwillbe mountedattheCassegrainfocus. withintegratedshutterandmagazinescanbeseeninFig.3.The filtersarestoredintwomagazineswhichcanmoveupanddown, eithersideofthefocalplane,throughlargeshaftsinthehousing. Alinearstageslidesfiltersintothebeam,wheretheyarelocked intoplacebymeansofmovablenotches.Thetotalnumberofmo- torsusedinthefilterexchangeandpositioningmechanismis7. Highprecisionfilterpositioningensuresthatintensityvariationsin theflatfieldsduetoopticalimperfectionsinthefilters(dustgrains, etc.)arelessthan0.1%.Thefilterexchangeunitisbuiltinsucha waythatitallowsonefiltertobepulledintothebeamwhilethepre- viousoneispushedout,allowingefficientobservinginspiteofthe ratherlargedistancethefiltershavetotravel.Filterexchangetime dependsonhowfarthemagazinesneedtomoveandwhetherthe incomingandoutgoingfilterbelongtothesamemagazineornot. Provisionsforincreasingtheefficiencyofobservationsaremade insoftwareandplannedforinstrumentoperations(e.g.optimized distributionoffilterinmagazines).