Optical design and calibration of a medium size telescope prototype for the CTA

ABSTRACT The Cherenkov Telescope Array (CTA) is designed to make a maj or improvement in the sensitivity of ground basedVHE (Very High Energy, den ed as > 20GeV to 100s of TeV) gamma-ray telescopes. Not only will the di erential-u xsensitivitybeanorderofmagnitudebetterthanthoseofthe currentlyoperatingCherenkovtelescopes,buttherewilla lsobesignic ant improvementsin the energy, spectral and angula r resolution. Delivering these features cost-e ectively requiresseveral telescope sizes and designs - a few large size telesc opes (23m diameter) are designed for the lowest energies, alarge number of small size telescopes (4–7m diameter) to inc rease the overall collection area which helps at the highestenergies and a number of Medium Size Telescopes (MST, 9 :5m or 12m diameter, depending on the mirror design) toprovidethe greatest sensitivity at  1TeV. To providecomplete sky coverage,CTA will have both a N orthern Hemisphereand a Southern Hemisphere site.A prototype for an MST design (currently under development) will be build in Berlin by 2012. This MST prototypehas a modie dDavies-Cottondesignwith a tessellated mirro r,with individualfacets of  1:2m in diameter. The facets arethree-pointmountedon the optical supportstructure, havi ngtwo poweredactuators for alignmentadjustments. In addi tiona number of CCD cameras will be mounted at various positions o n the dish and will be used for calibration. Here wepresent the various optical calibration tasks - optimizati on of the optical point-spread-function (PSF) and the point ing ofthis MST prototype,along with initial results.Keywords: CTA, Cherenkov,Telescope, Array,MST prototype,optical, calibration, PSF