The design and alignment of the DECam lenses and modelling of the static shear pattern and its impact on weak lensing measurements

The Dark Energy Survey (DES) will produce high quality images covering over 5000 square degrees of the sky, with precise photometric redshifts between z = 0.2 to z = 1.3, using g, r, i, z and Y filters. The Dark Energy Camera (DECam), under construction for this survey, consists of wide field corrector optics and a CCD detector array that will give a 2.2 square degree field of view. It will be placed at the prime focus of the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile. The Optical Science Laboratory (OSL) at University College London (UCL) is undertaking the alignment of the five lenses in the imaging system. These lenses range in diameter from 0.60 - 0.98 meters. The lenses must be held within tight tolerance limits in order to meet the DES science requirements. The tolerances are especially driven by the accuracy in the measurement of the weak lensing signal. This paper details the design for the cells that will hold the lenses and the alignment procedure for the mounting of the lenses and cells. Also presented is the expected static shear distortion pattern that will be generated and the impact of this pattern on the weak lensing signal measurement.