Focus and alignment using out-of-focus stellar images at the Dark Energy Camera

The focus and alignment system of the prime focus Dark Energy Camera (DECam), for the Dark Energy Survey at the CTIO 4 meter Blanco telescope, is described. DECam includes eight 2K by 2K CCDs placed 1.5mm extra- and intra-focally for active control of focus and alignment, as well as for wavefront measurement. We describe an algorithm for out-of-focus star (donut) image analysis and present results on the use of donuts for focus and alignment. Results will be presented for both simulated DECam images and for images taken at the Blanco 4 meter with the current MosaicII camera.