Focus and alignment of the Dark Energy Camera using out-of-focus stars

The active optics system of the Dark Energy Camera (DECam) at the CTIO 4 meter Blanco telescope, built for the Dark Energy Survey, uses out-of-focus stars (donuts) to determine the camera's focus and alignment, as well as provide a measure of the wavefront. In this paper, we describe the donut analysis algorithm and present results on focus, alignment, and wavefront from a donut campaign conducted at the Blanco from 2010 to 2012, using the previous wide-field camera.