Focus and Alignment system "First Light"

The focus and alignment system of DECam will be automatic--the camera will actively control the focus and alignment so the user can sit back and enjoy in focus images.  Tonight was the first night we were able to begin testing this sophisticated system, designed by Stanford/SLAC.

The focus and alignment CCDs are defocused by either 1.5 mm extra- or 1.5 mm intra-focally.  Tonight, as exposures were taken, the donuts (out of focus star images) on each of these CCDs were analyzed automatically for coma, astigmatism and focus.  A Zernike analysis of the wavefront error at the focal plane measured the Zernike parameters.  These plots show the Zernike term 4 (Z4) fits over a 15 minute period. The camera was intentionally "out of focus" for some tests. When in focus, the Z4 term is +/- 10 "units". In the attached charts, we are stable at +15 and -5. Separated by 20 "units" (units are waves at 700 nm).

Even at this early stage, the system seems to be working nominally.  Great job to our focus and alignment team!