Construction and testing of the wavefront sensor camera for the new MMT adaptive optics system

This paper describes the construction and testing of the Shack-Hartmann wavefront sensor (WFS) camera for the new MMT adaptive optics system. Construction and use of the sensor is greatly simplified by having the 12 x 12 lenslet array permanently glued to the detector array, obviating the need for any further realignment. The detector is a frame transfer CCD made by EEV with 80 x 80 pixels, each 24 microns square, and 4 output amplifiers operated simultaneously. 3 x 3 pixel binning is used to create in effect an array of quad-cells, each centered on a spot formed by a lenslet. Centration of the lenslet images is measured to have an accuracy of 1 mu m (0.02 arcsec) rms. The maximum frame rate in the binned mode is 625 Hz, when the rms noise is 4.5 - 5 electrons. In use at the telescope, the guide star entering the wavefront sensor passes through a 2.4 arcsec square field stop matched to the quad-cell size, and each lenslet samples a 54 cm square segment of the atmospherically aberrated wavefront to form a guide star image at a plate scale of 60 mu m/arcsec. Charge diffusion between adjacent detector pixels is small: the signal modulation in 0.7 arcsec seeing is reduced by only 10% compared to an ideal quad-cell with perfectly sharp boundaries.