The probe beam is from a 633nm diode laser couple through a single mode fiber. The resulting probe beam power is 6 microWatt. The beam is sent into the vacuum chamber with a 30 micron waist at the input window (to avoid picking up any curvature induced by atmospheric pressure), and expanded to a waist of 3.5cm at the back face of the test optic (a test mass generously donated by the Caltech 40 meter interferometer). The beam is reflected back to the quarter wave plate (where is polarization is now rotated 90 degrees to the input beam) and directed by the polarizing beamsplitter into the wavefront sensor (a commercially available Shack-Hartmann type sensor from Wavefront Sciences Inc.). The CCD camera is used for alignment and probe beam characterization.