Polarization shearing laser interferometer for aligning segmented telescope mirrors

The Center of Curvature Alignment Sensor (CCAS) was the original instrument installed in the center of curvature (CoC) tower on the Hobby-Eberly Telescope (HET) for aligning the 91 primary mirror segments. The CCAS is a polarization shearing interferometer with HeNe and diode laser sources that illuminate the HET primary mirror with polarized coherent light. Returns from each mirror segment focus back at the CoC and can be viewed on a faceplate at the front of the instrument for coarse alignment of the primary mirror, or sent into the interferometer for fine alignment. Inside the interferometer, Wollaston prisms separate the HET primary mirror image into two polarization components which are spatially shifted by the distance of one mirror segment. This overlaps images of segments with their neighbors to generate interference fringes. The beam is then split into 4 legs, each of which introduces phase shifts to the polarization. Fringe patterns shifted by 0, 90, 180, and 270 degrees are recorded on each leg by a CCD camera. The intensity in each pixel is measured and used in the standard 4-bucket algorithm to calculate the relative phase shift between the two mirror segments, and thus their tip/tilt misalignment. Segment piston is determined from the location of the peak in the fringe contrast function, using all four camera images and light at four laser diode wavelengths. Although the CCAS has recently been replaced with a Shack-Hartmann sensor for mirror alignment on the HET, its operation and performance are described. Under less environmentally challenging conditions, such as laboratory or space-based applications, this instrument could be used for aligning segmented mirrors to high precision.