Polarization compensation during circumferential metrology of advanced x-ray astrophysical facility optics
The Advanced X-ray Astrophysical Facility (AXAF) is the third components of NASA's Great Observatory Program, following the Hubble Space Telescope and the Gamma Ray Observatory. It will be used to study high energy astronomical phenomena in the x-ray spectrum from 0.1 - 10 KeV (124 - 1.24 angstroms). The optical assembly is made up of a nested set of Wolter Type I x-ray telescopes, each consisting of a confocal parabola/hyperbola pair. The 12 near cylindrical, grazing incidence elements are 1 meter in length and range from approximately 0.5 - 1.2 meter in diameter. The instrumentation for circumferential metrology of the AXAF optics uses a standard laser displacement measuring system in order to make precision measurements over a large dynamic range. A unique requirement of this laser system is that it must rotate on a precision bearing during data acquisition. A major error source minimized during the design of the instrument is the thermal load introduced by the laser head. In the final design, a series of quarter wave plates is used to decouple the laser from the rotating optical system while at the same time maintaining the desired polarization states of the incident laser beams. These polarization optics also introduce an angularly dependent path length error into the measurement system that, uncorrected, exceeds the requirements for the metrology system as a whole. This paper describes the details of the system, as well as the calibration method used to eliminate this error source.