ALIGNMENT FACILITY

All 336 grating facets are installed onto the HETG support structure and aligned to a reference grating. For the "roll alignment": A polarization modulated laser source is located at the AXAF on-axis image position relative to HETG. The time average modulation direction is determined with respect to an alignment reference polarizer attached to the alignment reference mirror fixture by detecting the second harmonic of th ewave transmitted through the reference polarizer. The transmitted wave is detected with a lock-in amplifier as described by Anderson, Levine and Schattenburg in "Transmission X-ray diffraction grating alignment using a photoelastic modulator." For "tip" and "tilt" alignment: The reflected laser beam is located with respect to the nominal HETG focal point location and the X and Y distances from this origin are noted.