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.