The NASA Infrared Telescope Facility is engaged in a long-term program to improve the image quality of the telescope.
One element of the program is to minimize the static aberrations. The largest static aberration is spherical aberration,
although aberrations caused by zonal polishing rings and support-pad print-through on the primary mirror are also
significant. To correct these static wave front errors, a new secondary mirror is being fabricated with a custom, phase
compensating surface. Since the as-built optical specifications for the IRTF mirrors have been lost, a configurable multimode
instrument was fabricated for use at both the prime and Cassegrain foci to characterize the primary mirror and to
measure the wave front errors at both foci. The instrument modes include a focal plane camera, a knife-edge tester, a
pupil viewer, a Hartmann wave front sensor, a calibrator, and an on-axis guider. Test results from the prime focus show
that the primary mirror has an incorrect conic surface and is poorly supported, which results in a fixed amount of
spherical aberration and variable amounts of astigmatism, coma, and trefoil. Cassegrain focal plane results show that the
original secondary mirror mount system also induces aberrations. Two new secondary mirrors have been made and at
least one of the mirrors will have a custom surface, using ion beam polishing methods, to correct these static aberrations.
An analysis is presently underway to determine the optimum compensating surface to be applied by ion beam polishing.