<title>COSTAR Phase I alignment description</title>

Abstract

The corrective optics space telescope axial replacement (COSTAR) configuration contains mechanisms in each science instrument channel that allow for on-orbit correction for image plane focus and for lateral and axial mapping of the Hubble Space Telescope (HST) primary mirror onto the aspheric corrector mirrors. The optical alignment of the COSTAR optics is accomplished in two phases. In Phase I, the mirror bezel tilts and lateral positions are determined through the use of surrogate flat mirrors with the mechanism's positions held at the mid-range of their travel. The Phase I alignment is followed by Phase II interferometric optimization of all five optical channels. At the conclusion of the Phase I alignment, the optics are positioned accurately enough to allow simultaneous correction of most channels on orbit through the use of the mechanism compensation and telescope fme-pointing control. Individual mirror positions and orientations are determined through the use of alignment telescopes, theodolites, alignment lasers, and reference fiducials incorporated into the COSTAR Alignment System (CAS). The proper angles and positions are transferred from the surrogate mirrors to the flight optics, bezels, and shims through use of the alignment transfer fixture (AlT). The flight optics are then installed on the mechanism arms and aligned in decenter and roll. The tolerances on the mirror positions at the conclusion of Phase I alignment are mm in lateral position, 1 mm in axial position, arc sec in azimuthal and elevation angle, and degrees in roll angle.