1.1.6 Aberration Correction and Optical Alignment

WFPC2 contains internal corrections for the spherical aberration of the HST primary mirror. These corrections are made by highly aspheric surfaces figured onto the Cassegrain relay secondary mirror inside each of the four cameras. Complete correction of the aberration depends on a precise alignment between the OTA pupil and these relay mirrors.

Mechanisms inside WFPC2 allow optical alignment on-orbit. The 47° pick-off mirror has two-axis tilt capabilities provided by stepper motors and flexure linkages, to compensate for uncertainties in our knowledge of HST's latch positions (i.e., instrument tilt with respect to the HST optical axis). These latch uncertainties would be insignificant in an unaberrated telescope, but must be compensated for in a corrective optical system. In addition, three of the four fold mirrors, internal to the WFPC2 optical bench, have limited two-axis tilt motions provided by electrostrictive ceramic actuators and invar flexure mountings. Fold mirrors for the PC1, WF3, and WF4 cameras are articulated, while the WF2 fold mirror has a fixed invar mounting. A combination of the pick-off mirror and actuated fold mirror (AFMs) has allowed us to correct for pupil image misalignments in all four cameras. Since the initial alignment, stability has been such that mirror adjustments have not been necessary. The mechanisms are not available for GO commanding.