Point Spread Function

5.2 Aberration Correction

WFPC2 has corrective figures on the relay secondary mirrors where the primary mirror is imaged; this optical correction recovers near-diffraction limited images over the entire CCD fields-of-view. Proper correction requires tight optical alignment tolerances, which are facilitated on-orbit by actuated optics. The corrective optics enable essentially all of the scientific objectives of the original WF/PC-1 to be met.

Table 5.1: Wavefront Error Budget.

Through a number of independent analyses, based on investigations of star images obtained on-orbit, and the examination of fixtures used during the figuring of the primary mirror, the aberrations of the HST optics were accurately characterized. The primary mirror was figured to an incorrect conic constant: -1.0139 +/- 0.005 rather than the -1.0023 design requirement, resulting in a large amount of spherical aberration. The optical design of WFPC2 creates an image of the OTA primary mirror near the surface of the relay Cassegrain secondary mirror in each of its channels. This design minimizes vignetting in the relay optics, but more importantly, facilitates correction of spherical aberration in the OTA primary by application of the same error (but with opposite sign) to the relay secondary. The optical figure of the WFPC2 secondary mirrors contains a compensating "error" in the conic constant. By adopting a prescription within the error bars for the HST primary mirror, corrective secondary mirrors were made with sufficient accuracy that the residual spherical aberration in the WFPC2 wavefront is small compared to other terms in the WFPC2 optical wavefront budget.

On the other hand, new and stringent alignment requirements were created by the steep optical figure on the corrective relay secondary mirrors. The primary mirror image must be accurately centered on the corrective mirror, and must have the correct magnification. Centering is the most demanding requirement. A failure to center accurately would create a new aberration in the form of coma. A misalignment of 7% of the pupil diameter introduces as much RMS wavefront error as was present in the form of spherical aberration prior to the introduction of corrective optics. The new requirements for alignment accuracy and stability led to the introduction of a tip-tilt mechanism on the pick-off mirror, to compensate for camera alignment uncertainties with respect to the OTA, and actuated fold mirrors which can compensate for internal misalignments. There was an additional term in the CEIS specification of the overall instrument wavefront error budget for alignment stability. It is lambda/25 RMS at 6328Å, as shown in Table 5.1.

"Design error" refers to the aberrations inherent in the design itself, which would be seen if the optics conformed perfectly to their specifications. All of the optics were fabricated and integrated into the WFPC2 optical bench. It was established on the basis of component tests, end-to-end optical interferometry, and through focus phase retrieval, that the WFPC2 optical system performed within the stated tolerances for "fabrication and alignment" in the laboratory environment. What remained was to demonstrate the stability of the optical alignment after launch vibration and in response to the thermal environment on-orbit. The "stability" line anticipated these uncertainties, and has been verified during early science operations.