5.3 Wavefront Quality

The conclusion of the extensive optical testing in Thermal Vacuum was that the cameras are well corrected to within the specifications. The dominant problem was a small difference in focus between the four cameras (Krist and Burrows 1995). The actuated fold mirrors and pick-off mirror mechanism performed flawlessly in correcting residual coma aberrations in the image, and enabled the on-orbit alignment procedures. Using out-of-focus images, a very accurate alignment of the cameras was accomplished. A side product was that the aberrations in each camera were measured (Krist and Burrows, Applied Optics, 1995). The results are given in Table 5.2. These values were used in generating the simulated PSFs given in Model PSFs. The WF3 wavefront error is higher than that of the other chips because it is the most out-of-focus relative to the PC (which is assumed to be in focus). It is the equivalent of about 10 microns of breathing out-of-focus.

Table 5.2: Aberrations in Each Camera. The numbers quoted are RMS wavefront errors in microns over the HST aperture (Zernike coefficients).

	Aberration	PC1	WF2	WF3	WF4
Z4	Defocus	0.0000	0.0410	0.0640	0.0480
Z5	0° Astig	0.0229	0.0109	0.0126	0.0163
Z6	45° Astig	0.0105	0.0041	0.0113	0.0190
Z7	V2 Coma	0.0000	0.0012	-0.0037	-0.0090
Z8	V3 Coma	-0.0082	0.0061	-0.0100	0.0019
Z9	X Clover	0.0063	0.0121	0.0010	0.0096
Z10	Y Clover	0.0023	0.0091	0.0130	0.0042
Z11	Spherical	-0.0131	-0.0215	-0.0265	-0.0247
Z22	5th Spherical	0.0034	0.0034	0.0036	0.0029
	Zonal Errors	0.0180	0.0180	0.0180	0.0180
	Total (RSS)	0.0353	0.0537	0.0755	0.0637
	Budget	0.0813	0.0794	0.0794	0.0794