ACS Encircled Energy Verification

George Hartig
26 Oct 2001

1. INTRODUCTION

The ACS image quality CEI specification is stated in terms of encircled energy (EE) of a point source within a diameter of 0.25 arcsec (0.10 arcsec for SBC), as projected on the sky through the HST OTA. For the WFC and HRC channels the requirement is 75% EE over the field at 632.8 nm, with a goal of 80%; for the SBC, the requirement is 30% at 121.6 nm. The WFC and HRC requirements have been directly verified with point source measurements at the specified wavelengths. However, the SBC image quality has not been tested at 121.6 nm and we rely on a set of images at the long wavelength end of its spectral range, together with modeling, to demonstrate likely compliance with the specification. A direct verification of the SBC image quality will be obtained, using observations of a star field through the F122M filter, after deployment of ACS in HST.

2. WFC AND HRC VERIFICATION

Verification of the WFC and HRC EE performance was performed in Feb 2001 in the RAS/HOMS OTA simulator at Ball Aerospace. ACS was in its flight configuration (less some hardware to be mounted external to the instrument enclosure), with WFC detector build 4 and HRC build 1, which were operated near the expected flight temperatures. Point source images were generated simultaneously at 9 field points, including field center and the four corners, by 9 He-Ne laser-fed single mode optical fibers for the WFC. A single fiber, moved in the RAS source plane for successive images at field center and near the 4 field corners, was used for the HRC.

For each image the encircled energy was determined using IDL routine qee.pro, which finds the image center, adjusts and subtracts the background level (assumed flat over the region of the image being analyzed), corrects for first-order geometric distortion, and computes the enclosed energy as a function of radius.

The results are summarized in Figures 1 (HRC) and 2 (WFC), which show the location of the images in the fields and their EE. In all cases the ACS CEI specification requirement is exceeded and the goal is met, indicating superb optical performance of the flight instrument over its fields of view. In each of these figures, the images are shown magnified and at their relative locations within their respective fields of view, which are outlined.

The excellent optical quality of the as-built ACS instrument is also evidenced in the aberration analysis presented in Tables 1 and 2, which list phase retrieval analysis results. Only the lowest order aberrations were fit and the results expressed as wavefront error in microns (RMS). At all field points of both channels the RSS of the coefficients of these terms is less than .05 waves at 633 nm, and are at levels predicted by the optical design. However, the image quality is degraded, as expected, by the CCD pixel response functions, due predominantly to charge diffusion in the silicon. This effect is particularly noticeable in comparison of the WFC and HRC image widths. The full width half maximum values presented in the tables were determined by fitting a gaussian, as integrated onto the pixels, to the images cores. The smaller physical size (15 vs. 21 microns) of the WFC pixels and the poorer sampling of the Airy disc combine to increase the core diameter of the WFC images to about 80 mas (1.6 px), while the HRC images enjoy diameters of about 60 mas (2.2 px), which is close to the theoretical optical limit. This can be seen in the figures, where the images for both the HRC and WFC are shown at about the same angular scale; while the HRC images clearly show the first diffraction ring, the ring is blended with the Airy disk in the WFC images.

• Fig. 1 : ACS/HRC Encircled Energy vs Field Position
• Fig. 2 : ACS/WFC Encircled Energy vs Field Position

Table 1 : ACS/HRC Image Quality Evaluation RAS/HOMS 22 Feb 01

Table 2 : ACS/WFC Image Quality Evaluation RAS/HOMS 22 Feb 01

3. SBC VERIFICATION

Since the SBC utilizes only the first 2 mirrors of the HRC optical train to correct and reimage the HST OTA focal plane to its MAMA detector, the excellent performance demonstrated by the HRC (which employs a third plane mirror to fold the optical path to the detector) is an indicator of the image quality that may be expected of the SBC. However, the confocality of the HRC and SBC must be established at both field center and over the field (match of the detector plane with the focal plane), and the performance of the MAMA detector must also be verified.

SBC image quality measurements were made on 11-12 May '99 at GSFC, using the RAS/Cal OTA simulator, with its 4 micron pinhole, illuminated with a deuterium lamp, serving as a point source. Although the ACS and RAS/Cal were kept under dry nitrogen purge, the flux in the resulting images is completely dominated by the strong Hg line at 184.9 nm emitted by the Hamamatsu deuterium lamp, as was shown in spectra produced by the SBC prisms. As was done for the HRC, focus scans were obtained, using the HRC/SBC corrector mechanism, at field center and near each corner of the SBC field, through filter F150LP. The resulting images showed that the MAMA detector was well aligned with the SBC focal plane, yielding good image quality at all 5 field points at the same corrector focus setting that optimized the HRC images. The encircled energy within the 0.15 arcsec diameter used for the focus curve assessment varied between .72 and .73 for all of the field points. The HRC detector was subsequently changed out, and the 2nd mirror in the HRC/SBC path was remounted to improve image stability, hence the confocality of the HRC and SBC was re-assessed on 5 Apr '01, with the ACS in its final flight configuration. These measurements of RAS/Cal images at field center only again demonstrated that the optimal focus for both channels is within +/-0.21 mm (in the detector plane) of one another, which, at the f/68 optical speed of these channels, permits a common focus setting with negligible effect on image quality.

To infer the encircled energy performance of the ACS SBC with the HST OTA at the requisite FUV wavelength (121.6 nm), we must rely on modeling, as supported by the phase retrieval estimates of the aberration content of the as-built HRC channel and of the OTA wavefront as well as OTA micro-roughness estimates. Using an IDL PSF modeling suite based on routines developed by John Krist for phase retrieval, and enhanced to include the laboratory-measured point response function for the MAMA detector, the encircled energy performance measured at 185 nm is matched with about .035 microns RMS of focus wavefront error (with no other aberrations). Adding the OTA micro-roughness and mid-frequency error maps determined from previous on-orbit measurements and modeling the resulting PSF at 122 nm shows that we may expect the encircled energy in the specified .10 arcsec diameter to be approximately .36, well in excess of the .30 requirement and approximately matching the goal of .35. The model image and its parameters are illustrated in Figure 3.

• Fig. 3 : SBC PSF Model at 122 nm with OTA