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4.1 Spatial Resolution and PSF

COSTAR has restored many of the FOC's envisioned capabilities, in that the COSTAR-corrected PSF contains more than 75% of the visible light within a radius of 0.1 arcsecond, while losing less than 20% of the light to the two reflections at the two extra mirror surfaces. The net increase in sensitivity is a factor of approximately 3-4 at visible wavelengths. COSTAR's improvement of the FOC PSF is illustrated in Figure 4.1, which shows the radial profile of an aberrated PSF image and a COSTAR-corrected image at 4860Å. Archive users should consult the FOC Instrument Handbook, version 3.0 for more details on the pre-COSTAR characteristics of the FOC.

Figure 4.1: PSF Before and After COSTAR

The encircled energy fraction

(

) is tabulated in the FOC Instrument Handbook, version 7.0 (Table 9) for various circular apertures. This quantity is normalized so that the encircled energy is 1.0 at a radius of 1 arcsecond (70 pixels). Figure 4.2 compares the encircled energy curves of the aberrated OTA, the COSTAR-corrected OTA, and a perfect diffraction-limited image from a 2.4m circular aperture with a 0.33 central obstruction, showing that the COSTAR-corrected FOC PSF approaches that of an ideal imaging system in both encircled energy performance and in the FWHM of the PSF core.

Figure 4.2: Encircled Energy Fraction and PSF Profile Before and After COSTAR

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