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4.1 Spatial Resolution and PSF

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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 ε(λ) 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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Last updated: 11/13/97 16:36:34