6.3 Image Characteristics

A two-dimensional Gaussian fit to a typical imaging point-spread function (PSF) has a FWHM of 1.97 pixels (47.1 mas), with 13.4% of the light in the brightest pixel. Because the HST focus varies with orbital phase, FWHM values can range from 1.8 to 2.4 pixels.

As discussed in Section 3.3, mid-frequency wavefront errors (MFWFEs) contribute significantly to the PSF wings at wavelengths < 2500 Å, so the spatial resolution of a point source will depend somewhat on its spectral energy distribution. For an M star, which has little photospheric flux at the shortest wavelengths, the image would be close to diffraction limited. For a hot white dwarf, the MFWFEs would have the maximum impact on the spatial resolution.

Deep images reveal the detailed shape of the COS imaging PSF. Figure 6.4 shows the PSF, averaged over 180 degrees of azimuth and plotted on a log scale to reveal its extended wings. Figure 6.5 shows a 2-dimensional grayscale image of a deep imaging observation that reveals two low-level "ghosts" located approximately 20 pixels to the right and the upper left of the center of the PSF.

The peak intensity of the brightest of the two ghosts is roughly 0.1% of that of the main PSF. These features may complicate the analysis of faint objects located in the wings of a brighter object.