‘corrected to sky’ give our estimates of FWHM in arcseconds on the sky. The values are much as expected, except for the visible wavelength detector, which is about twice the expected value. The longer wavelengths of the VIS channel will cause more diffraction through the mask apertures but will not explain this amount of difference. The scientific impact is small, however, as the telescope optical resolution is poor because of passing through a beamsplitter. The main use of the optical channel is for tracking spacecraft drift via star centroids, and there will be no impact on this performance, since the much larger integrate mode images will be used.

We note also that the resolution as measured from images with low MCP gain is very similar. The percentage numbers given for these are the signal reduction when using low gain, as it is intended for use with bright stars which would cause detector damage at full gain.

In Figure 9, we illustrate the resolution in the case of two closely spaced point source images. The profile is approximated by a gaussian, but an average spot image is shown, and has a slightly narrower peak and some weak broad wings. The plot 