Histogram of the FWHM in the AL direction for all combinations of field of view, CCD and gate in the AF instrument at a mean colour, derived from a preliminary PSF calibration for two-dimensional windows. The solid black line shows the total; the dashed and dotted lines show the preceding and following fields of view respectively. The median FWHM is 103 mas (1.75 pixels); in most cases the FWHM is below 108 mas, although there is a tail to 132 mas populated by CCDs at one corner of the focal plane.