The plot shows astigmatism of the psf across the image plane. A short single exposure of the open cluster M7 was used, exposure time = 60 seconds. Ellipticity is shown by the length of the lines in the lower plot — and the lines are aligned with the position angle of the sources. The upper plot shows that ellipticity rises pretty sharply towards the CCD edges — from about 0.05 in the center to about 0.4 at the edge. n.b. ellipticity is defined as e = 1 – b/a, where a is the semimajor axis and b the semiminor axis. Ellipticity at the center of the field is about 0.05 – not 0.0. These results are for the ellipticity of the inner core of the PSF (it’s measured out to 3 pixels radius). Whether the power-law halo of the PSF is non-round on any scales is still under study.
