A typical Fizeau interferogram of an eroded window is shown in Fig. 7. Such interferograms were made while the window was fitted to the laser, using the outside surface of the window (flat to X/10) as the reference. From the shapes of the interference fringes, the surface radii rt and rs were computed. The astigmatism was calculated from a knowledge of the mirror radii and the angle between the cavity axis and the window. In actual operation, the laser permits considerable variation of this latter angle (by adjusting the mirror tilt); this adjustment was used to make fine variations of the astigmatism.