This sight describes the Lyot test pretty nicely, and with regard to the phase contrast images we just saw on this list, have a peak at the images shown here.... http://www.optics.arizona.edu/jcwyant/Short_Courses/SIRA/4-MeasurementOfSurfaceQuality.pdf Dominic-Luc Webb _______________________________________________ ATM mailing list http://www.atmlist.net/