Contours of constant optical depth to the disk of M31. These are given in (dimensionless) units of ﹩10^{-6}﹩. The isophotes (thick lines) are taken from Hodge & Kennicutt (1982), and are 19, 20, 21, 22, 23, 24, and 25 B mag arcsec⁻². The rectangular boxes show the field of view of the CFHT 12k CCD, centered on M31 and aligned along both the major and minor axes.