Thanks for the follow-up on the A1.0 photometry. Despite Dave Monet's comments to the contrary, it has seemed to me that the bright stars (in the mag. 10 -13 range) were in fact no worse than fainter ones, and your analysis seems to bear this out. But clearly the non-linearities you have found for southern stars means the situation is complex. My compilation of faint sequences may be dense enough to allow one to make minor adjustments in each field, using the relevant GSPC sequence plus any others that may happen to fall on the same sky survey plate. \Brian