These are some useful Excel spreadsheets.

visibility_curve.xls (note: to process the Bessel function, you need to have the Analysis ToolPak Add-In enabled.)
Ever had a problem converting from JD to PTI day-of-year to calendar date (since PTI uses all 3?) This might help, maybe - not really updated for years after 2001.
Useful, maybe, for finding known good PTI calibrators.