ZTAU is a CLASS macro that estimates the zenith opacity based upon the
calibration scans.  It was a little tricky to write because the CLASS header
documents the opacity at the elevation of the current calibration not the
zenith, and the CLASS header for a calibration scan gives opacities for the
previous calibration (since the calibration calculation can't be done until
after the observation is over). 

The macro is named ZTAU and is in the FDATA:[OBS] directory.  Since it uses the
reduced CLASS spectra, you must first do a FILE IN on the CLASS file.  To run it
type 

	@FDATA:[OBS]ZTAU <first scan> <last scan>

where <first scan> and <last scan> specify the scan number range to consider.
It first asks for a backend (AOSA, AOSB, AOSC, or FBS).  (If you're running
CLASS on Frankenheim, chances are that you are in you "initials" directory.
If so, just type @[-]ZTAU ...)  ZTAU then FIND's all the calibration scans 
made with the specified backend in the scan number range.  Next it looks for
the first non-calibration scan (with that backend) that follows each of the
Cal's that it found.  Finally it prints the opacity scaled to the zenith, the
rest frequency, the UT hours, and the two scans used (Cal & non-Cal).