
cals: Plot residuals for reference stars. 

 This function takes the currently displayed region and looks for all
 stars which have 'reference' photometry and plots the difference
 between the reference magnitude (in the given filter) and the
 observed magnitude (same filter) versus one of several options:  the
 star color (based on the 'reference' photometry), the star magnitude,
 the airmass, time, secqunce, etc.

 These plots can use one of two options for the 'observed' magnitude.
 Either, the magnitude for individual measurement, using the currently
 set values for the CCD zeropoints (from the photometry database), or
 using the relative photometry determined for the particular star.  


dmags: plot difference between two measured magnitudes

resid: 

abszero

zeropts

cmd

