order to calibrate the photoelectric photometry we need to determine
the transformation coefficients of the colour equations linking the
instrumental response of each band to the international standard bands
of the photometric system adopted. This
is accomplished by plotting some proper quantities and by computing (by
means of least squares method) the coefficients of the colour
equations. For more details read 'Astronomical Photometry' by Henden
& Kaitchuck, or 'Photoelectric photometry of variable stars' by
Hall & Genet.
At OAC we have done this tens of times; below you can see an example of the
graphs obtained for each of the UBVR bands.