The following calibration format can be used directly in 
IRAF.digiphot.photcal.invertfit. The instrumental magnitudes
mU, mB, and mV are -2.5*log(counts/s)+25.

For the these images, the average parameters for UBV are the following:
Airmass---1.73, 1.485, 1.67
Integration time (s)---1800, 1200, 900
Pixel scale (arcsec)---1.134, 1.134, 1.134

begin UFIT
      status 0
      parameters 2
              u1
              u2
      derivatives   2
              0.1
              0.1
      values 2
             5.7383
             -0.04809
      errors 2
            0.067
            0.089

begin BFIT
      status 0
      parameters 2
              b1
              b2
      derivatives   2
              0.1
              0.1
      values 2
             3.8163
             0.01101
      errors 2
            0.021
            0.024

begin VFIT
      status 0
      parameters 2
              v1
              v2
      derivatives   2
              0.1
              0.1
      values 2
             3.2426
             0.05024
      errors 2
            0.016
            0.021