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---1.15, 1.19, 1.44
Integration time (s)---1800, 2700, 1200
Pixel scale (arcsec)---1.134, 1.134, 1.134

begin BFIT
      status 0
      parameters 2
              b1
              b2
      derivatives   2
              0.1
              0.1
      values 2
             3.918024
             -0.0270041
      errors 2
            0.00008
            0.00009

begin VFIT
      status 0
      parameters 2
              v1
              v2
      derivatives   2
              0.1
              0.1
      values 2
             3.273386
             0.01715033
      errors 2
            0.0115
            0.01237
# Thu 13:26:47 11-Jul-2002
begin	UFIT
	status	0	(Solution converged)
	variance	0.00348023
	stdeviation	0.05899347
	avsqerror	0.003478821
	averror		0.05898153
	avsqscatter	0.002500445
	avscatter	0.05000445
	chisqr		1.000405
	msq		0.003374769
	rms		0.05809276
	reference	mU
	fitting		(UB+BV+V)+u1+u2*XU+u3*BV+u4*UB*XU
	weights		photometric
	parameters	4
		u1	(fit)
		u2	(constant)
		u3	(constant)
		u4	(constant)
	derivatives	4
		0.1
		0.1
		0.1
		0.1
	values	4
		5.267
		0.453
		-0.125
		0.
	errors	4
		0.01
		0.
		0.
		0.