Summary of YALO optical color terms

I use tranformations of the form used by Stetson's CCDSTD program:

O1 = M1 + A0 + A1*I2 + A2*X + A3*T
O2 = M2 + B0 + B1*I2 + B2*X + B3*T
O3 = M3 + C0 + C1*I3 + C2*X + C3*T
O4 = M4 + D0 + D1*I4 + D2*X + D3*T
A3=0.
B3=0.
C3=0.
D3=0.

where X=airmass, T=UT time during the night, I= library index
(B-V,V-R, V-I) etc), M= library magnitude (B,V,R,I), and O= observed
aperture magnitude.

Put another way, I am solving the following multi-linear equations for
the coeffs A0,A1,A2,A3,B0, etc where bvri are the observed mags and
BVRI are the tabulated ones:

b_obs = f(B,B-V,X,T)
v_obs = f(V,B-V,X,T)
r_obs = f(R,V-R,X,T)
i_obs = f(I,V-I,X,T)

Thus, if A1 = -0.05, then

b_obs =  b0 + B + -0.05*(B-V)

where b_obs is the observed magnitude on the natural system (aperture
mag), b0 is the zero-point of the fit (not given here), and B/B-V are
the library values of the transformation.

1998

Data taken on three nights, 24/5 - 26/7 Jan 1998

Only 24/5 was really photometric. I reduced the data using a cloudy
solution that just calculates the color terms.

A1 =  -0.086   0.005 <<
B1 =   0.025   0.005 <<
C1 =  -0.403   0.009 <<   ** Old R filter
D1 =   0.051   0.005 <<

2000

I had data from 7 nights: feb04,feb05,feb06,feb07,feb09,feb11 (all
2000).

Six nights were photometric. New R filter.

The following are the average between the absolute reductions and the
cloudy reductions:

A1 = -0.061  0.005
B1 = +0.022  0.005
C1 = -0.017  0.009
D1 = +0.047  0.005

2001

Data taken on feb03,feb04,feb05 

feb03 and 05 were photometric and I reduced the data absolutely. Feb04
was not and I used a cloudy solution.

The averaged color terms are:

A1 = -0.079 0.005
B1 = +0.018 0.005
C1 = -0.030 0.005
D1 = +0.045 0.005

The B color term has changed by slightly more than I would have
expected.