2 Observations

All observations were made with the 1.0-m Ritchey-Chrétien telescope of the U.S. Naval Observatory, Flagstaff Station. A Tektronix/SITe $1024 \times1024$thinned, backside-illuminated CCD was used, along with Johnson UBV and Kron-Cousins RI filters. Images were processed using IRAF, with nightly median sky flats and bias frames. Aperture photometry was performed with routines similar to those in DAOPHOT (Stetson 1987). Astrometry was performed using SLALIB (Wallace 1994) linear plate transformation routines in conjunction with the USNO-A2.0 reference catalog. Errors in coordinates were typically under 0.1 arcsec in both coordinates, referred to the mean coordinate zero point of the reference stars in each field.

The telescope scale is 0.6763 arcsec/pixel, with a total field of view of around $11.4\times 11.4$ arcmin. Typical seeing was $\sim$2 arcsec. A 9 arcsec extraction aperture with concentric sky annulus was commonly used.

The reported photometry only uses data collected on photometric nights (transformation errors under 0.02 mag). For each such night, symbiotic field observations were interspersed with observations of Landolt (1983, 1992) standard fields, selected for wide color and airmass range. The mean transformation coefficients (cf. Henden & Kaitchuck 1990, Eqs. 2.9ff) are:

$$\pm\ 0.007$$ V: -0.020 (1)

$$\pm\ 0.007$$ B-V:  0.949 (2)

$$\pm\ 0.018$$ U-B:  1.072 (3)

$$\pm\ 0.005$$ V-R:  1.017 (4)

$$\pm\ 0.013.$$ R-I:  0.971 (5)

Second order extinction was negligible except for B-V, where a coefficient of -0.03 was used.

The symbiotic field photometry was usually performed as the field transited. In a few rare cases, observations were made at higher airmass to the West. In these cases, care was taken to obtain extinction measures at equivalent or higher airmass. Each field was observed on at least three nights. Since all primary standards used the same aperture as the secondary standards being established, and the apertures were large, no aperture corrections were necessary.