SIMBAD references

This paper presents photometric redshifts for 431 Abell clusters imaged as part of the Palomar Abell Cluster Optical Survey, of which 236 are new redshifts. We have obtained moderately deep, three-band (Gunn gri) imaging for this sample at the Palomar Observatory 60 inch (1.5 m) telescope, as part of the photometric calibration of the Digitized Second Palomar Sky Survey. Our data acquisition, reduction, and photometric calibration techniques are described, and photometric accuracy and consistency are demonstrated. An empirical redshift estimator is presented, utilizing background-corrected median g-r colors and mean g magnitudes for the ensemble of galaxies in each field. We present photometric redshift estimates for the clusters in our sample with an accuracy of σ_z=0.038. These redshift estimates provide checks on single-galaxy cluster redshifts, as well as distance information for studies of the Butcher-Oemler effect, luminosity functions, mass-to-light ratios, and many other projects.