UBVRI multi-aperture photometry for 71 globular clusters in our own Galaxy

We present the results of a programme of multi-aperture photoelectric photometry in the Cousins UBVRI system for the integrated light of 71 globular clusters in our Galaxy. The apertures number nine and range in diameter from 9.6 to 270 arcsec. We present an analysis of the uncertainties introduced by all sources of error in studies of this nature. For most clusters, photon-statistical and pointing errors are negligible when compared with the sample size effects; our observations are in excellent accord with numerical simulations based upon known globular cluster luminosity functions. Amongst other things, we conclude that there is no evidence for significant colour gradients within globular clusters, with the sole exception of the cluster NGC 104 (47 Tuc). We discuss a simplified technique for determining the colour of globular clusters independently of a direct measurement of the sky brightness near the cluster; the method requires simply that there be no strong radial gradient of colour in the cluster and explicitly compensates for the line-of-sight field and sky contamination. We present the complete photometric data in tabular form.