HUBBLE-SPACE-TELESCOPE MEDIUM DEEP SURVEY WITH THE WIDE-FIELD AND PLANETARY CAMERA .1. METHODOLOGY AND RESULTS ON THE FIELD NEAR 3C-273

We present results from the Medium Deep Survey (MDS), a Key Project us ing the Hubble Space Telescope. Wide Field Camera images of random fie lds have been taken in ''parallel mode'' with an effective resolution of 0''.12 FWHM in the V(F555W) and I(F785LP) filters. The exposures pr esented here were targeted on a field 6' away from 3C 273, and resulte d in similar to 5 hr integration time in each filter. Detailed morphol ogical structure is seen in galaxy images with total integrated magnit udes down to V similar or equal to 22.5 and I similar to 21.5. Paramet ers are estimated that best fit the observed galaxy images, and 143 ob jects are identified (including 23 stars) in the field to a fainter li miting magnitude of I approximate to 23.5. We outline the extragalacti c goals of the HST Medium Deep Survey, summarize our basic data reduct ion procedures, and present number (magnitude) counts, a color-magnitu de diagram for the field, surface brightness profiles for the brighter galaxies, and best-fit half-light radii for the fainter galaxies as a function of apparent magnitude. A median galaxy half-light radius of 0''.4 is measured, and the distribution of galaxy sizes versus magnitu de is presented. We observe an apparent deficit of galaxies with half- light radii between similar to 0''.6 and 1''.5, with respect to standa rd no-evolution or mild evolution cosmological models. An apparent exc ess of compact objects (half-light radii similar to 0''.1) is also obs erved with respect to those models. Finally, we find a small excess in the number of faint galaxy pairs and groups with respect to a random low-redshift field sample.