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We report on a large HST imaging survey of BL Lac objects, at spatialresolution ~10 times better than previous ground-based surveys. We focus ondata reduction and analysis, describing the procedures used to model the hostgalaxy surface brightness radial profiles. A total of 69 host galaxies wereresolved out of 110 objects observed, including almost all sources at z < 0.5.We classify them morphologically by fitting with either an exponential disk ora de~Vaucouleurs profile; when one fit is preferred over the other, in 58 of 69cases, it is invariably the elliptical morphology. This is a very strong resultgiven the large number of BL Lac objects, the unprecedented spatial resolution,and the homogeneity of the data set. With the present reclassification of thehost galaxy of 1418+546 as an elliptical, there remain no undisputed examplesof a disk galaxy hosting a BL Lac nucleus. This implies that, at 99%confidence, fewer than 7% of BL Lacs can be in disk galaxies. The apparentmagnitude of the host galaxies varies with distance as expected if the absolutemagnitudes are approximately the same, with a spread of +-1 mag, out toredshift z < 0.5. At larger redshifts, only 6 of 23 BL Lacs are resolved so thepresent data do not constrain possible luminosity evolution of the hostgalaxies. The collective Hubble diagram for BL Lac host galaxies and radiogalaxies strongly supports their unification.

TheHubble Space TelescopeSurvey of BL Lacertae Objects. I. Surface Brightness Profiles, Magnitudes, and Radii of Host Galaxies