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We have carried out an independent quantitative evaluation of the galaxycomponent of the "COSMOS/UKST Southern Sky Object Catalogue" (SSC) and the"APM/UKST J Catalogue" (APM). Using CCD observations our results corroboratethe accuracy of the photometry of both catalogs, which have an overalldispersion of about 0.2 mag in the range 17 <= b_J <= 21.5. The SSC presentsexternally calibrated galaxy magnitudes that follow a linear relation, whilethe APM instrumental magnitudes of galaxies, only internally calibrated by theuse of stellar profiles, require second-order corrections. The completeness ofboth catalogs in a general field falls rapidly fainter than b_J = 20.0, beingslightly better for APM. The 90% completeness level of the SSC is reachedbetween b_J = 19.5 and 20.0, while for APM this happens between b_J = 20.5 and21.0. Both SSC and APM are found to be less complete in a galaxy cluster field.Galaxies misclassified as stars in the SSC receive an incorrect magnitudebecause the stellar ones take saturation into account besides using a differentcalibration curve. In both cases, the misclassified galaxies show a largediversity of colors that range from typical colors of early-types to those ofblue star-forming galaxies. A possible explanation for this effect is that itresults from the combination of low sampling resolutions with properties of theimage classifier for objects with characteristic sizes close to theinstrumental resolution. We find that the overall contamination by starsmisclassified as galaxies is < 5% to b_J = 20.5, as originally estimated forboth catalogs. Although our results come from small areas of the sky, they areextracted from two different plates and are based on the comparison with twoindependent datasets.Comment: 14 pages of text and tables, 8 figures; to be published in the  Astronomical Journal; for a single postscript version file see  ftp://danw.on.br/outgoing/caretta/caretta.p

A Quantitative Evaluation of the Galaxy Component of the COSMOS and APM Catalogs