A Recalibration of Optical Photometry: Tycho-2, Strömgren, and Johnson Systems

I use HST spectrophotometry to analyze the calibration of 3 opticalphotometry systems: Tycho-2 B_T V_T, Str\"omgren u v b y, and Johnson U B V.For Tycho-2, I revisit the analysis of Ma\'iz Apell\'aniz (2005) to include thenew recalibration of grating/aperture corrections, vignetting, and chargetransfer inefficiency effects produced by the STIS group and to consider theconsequences of both random and systematic uncertainties. The new resultsreaffirm the good quality of both the Tycho-2 photometry and the HSTspectrophotometry, but yield a slightly different value for ZP_B_T-V_T. For theStr\"omgren v, b, and y and the Johnson B and V filters I find that thepublished sensitivity curves are consistent with the available photometry andspectrophotometry and I derive new values for the associated ZP_b-y, ZP_m_1,and ZP_B-V. The situation is different for the Str\"omgren u and the Johnson Ufilters. There I find that the published sensitivity curves yield results thatare inconsistent with the available photometry and spectrophotometry, likelycaused by an incorrect treatment of atmospheric effects in the short-wavelengthend. I reanalyze the data to produce new average sensitivity curves for thosetwo filters and new values for ZP_c_1 and ZP_U-B. The new computation ofsynthetic U-B and B-V colors uses a single B sensitivity curve, whicheliminates the previous unphysical existence of two different definitions.Finally, I find that if one uses values from the literature where uncertaintiesare not given, reasonable estimates for these are 1-2% for Str\"omgren b-y,m_1, and c_1 and 2-3% for Johnson B-V and U-B. The use of the results in thisarticle should lead to a significant reduction of systematic errors whencomparing synthetic photometry models with real colors and indices.Comment: 49 pages, 15 figures, 10 tables, accepted for publication in the February 2006 issue of the Astronomical Journal. This second version includes small corrections to Figs. 6 and 8 and to Table