A recalibration of optical photometry: Tycho-2, Str\"omgren, and Johnson   systems
  I use HST spectrophotometry to analyze the calibration of 3 optical photometry 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 the new recalibration of grating/aperture corrections, vignetting, and charge transfer inefficiency effects produced by the STIS group and to consider the consequences of both random and systematic uncertainties. The new results reaffirm the good quality of both the Tycho-2 photometry and the HST spectrophotometry, but yield a slightly different value for ZP_B_T-V_T. For the Str\"omgren v, b, and y and the Johnson B and V filters I find that the published sensitivity curves are consistent with the available photometry and spectrophotometry 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 U filters. There I find that the published sensitivity curves yield results that are inconsistent with the available photometry and spectrophotometry, likely caused by an incorrect treatment of atmospheric effects in the short-wavelength end. I reanalyze the data to produce new average sensitivity curves for those two filters and new values for ZP_c_1 and ZP_U-B. The new computation of synthetic U-B and B-V colors uses a single B sensitivity curve, which eliminates the previous unphysical existence of two different definitions. Finally, I find that if one uses values from the literature where uncertainties are 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 this article should lead to a significant reduction of systematic errors when comparing synthetic photometry models with real colors and indices. 
J. Ma\'iz Apell\'aniz (STScI & ESA)