A cross-calibration between Tycho-2 photometry and HST spectrophotometry

I show that Tycho-2 photometry and HST spectrophotometry are accurate and stable enough to obtain a precise cross-calibration by analyzing a well-calibrated sample of 256 stars observed with both Hipparcos and HST. Based on this analysis, I obtain the following photometric zero points in the Vega magnitude system for Tycho-2: 0.020±0.001 (B_T-V_T), 0.078±0.009 (B_T), and 0.058±0.009 (V_T).