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https://heasarc.nasa.gov/W3Browse/all/tycho2.htmlВсего звёзд под миллион.
Bt_MagThe Tycho-2 BT magnitude, blank when no magnitude is available. Either BT (bt_mag) or VT (vt_mag) is always given. Approximate Johnson photometric magnitudes may be obtained as follows: V = VT - 0.090*(BT-VT) B-V = 0.850*(BT-VT) Consult Sect 1.3 of Vol 1 of "The Hipparcos and Tycho Catalogues", ESA SP-1200, 1997, for more details.Bt_Mag_ErrorThe standard error in the BT magnitude.Vt_MagThe Tycho-2 VT magnitude, blank when no magnitude is available. Either BT (bt_mag) or VT (vt_mag) is always given. Approximate Johnson photometric magnitudes may be obtained as follows: V = VT - 0.090*(BT-VT) B-V = 0.850*(BT-VT) Consult Sect 1.3 of Vol 1 of "The Hipparcos and Tycho Catalogues", ESA SP-1200, 1997, for more details.Vt_Mag_ErrorThe standard error in the VT magnitude.