  We take advantage of the availability of precision parallax data from Gaia
Data Release 2 together with machine learning to develop a set of equations for
transforming Tycho-2 (VT, BT) magnitudes into the Johnson-Cousins (J-C) system.
Starting with data for 558 standard stars with apparent magnitudes brighter
than 11.0, we employed one step supervised learning with weight decay
regularization and 10-fold cross validation to produce a set of transformation
equations from Tycho-2 into J-C, which in turn were used to derive
transformations of the Tycho-2 standard deviations into the J-C system. Both
the aggregated cross validation data sets and the in-sample results from the
final training were essentially unbiased (average errors << 1 mmag in both B
and V) and had error standard deviations comparable to those of the input data.
Comparison of errors in- and out-of-sample indicate modest generalization error
growth. Moreover, testing of the distributions of the normalized errors
indicated that the predicted standard deviations are accurate, enabling them to
be reliably employed in the suitability ranking of comparison star candidates.
These results thus enable utilization of a substantial portion of the 2.5
million star Tycho-2 data set as comparison stars for two-color bright star
ensemble photometry.
