# Tycho-2 photometric reference subset
# Created Jan 27, 2003 by Michael Richmond
#
# This file contains information on a subset of stars from the Tycho-2
# catalog.  The stars were selected to be good photometric references,
# by satisfying all of these conditions:
#
#         magnitude      Bt  >  1.0    (discards incorrect "zero" magnitudes)
#         magnitude      Bt  < 11.8    (discards faint stars)
#         magnitude      Vt  >  1.0    (discards incorrect "zero" magnitudes)
#         magnitude      Vt  < 10.7    (discards faint stars)
#         uncertainty in Bt  <  0.05
#         uncertainty in Vt  <  0.05
#         color   -0.2 < (Bt - Vt) < 1.8
#         no other Tycho-2 star within 50 arcsec
#         "solution" flag does not indicate double-star astrometric solution
#      
# For each such star, I estimated Johnson-Cousins magnitudes based on
# on message from Arne Henden:
#
#                 B  = Bt + 0.018  - 0.2580*(Bt - Vt)
#                 V  = Vt + 0.008  - 0.0988*(Bt - Vt)
#                 I  = Vt - 0.039  - 0.9376*(Bt - Vt)
#
# This file contains 360,741 stars.
# If spaced equally over the sky, they should provide about 9 stars
# per square degree.
#
# The RA and Dec are both expressed in decimal degrees, equinox 2000.
#
#     RA          Dec     B      V      I