What is Tycho photometry?
Tycho was an instrument on the Hipparcos satellite launched in 1989. Tycho was a scanning star-mapper that was less precise than main instrument although still extremely precise compared to most ground-based measurements but that allowed somewhat fainter stars to be measured in two colours, roughly approximating to the B and V bands. The Tycho-2 catalogue, the most recent and definitive, includes 2 539 913 stars (about 60 per square degree of sky) and is almost complete to magnitude V=11. Although typically an individual Tycho measurement had rather large errors, Tycho took many tens of measures on average one hundred and thirty thus the uncertainty in the final Tycho magnitude is normally about a factor of ten better than the standard deviation of the measures for any star. Note that only the Tycho-2 catalogue should be used. Previous Tycho photometry suffered from some difficulties, as is logical in such a vast undertaking and these have had to be ironed-out progressively with time.
