Deriving V Magnitudes from USNO-A2.0 Photographic Photometry

Eric Mamajek (U. Rochester)

Last updated: 26 April 2010

The USNO-A2.0 catalog (Monet et al. 1998) provides photographic magnitudes in a blue "B" and red "R" band. These magnitudes were derived from conversions between Tycho BV photometry and photographic photometric systems for various emulsions (O, E, J, F, etc.) constructed by Arne Henden (see documentation on the photometric calibration of USNO-A efforts). For some references for photometric conversions between various systems, see Seiichi Yoshida's Magntiude Systems and Color Conversions webpage. I was interested in some simple relations to convert the photographic magnitudes into standard V mangitudes (despite the sizeable uncertainties that typically accompany photographic photometry). Based on a (now defunct) website posted by Project Pluto, I had been previously using the following relations: VJohnson = 0.375*BA2.0 + 0.625*RA2.0, and (B-V)Johnson = 0.625*(B-R)A2.0.

I cross-referenced 678 unique UBVRI photometric standard stars from Landolt (1992, 2007) and Landolt & Uomoto (2007) with the USNO-A2.0 catalog to determine photometric conversions between the USNO-A2.0 photographic photometry and the standard system defined by Landolt. The Landolt UBVRI photometry is ultimately traceable to the UBV photometric system of Johnson & Morgan (1953) and the RI system of Cousins (1976) (see discussion in Landolt 1992). The coordinates from Landolt's papers were queried against the USNO-A2.0 catalog using Vizier, adopting a 10" search radius. To minimize against the effects of neighboring stars and misidentifications, I selected only those stars that had a single, unique match A2.0 entry within 10", and those that were not flagged as "ACT" (i.e. bright stars in the Tycho catalog, as one would not bother with photographic magnitudes for these brighter stars). Some of the Landolt standards are fast-moving proper motions stars, but no attempt was made to track down stars that had moved more than 10" between the USNO-A2.0 mean epoch and the epoch quoted by Landolt. I found 448 convincing matches between Landolt's standard list and an entry in the USNO-A2.0 catalog with a useful B and R magnitude listed. I did a linear Z(X,Y) fit to these data and found that 6 stars were clear statistical outliers using Chauvenet's criterion (there were most likely stars with incorrect A2.0 counterparts). After this single round of clipping, I derived the following color-magnitude relations based on the photometry for the remaining 442 Landolt standards:

VLandolt = 0.429 + 0.325*(BA2.0) + 0.654*(RA2.0)
{range: 11.6 < B < 18.0, 10.0 < R < 17.5 ; rms = 0.35 mag}

(B-V)Landolt = 0.546*(B-R)A2.0 + 0.235
{range: -1.5 < (B-R) < 3.5 ; rms(B-V) = 0.26 mag}

Top plot: A2.0 B-R color vs. Landolt B-V color. Clearly there is a discontinuity around ~A0 where B-V ~ 0.

Bottom plot: 3D plot showing USNO B magnitude on the X-axis, USNO R magnitude on the Y-axis, and Landolt V magnitude on the Z-axis.