Re: Re: USNO A2.0 "R"

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Michael Koppelman asked:

> It brings me back to my question, which I haven't found an 
> answer for, which is what does USNO mean by R?

  Going to google, I searched on "usno a2.0 catalog".  The third
item on the returned list was

       http://tdc-www.harvard.edu/software/catalogs/ua2.html

Section 3 of the document is a brief description of the photometric
calibration of the catalog.  The whole section is only about a page long,
so it's easy to read.  Here's an excerpt:


    The Tycho catalog provides B and V magnitudes for its stars. USNO-A2.0
    uses these and Henden's photometric conversion tables between (B,V) and
    (O+E+J+F) to set the bright end of the photometric 
    calibration for each plate.  This is an improvement over USNO-A1.0. 

    Unfortunately, GSPC-II and other large catalogs of faint photometric
    standards are not available, so the faint end of the 
    photometric calibration came from the USNO CCD parallax fields 
    in the North, and from the Yale Southern Proper Motion CCD 
    calibration fields (van Altena et al. 1998) for fields near
    the South Galactic pole. Hence, the faint photometric calibration 
    of USNO-A2.0 may not be any better than for USNO-A1.0. 
    Sorry. When better sources of faint photometric calibration data 
    become available, new versions of USNO-A will be compiled. 

  So the USNO R-band values are -- for bright stars -- derived from 
Tycho Bt and Vt.  How familiar :-)

                                              Michael Richmond