Re: USNO A2.0 "R"

[aah@nofs.navy.mil]

Oh, yes, one more important fact.  Michael K. wrote:
>I'm sure I could find this somewhere, but I'm wondering how the 
>USNO A2.0 "R" compares to the Cousins R. The reason I ask is 
>when I use the R magnitudes on the comp stars of the star I am 
>working on, they don't really match up. Using the USNO A2.0 R 
>magnitudes the difference between my two comp stars should be 
>0.7. With my data, using an average of over 200 measurements, 
>the difference is 1.2. When you split the difference I get 9.95 
>vs. USNO's 10.2 and 11.15 vs. USNO's 10.9.
  You need to understand the basic USNO-A catalog before you
start using any of the photometry.  USNO-A is a scan of the
POSS-I O(blue) and E(red) photographic plates.  These plates are
completely saturated for stars brighter than V=14.  It turns out
that at some level, you can use the star diameter as a measure
of the brightness and recover photometry for brighter stars.  That
is what is done in USNO-A, but a tenth magnitude star is about
30arcsec across on the plates.  Use Tycho2 and the R-band transformation
coefficients rather than USNO-A for such bright objects.  Another
useful source would be UCAC, since their magnitudes lie inbetween
V and Rc and are not saturated at 10th.
Arne