[vsnet-chat 951] Re: More on VRI photometry

[aah@nofs.navy.mil]

  I've been out of town, so haven't been able to comment on the discussion by
Brian Skiff about Johnson vs. Cousins R,I photometry.  My own personal
experience has been the same as Brian's: all professional work is being
done with Cousins R,I filters.  The two main reasons are the excellent
quality of standards (Cousins and Landolt) and the accessibility,
magnitude and color range of Landolt's equatorial standards.  Astrophysically,
there is little difference between Johnson and Cousins filters, but Cousins
is preferred because you can calibrate your data better.
  If you already have Johnson R,I filters, can you transform the data taken
through those filters into the Cousins system?  The answer is provisionally
"yes".  You will have a fairly large color term (not normally wise, but
acceptable), but should get reasonable results for normal stars.  You will
have much larger problems for Miras and other red objects, or for unusual
objects such as SNe.  Bottom line:  if you already have Johnson filters,
and cannot afford purchasing Cousins R & I replacements, then use Landolt
standards to determine your transformations and note in any publication
of your photometry that your instrumental magnitudes were obtained through
the Johnson filters.
  As Brian stated, if you had to choose two (and only two) filters for
your CCD system, probably the best choice would be V and I, since the I
bandpass misses most of human light pollution and is less affected by
moonlight.
  Note that the Cousins I filter cuts off around 885nm (fwhm), so there is
no filter to cover the red extension (to 1100nm) available with a CCD.
It would be nice to add a very red filter, such as the Gunn z' (925nm,
with 145nm width) to the UBVRI complement.  Unfortunately, there are no
current calibrating standards for this filter at the level of the Landolt
standards.  SDSS (which uses Gunn filters) is using our 1.0m telescope to set
up their standards, but the results will not be available for a couple of
years.
Arne
--------------------------------------------------------------------
Arne Henden                               Instruments/software/CCDs
US Naval Observatory Flagstaff Station    Cepheids/photometry/IR
P.O. Box 1149                             http: ftp.nofs.navy.mil  
Flagstaff, AZ 86002-1149                  Voice: (520)779-5132
aah@nofs.navy.mil                         FAX:   (520)774-3626

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