images taken contiguously and showed a radial variation no
greater than those in R. In addition, the C color term was found to be
~0.1 with the CTIO Tek 2K #4 CCD, which generally gave values up to
twice as large as this with the previous C filter.
A new set of 4-inch Washington C and M filters are available at both
KPNO and CTIO. These filters follow the recent prescription of Geisler
(1996 AJ, 111, 480)
designed to improve their characteristics. They
are all Schott glass filters and are thus not only cheap (you can buy a
set for about $1K) but have guaranteed optical quality, unlike the
Corning filters used previously. In addition, the C filter is about 20%
more efficient and is also a closer match to the standard filter, thus
resulting in significantly smaller color terms. This filter does have a
small red leak (peak of 0.15%) between 6700 Å and 7700 Å but this should
not cause significant errors (<< 1%) except for M stars or very
reddened objects. The M filter response is very similar to that of
previous M filters and has a comfortably small color term and
negligible red leak. Observations taken with both the CTIO and KPNO C
filters indicate that images are very good and that the color term has
indeed been improved. For example, during good seeing conditions at the
KPNO 4-m, C images actually had smaller FWHM and aperture corrections
than R images taken contiguously and showed a radial variation no
greater than those in R. In addition, the C color term was found to be
~0.1 with the CTIO Tek 2K #4 CCD, which generally gave values up to
twice as large as this with the previous C filter.
Everyone interested in obtaining Washington photometry should use these
new filters. They are designated as C and M 4 x 4 "set 2" at CTIO and
as KPNO filters 1580 (C) and 1581 (M). As noted in Geisler (1996), the
R and I (Kron-Cousins) filters are excellent substitutes for the
Washington T
and T
filters.
Doug Geisler, Ed Carder