[vsnet-chat 5789] Re: [AAVSO-DIS] survey, pixy etc

[Arne Henden <aah@nofs.navy.mil>]

Taichi Kato wrote:
> ...If one feels a need
> to discriminate K and Ks (or K'), the discrimination between Rj and Rc
> is a matter of comparable significance.
   While I understand the basic intent here, I don't think these two
bandpass discriminations are of "comparable significance."  The difference
between Rc and Rj is small, and other than a zeropoint shift, the two
systems are reasonably close for moderate-color stars.  For the
K bandpass, one should remember that the K window is almost as wide
as the entire visible window.  Ks or K' is used to denote a filter
that covers the blue end of the window, dropping the last 200nm or
so on the red end.  That red end is where the thermal background due
to the atmosphere and telescope structure rises sharply, and removing
it from the bandpass drops the sky background by at least a factor of
two.
   There are four regimes of "filtered" photometry:  narrow-band, such
as Halpha, where everyone agrees with each other since the bandpass
is determined by the filter; medium-band, such as the Stromgren uvby system,
again pretty much determined by the filters; wide-band, such as Johnson
or SDSS, where you have to be careful how the filters/CCD are specified
to get even an approximate match, with transformations often necessary; and
unfiltered, where the response is entirely due to the CCD and atmosphere
and matching with anyone else is a real problem.  At the same time,
the amount of detected light increases as you widen the bandpass, so
it is always a compromise as to how photometry is performed.  For most
amateurs, going Johnson/Cousins is your best choice for all-around
general-purpose photometry.
Arne

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