[vsnet-chat 3477] Re: re 2MASS data format ... also broad to narrow passband transformations

[aah@nofs.navy.mil]

I compare the current state of IR standardization to the
1950's period in photoelectric photometry: a few standards
have arisen, but the quality of the standard stars still
suffers.  Brian suggests Persson's standards have 0.5percent
quality; I think the jury is out on this one.  Suffice it
to say that several recent papers (and at least one in
preparation) are starting to agree at a much more precise
level than in the past.  However, systematics have always
played an important role in IR photometry, so don't trust
anyone yet!  Remember also that IR observations are taken
through atmospheric 'windows' that have real problems with
H2O, CO2, and emission lines; there is no such thing as
being able to get 0.01mag quality photometry from a single
night of observation since the extinction coefficients
and transparency vary throughout the night.
  The two short-wavelength versions of the K-bandpass
(K-short or Ks, and K') were primarily created to decrease
the thermal contribution from the sky, which is increasing
rapidly in the 2.3-2.4 micron region.  While several surveys,
such as 2MASS, use one of these shortened bandpass filters,
most high-precision work still uses the original Johnson
K filter (2.0-2.4 micron).  While you might think that
massive surveys like 2MASS and SDSS would take over the
world and redefine what filters are used, neither survey
is a photometric one and so the quality of the photometry
is certainly not 'standards' level.  Most professionals
that I know of just take the JHKs or u'g'r'i'z' data and
transform it into a system that they know and understand.
Arne

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