[vsnet-chat 3474] Re: re 2MASS data format ... also broad to narrow passband transformations

[Brian Skiff <bas@lowell.edu>]

     Actually the business with the definition of the near-IR JHK passbands
is a little more convoluted than John suggests.  Yes, folks have pretty
much switched to the "K-short" passband for the commonly-used K system.
But the driver was that the system was defined by Johnson and colleagues
so that the red side of the passband was defined by the atmospheric water-
vapor cutoff.  The problem arose that folks were trying to replicate the
system from places like Mauna Kea and near sea-level:  the very high sites
had _too_little_ H2O while the low sites had too much, and as a result you
ended up with "Mount Stromlo" JHK, "South Africa" JHK, CTIO JHK, Mauna Kea
(and UKIRT, IRTF...) JHK, etc.  An effort to redefine the passbands was
made by Andy Young (1994 A&A Suppl 105, 259) under an IAU working group, and
although I'm pretty usre he was ignored, the high-accuracy IR standards
published by Eric Persson (1998 AJ 116,2475) ended up on nearly the 
recommended scale.  The Persson et al. standards define a system to the 
half-percent level for the first time; prior to that the primary standards
were good to only 0.03-0.05 mag. at best, and system-to-system differences
were on the same order.
     It could be that once the Sloan-Gunn system stuff standardized, the
bandwagon could shift by its sheer bulk so that the traditional UBVRIJHKLMN...
colors will fall by the wayside, and we'll be forced to shift to the Gunn
system.

\Brian

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