L and T dwarfs emit most of their radiation in the near infrared and their
spectral energy distributions are dominated by strong molecular absorption
bands. These highly structured energy distributions lead to JHK magnitudes that
are extremely sensitive to the exact filter bandpass used. In the case of the T
dwarfs, the differences between commonly used photometric systems can be as
large as 0.4 mag at J and 0.5 mag at J-K. To address this problem, we have
synthesized J,H and K magnitudes for some of the common photometric systems and
present transformation equations. If the spectral type of the dwarf is known,
our transformations allow data to be converted between systems to 0.01 mag,
which is better than the typical measurement uncertainty. Transforming on the
basis of color alone is more difficult because of the degeneracy and intrinsic
scatter in near-infrared colors of L and T dwarfs; in this case J magnitudes
can only be transformed to < 0.05 mag and H and K to < 0.02 mag.Comment: 26 pages including 9 figures, uses aastex, to be published in PASP
Jan 200