(atmospheric window centered around 2.2 microns, or 136 THz)
Within astronomy, the term J band refers to an atmospheric window
within the near-infrared range centered on 1.22 microns (1220 nm),
i.e., 240 THz, and a photometric systempassband aimed at this
range, e.g., with a full width at half maximum of 230 nm.
In addition to infrared-specific telescopes,
many optical telescopes can observe in this range.
The letter is also used to indicate the magnitude of the signal
observed through the passband filter, e.g., citing "J = 1.5".
The passband is incorporated into the 2MASSJHK photometric system, as
well as other infrared photometric systems.
Note that there is some variation in such bands, regarding
the central wavelength and the FWHM, between different systems and
surveys, but versions of the J band are generally
designed to observe through the J atmospheric window.
The term J band also has other meanings outside astronomy,
one being a radio communications band, which is
nowhere near the above infrared band.