K band

Within astronomy, the term K band refers to an atmospheric window within the near-infrared range roughly centered on 2.2 microns (2190 nm), which is about 136 THz. The term K band is also used for passbands of various photometric systems designed to use this window, e.g., with a full width at half maximum of 390 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 filter, e.g., citing "K = 1.5". The passband is incorporated into the 2MASS JHK photometric system, as well as other infrared photometric systems. Note that there is a little bit of variation in such bands regarding the central wavelength and the FWHM between different systems and surveys, but versions of the K band are generally designed to observe through the K atmospheric window.

The term K band also has other meanings outside astronomy and there are at least two K bands defined for radio communications, which are nowhere near the above infrared band. At least one of these radio-band terms is also used in radio astronomy, e.g., the K-band focal plane array (KFPA, 18-26.5 GHz) formerly on the Green Bank Telescope, which observed a range roughly falling within a defined communications K band (20-40 GHz).