In the infrared, as in the optical, the means of reporting source brightnesses and the units employed have varied considerably. In recent years, however,
magnitude systems have been used less frequently, and the most popular unit for expressing brightnesses, both for point source fluxes and surface brightnesses, is steadily becoming the Jansky. We have adopted the Jansky as the standard flux unit (and Jansky/arcsec
2 for surface brightness measurements) for NICMOS in our documentation and in observer-oriented software. Here we provide some simple formulae and tables to facilitate the conversion from other units into Jy
. A Unit Conversion Tool is also available on the NICMOS WWW site, at the following URL:
Infrared astronomy really began in the 1960s, when the vast majority of astronomy was still carried out in the visual region. Flux measurements were routinely reported in the UBV
magnitude system, and to attempt to integrate IR astronomy into this system, Johnson (Ap.J.,
134, 69) defined the first IR
magnitude system. This involved four new photometric bands, the J, K, L and M bands which were centered on wavelengths of 1.3, 2.2, 3.6 and 5.0 microns. These bands were defined not only by the filter bandpasses, but also by the wavebands of the ‘windows’ of high transmission through the atmosphere. In this system, all measurements were referred to the Sun, which was assumed to be a G2V star with an effective temperature of 5785K, and was taken to have a V–K color of roughly +2.2. From his own measurements in this system, Johnson determined Vega to have a K
magnitude of +0.02 and K–L=+0.04.
Until the early 1980s IR astronomical observations were restricted to spectra or single channel photometry, and most photometry was reported in systems at least loosely based on Johnson’s system. These systems added a new band at 1.6 microns known as the H band and two bands were developed in place of the one formerly defined by Johnson as the L band; a new definition of the L band centered on 3.4 microns, and a rather narrower band known as L' centered on 3.74
microns.
As the new science of infrared astronomy rapidly expanded its wavelength coverage, many new photometric bands were adopted, both for ground-based observations and for use by the many balloon- and rocket-borne observations and surveys. The differing constraints presented by these different environments for IR telescopes resulted in systems with disappointingly little commonality or overlap, and today the IR astronomer is left with a plethora of different systems to work with.
The IRAS survey results, which were published in 1986, presented observations made photometrically in four bands in the mid- and far-infrared, and mid-infrared spectra, and all were presented in units of Janskys, rather than defining yet another new
magnitude system. Since then, IR data from many sites around the world have been increasingly commonly presented in Janskys (Jy), or in Jy/arcsec
2 in the case of surface brightness data. IRAS maps are often presented in units of MJy/steradian.
Ground-based mid-IR photometry is usually carried out using the N and Q photometric bands, which are themselves defined more by the atmospheric transmission than by any purely scientific regard.
IRAS, freed of the constraints imposed by the atmosphere, adopted its own 12 micron and 25 micron bands, which were extremely broad and therefore offered high sensitivity. Similarly, NICMOS, being above the atmosphere, is not forced to adopt filter bandpasses (See
Chapter 4 and ) like those used at ground-based observatories, but instead has filters constrained purely by the anticipated scientific demands. Thus in practice NICMOS does not have filters precisely matched to any of the standard ground-based photometric bands. The remaining sections contain simple formulae to convert between systems (
magnitudes to Jy, etc.) and look up tables.
