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: http://www.stsci.edu/instrument-news/handbooks/nicmos/c11_reference.doc2.html
Дата изменения: Thu Aug 7 20:42:45 1997 Дата индексирования: Tue Feb 5 12:33:22 2013 Кодировка: |
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, though with the addition of a new band at 1.6 microns known as the H band and the development of two bands 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/arcsec2 in the case of surface brightness data (although IRAS maps are often presented in the rather grandiloquent 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) 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 matched to any of the standard ground-based photometric bands.
We are aware that some observers do not routinely use these units, and therefore below we give a set of simple formulae to use to convert between systems, and some conversion tables. In the future we will also make available, via the World Wide Web, software to perform conversions between different systems and units.