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Near Infrared Camera and Multi-Object Spectrometer Instrument Handbook for Cycle 17 > Appendix B: Flux Units andLine Lists > B.2 Formulae

B.2 Formulae
B.2.1
One Jansky (Jy) is defined as 10–26Wm-2Hz-1, so it is a unit of measurement of the spectral flux density, Fν.
To determine Fλ from Fν in Jy, use the following formula:
where λ is the wavelength in microns (μm), and β is a constant chosen from Table B.1 and depending on the units of Fλ. (This is simply derived, using the fact that dν/dλ = c/λ2.)
Table B.1: Constants for Converting Fλ and Fν.
Fλ measured in
Wm-2μm-1
310–12
Wcm-2μm-1
310–16
erg sec-1 cm-2 μm-1
310–9
erg sec-1 cm-2 -1
310–13
Remember that 1W=107erg sec-1, and 1μm=104.
B.2.2
The spectral flux density Fν can be calculated from its magnitude as
where m is the magnitude and Fo the zero-point flux for the given photometric band. We list the central wavelengths and zero-point fluxes for the more commonly encountered photometric bands below in Table B.2. The CIT system was originally based on Beckwith et al. (1976, Ap.J., 208, 390); the UKIRT system is in fact based on the original CIT system, but with adjustments made largely owing to different filter bandpasses. It should be noted that for a given photometric band there will be small differences in the effective wavelength and zero-point flux from one observatory to another, and for astronomical objects with radically different colors, so these figures can only be treated as approximate.
λ[μm]
Fo[Jy]
Fo[Jy]
B.2.3
Surface brightnesses are generally measured in Janskys arcsec-2, MJy steradian-1 or magnitudes arcsec-2. If you have a surface brightness Sν in MJy steradian-1, then you can use:
.
If you have Sν in magnitudes arcsec-2, you can simply use the formula and zero-points as given in the previous section for point sources.

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