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B.2 Formulae
 B.2.1 Converting Between Fν and Fλ
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.)
 Fλ measured in β Wm-2μm-1 3×10–12 Wcm-2μm-1 3×10–16 erg sec-1 cm-2 μm-1 3×10–9 erg sec-1 cm-2 Å-1 3×10–13
Remember that 1W=107erg sec-1, and 1μm=104Å.
 B.2.2 Conversion Between Fluxes and Magnitudes
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.
 Band λ[μm] Fo[Jy] (CIT) Fo[Jy] (UKIRT) V 0.56 3540 3540 R 0.70 2870 — I 0.90 2250 — J 1.25 1670 1600 H 1.65 980 1020 K 2.2 620 657 L 3.4 280 290 L' 3.74 — 252 M 4.8 150 163 N 10.1 37 39.8 Q 20.0 10 10.4
 B.2.3 Conversion Between Surface Brightness Units
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.