One Jansky (Jy) is defined as 10–26Wm

-2Hz

-1, so it is a unit of measurement of the spectral flux density, F

ν.

To determine Fλ from F

ν 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 F

λ. (This is simply derived, using the fact that d

ν/d

λ = c/

λ2.)

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.

Surface brightnesses are generally measured in Janskys arcsec-2, MJy steradian

-1 or magnitudes arcsec

-2. If you have a surface brightness S

ν 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.