Space Telescope Science Institute  B.1 Infrared Flux Units B.3 Look-up Tables

B.2 Formulae


B.2.1 Converting Between F and F

One Jansky (Jy) is defined as 10-26Wm-2Hz-1, so it is a unit of measurement of the spectral flux density, F.

For F in Jy, use the following formula:

where is the wavelength in microns (µm), and is a constant chosen from Table B.1 and depending on the units of F. (This is simply derived, using the fact that d/d= c/2.

Table B.1: Constants for Converting F and F
F measured in
Wm-2µm-1
3x10-12
Wcm-2µm-1
3x10-16
erg sec-1 cm-2 µm-1
3x10-9
erg sec-1 cm-2 Å-1
3x10-13
)

Remember that 1W=107erg sec-1, and 1µm=104Å.

B.2.2 Conversion Between Fluxes and Magnitudes

The spectral flux density F 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.

Table B.2: Effective Wavelengths and Zero-points for Photometric Bands
Band
[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 S in MJy steradian-1, then you can use:

.

If you have S in magnitudes arcsec-2, you can simply use the formula and zero-points as given in the previous section for point sources.


B.1 Infrared Flux Units B.3 Look-up Tables
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