NICMOS Instrument Handbook for Cycle 11

Formulae

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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 A2.1 and depending on the units of F. (This is simply derived, using the fact that d/d= c/2.

Table A2.1: 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.

Conversion Between Fluxes and Magnitudes

The spectral flux density F can be calculated from its magnitude as

where m is the magnitude and F_o 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 A2.2. 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 A2.2: 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

 

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.