Spitzer Documentation & Tools
Spitzer Telescope Handbook


Appendix A.                   Infrared Astronomy

Infrared Flux Units

The infrared flux density from a point source is most commonly given in units of Jansky (Jy) where:

1 Jy = 10-23 erg s-1 cm-2 Hz-1 = 10-26 Watts m-2 Hz-1 =


The conversion between Janskys and flux density in Wm-2 per unit wavelength is given by:


The infrared flux density from an area on the sky, such as the surface brightness of an extended object, or the background emission, is commonly given in 106 Jy steradian-1 = 1 MJy sr-1.  Another common unit is Jy per square arcsecond; 1 MJy sr-1 = 2.350443 × 10-5 Jy arcsec-2.


Tables 7.1 – 7.3 list the zero magnitude fluxes for various common optical and infrared filters.  Note that the photometric system of filter sets can vary, depending on the manufacturer.  The magnitude of a source can be converted to a flux density using:


There is an online Javascript tool available from the SSC Web site that interactively converts Janskys to magnitudes (or any of a variety of flux units) and vice versa.

Table A.1. 2MASS system zero points

Passband Effective wavelength (microns) Zero point (Jy)
J 1.235 1594
H 1.662 1024
Ks 2.159 666.7

References: Table 2 from Cohen, Wheaton, & Megeath 2003, AJ, 126, 1090; see also 2MASS All-Sky data release document: http://www.ipac.caltech.edu/2mass/releases/allsky/doc/explsup.html


Table A.2. Johnson system zero points

Passband Effective wavelength (microns) Zero point (Jy)
U 0.36 1823
B 0.44 4130
V 0.55 3781
R 0.71 2941
I 0.97 2635
J 1.25 1603
H 1.60 1075
K 2.22 667
L 3.54 288
M 4.80 170
N 10.6 36
O 21.0 9.4

References:  Allen’s Astrophysical Quantities, Fourth edition, 2001, Arthur N. Cox (ed.), Springer-Verlag; Campins, Rieke, & Lebofsky 1985, AJ, 90, 896; Rieke, Lebofsky, & Low 1985, AJ, 90, 900.

Table A.3 UKIRT system zero points

Passband Effective wavelength  (microns) Zeropoint (Jy)
V 0.5556 3540
I 0.9 2250
J 1.25 1600
H 1.65 1020
K 2.20 657
L 3.45 2.90
L' 3.80 2.52
M 4.8 163
N 10.1 39.8
Q 20.0 10.4

References: UKIRT Web page:


Infrared Backgrounds

Various astronomical sources emit radiation in the infrared part of the spectrum.  Cool stars (M class) have their peak emission just short of the near infrared.  However, stars with dusty envelopes or shells and circumstellar disks can be quite bright in the infrared.  Regions of star formation, HII regions, and planetary nebulae are strong infrared sources.  The (relatively) cool interstellar medium in galaxies has an infrared component.  There are also ultra-luminous infrared bright galaxies that are very strong sources of infrared radiation.


As in the optical, the infrared zody is concentrated toward the ecliptic with weaker emission, by approximately a factor of 4, toward the ecliptic poles.  The infrared zody is strongest from about 5 microns to about 30 microns with peak emission at about 10 microns.  The infrared zody has structure on most scales and, as observed from Earth, varies from season to season.  The intensity of the infrared zody will also vary with solar elongation, or how close to the Sun one is pointed.  The infrared zody is difficult to model.


As one moves to longer wavelengths (100 microns), diffuse Galactic emission from dust clouds in the interstellar medium becomes the dominant contribution to the infrared background.  This infrared cirrus is patchy, with higher concentrations found in the Galactic disk and toward the Galactic center.  However, it is important to realize that the cirrus is ubiquitous, and it is critical to examine the IRAS maps or radio maps of cirrus tracers when planning longer-wavelength observing.  Far-infrared emission from external galaxies in the field of view will add to the overall background flux.

Solar System Objects Included in Bright Object Avoidance








4 Vesta

6 Hebe

1 Ceres

7 Iris

15 Eunomia