2.2.2 What is the IR background?

In general, the IR background contribution is a combination of the zodiacal light, interstellar medium, and cosmic background radiation. At the shorter wavelengths, the background is dominated by zodiacal light, while at the longer wavelength end of the Spitzer spectrum, each of the three components may be contributing.

For IRAC and IRS, which lie at the shorter wavelength `blue' end of Spitzer instruments, the background is dominated by zodiacal light, so a general rule-of-thumb is that the background is (qualitatively) ``low'' near the ecliptic poles (absolute ecliptic latitude greater than about $60^\circ$), ``high'' if it is in the ecliptic plane (absolute ecliptic latitude less than $30^\circ$, say), and ``medium'' for all other ecliptic latitudes.

For MIPS, which lies at the longer wavelength `red' end of Spitzer instruments, the ISM dominates near the Galactic plane, and at high Galactic latitudes, observations can be limited by confusion of your target with background sources, the so-called confusion limit.

At present, the SSC has calculated the expected background for three lines of sight, with a range of ecliptic and Galactic latitudes, and quantified the background levels that correspond to the qualitative labels of ``low,'' ``medium,'' and ``high'' introduced above. The results are tabulated and plotted at:

g http://ssc.spitzer.caltech.edu/obs/bg.html

As we will see in §2.4, we can obtain a quantitative estimate of the background for any target position either within Spot, or with IRSKY. How to do this is described in detail in the worked examples in the chapters that follow. See §9.5.1 in particular.