IRAS Explanatory Supplement
I. Introduction
C. Overview of Infrrared Sky

The various depths of coverage by the IRAS survey are displayed in Fig. I.C.1. The clear areas in the middle plot were covered with at least two sets of confirming scans, while the clear areas in the bottom plot were covered with confirming scans three or more times. Because the basic requirement for inclusion in the IRAS catalogs was that an object had to be observed with at least two sets of confirming scans, the clear portion of the middle plot represents the basic area covered by the IRAS survey. The shaded areas in the top plot show the areas of sky that were missed entirely.

The general distribution of well-confirmed point sources observed by IRAS is shown in Galactic coordinates in Fig. I.C.2 to I.C.4. Three classes of source covering almost all objects in the point source catalog can be defined according to spectral energy distribution: most of 130,000 sources that are brighter at 12  µm than at 25  µm are stars (Figure I.C.2); most of the 50,000 objects that are brighter at 60  µm than at 25  µm and which are located more than 20 degrees from the Galactic plane are external galaxies (Fig. I.C.3); most of the 35,000 sources detected only at 100  µm are cold, dense clumps within the interstellar cirrus (Fig.I.C.4).