VIII.A. Introduction

IRAS Explanatory Supplement
VIII. Sky Coverage, Confusion, Completeness and Reliability
A. Introduction

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The main results concerning the sky coverage, completeness and reliability of the IRAS survey can be summarized by noting that: 1) 96% of the celestial sphere was covered sufficiently to appear in the catalog; 2) at Galactic latitudes |b| > 20° and at wavelengths of 12, 25 and 60 µm the catalog is essentially complete for sources brighter than about 1.5 Jy; and 3) fewer than one source in 1000 having two hours-confirmed sightings is a spurious object.

At 100 µm the situation is more complicated due to the effects of "cirrus"-like emission (cf. Low et al. 1984) that often affects point source fluxes and completeness and reliability even at high Galactic latitudes. Whether a particular point source is an independent entity such as a galaxy or a dense condensation in a cirrus wisp has to be carefully considered. It is advisable to check the values of the cirrus and confusion flags for 100 µm flux densities because these may indicate extended emission at the point source position (see Sections V.H.4, VII.H.2 and X.B.l).

In regions of high point source density such as the inner parts of the Galactic plane and the Magellanic Clouds, the completeness of the catalog is likely to be poor. More stringent criteria for the selection of catalog sources were applied in these regions to ensure a high reliability of the sources. It is hard to arrive at good estimates of the completeness and reliability, particularly since these are also regions of strong extended emission. The completeness is very low in regions affected by the Galactic plane "shadow" caused by the lagging of the noise estimator.

The catalog of small extended sources is considerably less complete and reliable than the point source catalog due to the cruder algorithms used and to the effects of confusion from larger extended structures. These results are discussed in Section VIII.E.

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