Ninety-six percent of the sky was covered with the two or more HCONs required for sources to be considered for inclusion in the catalog. A three HCON coverage was achieved over 72% of the sky. Roughly l5% of the sky received more than three HCON coverages. Areas receiving more than three HCON coverages arose mainly from the minisurvey (see below), from the overlap between lunes used in the survey strategy, from rescans scheduled to fill in coverage holes and from the tendency of scans to overlap at high ecliptic latitudes.
From the standpoint of the IRAS survey itself the area of the "minisurvey" was the most important area receiving extra coverage (Section III.C.11). The particular area was selected because it was available immediately after the telescope cover was ejected. The region was surveyed intensively to check the source detection and confirmation algorithms and to verify that the chosen survey strategy provided the desired completeness and reliability; see Rowan-Robinson et al. (1984). The minisurvey consisted of two strips of the sky centered approximately at ecliptic longitudes 60° and 250° and contained areas both of high source density near the Galactic plane and of reasonably low source density. Parts of the minisurvey received as many as four HCON coverages during the minisurvey proper. The whole minisurvey area received three more HCON coverages during the main survey. Thus some regions received as many as seven HCONs.
The part of the sky with no HCON coverage is entirely contained within two strips on opposite sides of the sky each roughly 5° wide and 60° long. This part of the sky was not observed because of the operational difficulties explained in Section III.D.5. At the borders of the gap there are strips 0.5° wide that received only one HCON coverage. In addition, there are various small areas of the sky which received only one HCON coverage. for the single HCON areas, comprising in total some 500 sq. deg, there are no entries in the catalog. The main cause of the small holes was the inability to take useful data when the spacecraft passed through the South Atlantic Anomaly (the SAA, see Section III.D.4). These areas are mainly concentrated in the south (see the detailed area coverage plots in Chapter XIII). Hours-confirmed detections made in the single HCON sky are contained in the catalog of rejected point sources.