IRAS Explanatory Supplement
I. Introduction
A. General Overview

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  1. The IRAS Mission
  2. The Explanatory Supplement
  3. Cautionary Notes

A.1 The IRAS Mission

The primary mission of the Infrared Astronomical Satellite (IRAS) was to conduct a sensitive and unbiased survey of the sky in four wavelength bands centered at 12, 25, 60, and 100 µm. The project was initiated in 1975 as a joint program of the United States, the Netherlands, and the United Kingdom. Launched in January 1983, IRAS ceased operations in November 1983 after having successfully surveyed more than 96% of the sky.

The results of several portions of the IRAS mission are given in a catalog of infrared point sources, in a catalog of extended sources smaller than 8', in a catalog of low-resolution spectra, and in an atlas of absolute surface brightness images of the entire infrared sky. These catalogs give the characteristics of some 250,000 point sources and 20,000 small extended sources down to a limiting flux density, away from confused regions of the sky, of about 0.5 Jy at 12, 25 and 60 µm and about 1.5 Jy at 100 µm for point sources, and about a factor of three brighter than this for small extended sources. The angular resolution of the instrument varied between about 0.5' at 12 µm to about 2' at 100 µm. The positional accuracy of sources detected by IRAS depends on their size, brightness and spectral energy distribution but is usually better than 20". Approximately 5000 8-22 µm spectra of survey sources brighter than 10 Jy at 12 and 25 µm are available.

A.2 The Explanatory Supplement

This Explanatory Supplement is intended to be a complete and self-contained description of the IRAS mission in relation to the products of the survey. In Chapter II, the IRAS satellite, telescope and focal plane instrumentation are reviewed. The elements of the mission profile--the constraints, the design features, and the in-flight modifications to that design--are described in Chapter III and are accompanied by a chronology of the events of the mission. In-flight tests of those aspects of the performance of the instrument directly associated with the survey are presented in Chapter IV. Chapters V and VI describe the processing performed on the data; the summaries that precede the detailed discussions should be sufficient to acquaint the user with the contents of the catalogs. Since the flux reconstruction and calibration of the instrument probably hold intrinsic interest for many readers, these are described separately in Chapter VI.

A preliminary analysis of some of the statistical properties of the catalogs is given in Chapter VII. Emphasis is placed on general statistics, such as positional and photometric accuracy and on easily derived number counts. A preliminary analysis of the sky coverage and of the completeness and reliability of the catalog is given in Chapter VIII. The low-resolution spectrometer and the analysis of its measurements are described in Chapter IX. Chapter X explains the format and meaning of each of the entries in the catalogs. Each printed volume of the catalogs repeats the description of the formats of that catalog.

In order to produce the catalogs in a timely fashion, some processing errors and anomalies could not be fixed; those which were discovered before the release of the data in November 1984 are described in Chapter XI. Chapter XII gives the errata and revisions as of 1987, including the important flux overestimation correction used for version 2.0 of the Point Source Catalog. A compilation of the names of people who worked on the IRAS project comprises Chapter XIII. The last chapter provides a series of plots giving the details of the coverage of the sky by the IRAS survey.

Each chapter of the Supplement was written by those members of the IRAS team whose names are appended to that chapter. The work described was obviously the result of efforts by many individuals and should not be ascribed to the authors alone.

A.3 Cautionary Notes

While it is unlikely that all aspects of the instrumental performance or the data processing will be of interest to all readers, even casual users should familiarize themselves with the various caveats described in the chapters appropriate to the type of data in question. All users of IRAS data should be cognizant of the following crucial facts:
  1. The sky at 100 µm is dominated by filaments termed "infrared cirrus" which, although concentrated near the Galactic plane, can be found almost all the way up to the Galactic poles (Fig. I.C.4). The primary, deleterious effects of the cirrus are that it can generate well-confirmed point and small extended sources that are actually pieces of degree-sized structures rather than isolated, discrete objects and that it can corrupt 100 µm, and occasionally 60 µm, measurements of true point sources (Sections V.H.4, VIII.D.2).

  2. The spectral bandwidths of the detectors were sufficiently wide that the quoted flux densities depend on the assumed energy distribution of the source. For the catalogs, the energy distribution was taken to be constant in the flux per logarithmic frequency interval. If the source has a different energy distribution than this, a color conection, as large as 50% in extreme cases, must be applied to the quoted flux densities (Section VI.C.3).

  3. The survey is clearly confusion limited within about 10° of the Galactic plane and in several areas of the sky such as the Ophiuchus and Orion-Taurus regions. Considerable effort has been made to select only highly reliable sources in such areas, at the expense of completeness. The flags associated with sources with possible confusion-related problems should be examined very carefully (Sections V.D.8, V.H.6, VIII.C, VIII.D and X.B).

  4. The algorithm used to estimate the detector noise suffered from a signifficant lag. This caused an under-estimate of the true noise when approaching regions of rapidiy changing noise and an over-estimate of the noise when leaving such areas. Regions with large and rapidiy varying numbers of sources, such as the Galactic plane, also produced this effect. Since the source detection algorithm (Section V.C) thresholded on signal to noise ratio, the overestimated noise level resulted in a dearth of sources, or a shadow, in the areas observed just after passage across the Galactic plane. At 60 and 100 µm, where the effect is worst, a "coverage hole" can extend as far as 2° from the plane. The density of detected sources can differ, totally artificially, by as much as a factor of ten from one side of the plane to the other due to this shadowing (Section VIII.D).

  5. While great pains were taken to confirm the reality of sources in the point and small extended source catalogs, no such attempt was made for the sky brightness images. Instead, separate images of the sky taken at times differing from weeks to months are given. It is the responsibility of the user to ensure that sources seen in the images are not due to transient sources such as asteroids.
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