ISSA Explanatory Supplement
A. General Overview
The Infrared Astronomical Satellite (IRAS) conducted a survey of 98% of the sky from low Earth orbit in four bands with effective wavelengths of 12, 25, 60 and 100 µm during a ten month period from January to November, 1983. The purpose of the survey was to produce an extremely reliable catalog of infrared point sources at a sensitivity unobtainable from within the Earth's atmosphere. The good stability of the IRAS infrared detectors allowed radiometry of extended astronomical sources with the IRAS survey data. The original extended emission atlas, known as SkyFlux and consisting of 16.5° square surface brightness images, was released along with the IRAS Point Source Catalog between 1984 and 1986 (IRAS Catalogs and Atlases: Explanatory Supplement, 1988, ed. C. A. Beichman et al. (Washington D.C.:GPO)). The IRAS Point Source Catalog is available for download.
The 16.5°× 16.5° SkyFlux images gave a broad view of infrared emission from the Galaxy and the solar system with high angular resolution and unprecedented sensitivity. It was clear, however, that large improvements in sensitivity and photometric accuracy were obtainable using knowledge gained in the production and analysis of the original IRAS data products. Accordingly, the Infrared Processing and Analysis Center (IPAC) reprocessed the IRAS data to produce improved images. The results of the reprocessing are now available as the IRAS Sky Survey Atlas (ISSA). The ISSA covers the sky with 430 fields. Each field is a 12.5°× 12.5° region centered every 10° along declination bands which are spaced 10° apart. There are two releases of the ISSA. The first release, in 1991, completely covers high ecliptic latitudes (||>50°) with some coverage down to || ~ 40°. The second release, in 1992, covers ecliptic latitudes of 50° >||>20° with some coverage down to || ~ 13°. The remaining fields between ecliptic latitudes -20° to 20° are released as a separate product, the ISSA Reject Set, so named because of their reduced quality compared to the rest of the ISSA. These fields are contaminated by zodiacal emission residuals and zodiacal dust bands. The ISSA Reject Set is usable for many applications but special care should be taken when using these data for photometric measurements (§ I.D.3 and §IV.F). This IRAS Sky Survey Atlas Explanatory Supplement describes in detail the production, analysis and formats of the Atlas.
The scientific motivation for ISSA is to present consistently calibrated infrared images of the entire sky from IRAS at spatial scales larger than 5'. The combination of calibration improvements (§III.A.2), removal of most of the zodiacal foreground (§III.C.2), and detector destriping (§III.C.3) result in a sensitivity increase of a factor greater than five over the SkyFlux images at short IRAS wavelengths. This is enough to reveal Galactic dust features previously invisible at 12 and 25 µm. Detector noise is the limiting noise of ISSA for small spatial scales at most locations above 20° ecliptic latitude. At latitudes within 20° of the ecliptic plane (the ISSA Reject region) the limiting noise is due to zodiacal dust bands and residual zodiacal emission.
The ISSA images are designed to give relative photometry for objects outside the solar system. They cannot be used for determining the absolute sky surface brightness. Section §II.B gives important details about the calibration of IRAS. The comparison of IRAS results with the Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) satellite should be understood before using ISSA images for quantitative photometric measurements (§IV.D.3).
The remainder of this Introduction provides a refresher on those aspects of the IRAS telescope and the IRAS survey needed to understand the ISSA images. It will define the terms and introduce the concepts used in this document. A collection of cautionary notes vital for the correct use of the ISSA images is also presented. An overview of the changes and improvements made since the SkyFlux atlas was released is presented in Chapter II. Chapter III gives a description of the processing used to produce the Atlas. Chapter IV presents results from analysis of the ISSA and the ISSA Reject images. Chapter V details the formats of the ISSA images.