IRAS Explanatory Supplement
III. The IRAS Mission
A. Requirements
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The IRAS mission was designed to observe the entire sky in the infrared and to insure that the resultant survey was reliable and complete down to a specifiable flux level. This requirement was specified numerically as a catalog of point sources over 95% of the sky, that was 98% complete in unconfused regions and 99.8% reliable. To create accurate maps of the total sky brightness, observations of diffuse emission also needed to be free of significant background variations due to local effects such as Sun and Earth shine. It was desirable to observe the same distant sky through different regions of the solar system so that the effects of solar system backgrounds could be separated from Galactic and other backgrounds.
Many non-astronomical sources of infrared radiation were detected by IRAS, including contaminants released by IRAS itself, Earth orbiting dust particles, Earth orbiting satellites and other space debris. Asteroids and comets were also detected. The difficulties caused by dust particles and debris can be highlighted by noting that a dust particle of diameter 100 µm radiating as a blackbody at a temperature of 200 K could be seen by the 12, 25 and 60 µm detectors with a signal to noise ratio of >3 for distances closer than about 18 km. The infrared detectors were also sensitive to impacts by cosmic rays and charged particles from the Earth's trapped radiation belts. The residual effects of these impacts could not be entirely removed by the on-board nuclear pulse circumvention electronics (Section II.C.5) and so were a potential source of difficulty.
The requirement for a reliable survey, along with the above considerations, necessitated multiple observations of the sky to discriminate between the fixed (distant) astronomical objects and the moving (nearby) objects in the solar system, and against noise sources such as particle radiation effects on the detectors. By reobserving the sky on a time scale of seconds the residual effects of the radiation hits and the fastest moving local material were recognized and rejected (seconds-confirmation); by reobserving on a time scale of hours slower moving but still relatively local sources were recognized (hours-confirmation); and by reobserving on a time scale of weeks and months, slowly moving solar system objects such as asteroids were recognized (weeks-confirmation).
It should be borne in mind that the purpose of multiple coverage was to enhance the reliability of the point source detections, not to coadd data to enhance sensitivity.
The demand that a source should be seen twice within seconds was fulfilled by the design of the focal plane. A mission design plan (McLaughlin 1984) and a survey strategy plan (Lundy 1984) to achieve the required, homogeneous, multiply-confirmed coverage of the whole sky were evolved subject to the instrumental and observational constraints. It is necessary to explain the observational constraints that had to be met as these are important for understanding the strategy and the reasons that, in practice, the sky coverage was neither completely uniform nor homogeneous.
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