I.B. The IRAS Survey

ISSA Explanatory Supplement
I. Introduction
B. The IRAS Survey


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Complete details of the designs of the IRAS telescope and instruments, the IRAS sky survey and the IRAS data processing system, along with extensive descriptions of the IRAS data products, are contained in the IRAS Explanatory Supplement 1988, hereafter referred to as the Main Supplement. Short descriptions of the IRAS survey instrument and the survey design are included here for easy reference.

IRAS was launched into a Sun-synchronous polar orbit at 900 km altitude over the Earth's terminator to facilitate long scans of the sky along portions of circles centered on the Sun (Figure I.B.1 and Main Supplement, §III.B). This orbit geometry would have allowed the IRAS telescope to view the whole sky in exactly six months if it had remained pointed exactly 90° from the Sun. The IRAS survey strategy used the ability of the satellite to point at varying angles from the Sun to complete two confirming surveys of 98% of the sky and a third confirming survey of 75% of the sky within the ten month operating period of the satellite.
Figure I.B.1 A schematic drawing of the orbital geometry. The orbital altitude, 900 km, and inclination, 99°, combined with the Earth's equatorial bulge, led to a precession of the plane of the orbit of about 1° per day. As a result, the orbit plane constantly faced the Sun as the satellite orbited near the Earth's terminator. By pointing the satellite radially away from the Earth, the cold telescope was shielded from the heat loads from the Sun and Earth while providing natural scanning motion across the entire sky in about six months. A sequence of hours-confirming scans on the celestial sphere is also shown.
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Each confirming survey, called an HCON for Hours CONfirmation (href="../../exp.sup/ch3/A.html">Main Supplement, ), consisted of a series of Sun-centered scans in which the 1/2°-wide focal plane array was moved by 1/4° between scans. In this way a double coverage of the sky was accomplished by scans separated by up to 36 hours, allowing point source detections from one scan to be correlated with other scans to confirm the reality of detections. Two HCONs (HCON-1 and HCON-2) were performed concurrently in the first six months of the IRAS mission, with the second HCON lagging behind the first by a few weeks. Solar elongation angles, epsilon, of the telescope line of sight were roughly confined to 80°-100° during HCON-1 and HCON-2. The third survey, HCON-3, was begun after the completion of the first two and used the full available range of solar elongation (60°-120°) in an attempt to cover the whole sky in less than six months. The third HCON was only 75% complete when it was terminated by exhaustion of the IRAS liquid helium supply.

A significant feature of the IRAS survey strategy is that zodiacal emission, arising from interplanetary dust in the solar system, presented a constantly changing source of foreground emission through which IRAS observed. Two observations of the same point on the celestial sphere separated by as little as a few days would measure significantly different surface brightnesses because the Earth moved in its orbit and changed the line of sight through the zodiacal dust cloud. This variation produced steep gradients in individual HCON images where adjacent locations on the sky were scanned at different times. This prevented direct co-addition of separate HCONs. The variation in zodiacal foreground was most troublesome at 12 and 25 µm (15% to 30% depending on the HCON) which fall near the peak wavelength of the zodiacal emission. At the longer wavelengths, diffuse Galactic emission becomes much stronger than zodiacal emission, reducing the effects of zodiacal variation.
Figure I.B.2. A schematic drawing of the IRAS focal plane. The numbered rectangles in the central portion each represent the field of view of a detector, filter and field lens combination. The image of a source crossed the focal plane in the Y direction as indicated. The filled-in detectors were inoperative, while the cross-hatched detectors showed degraded performance during the mission.
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The focal plane array of the IRAS survey instrument consisted of 62 detectors with either 15 or 16 detectors at each of the four IRAS wavelengths (Figure I.B.2). The telescope was oriented so that, during survey scans, the image of the sky moved across the array in the long direction at 3.85' s-1, producing complete coverage of a 0.5°-wide swath of sky. The four staggered rows of detectors in each wavelength band were designed to provide slightly more than 100% overlap of the detectors during a single scan. This provides slightly more than two samples per detector in the cross-scan direction, which substantially undersamples the telescope point spread function at the shorter wavelengths. Sampling rates of 16, 16, 8 and 4 samples per second of the 12, 25, 60 and 100 µm detectors, respectively, combined with the 3.85' s-1 scan rate and the detector widths of 0.75', 0.75', 1.5' and 3.0', gives about a 50% oversampling in the in-scan direction. All 62 detectors did not operate correctly in orbit. Two nearly adjacent dead 25 µm detectors and one dead 60 µm detector left holes in the focal plane swath. Four noisy or partially blind detectors affected the 12 and 25 µm arrays (Figure I.B.2).


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