This document was directly transferred over from the PDS3 IRAS catalog documentation. PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "2003-06-10, S.McLaughlin, Copied from CN catalog; inserted instrument summary from instrument host catalog file. 2004-12-15, S.McLaughlin, Resolved liens." OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "IRAS" INSTRUMENT_ID = "FPA" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "FOCAL PLANE ARRAY" INSTRUMENT_TYPE = "DETECTOR ARRAY" INSTRUMENT_DESC = " Instrument Overview =================== The IRAS Focal Plane Array (FPA) was located at the focal plane of the IRAS Ritchey-Chretien telescope. The mirror surfaces were beryllium, and the baffles were aluminum coated with Martin Optical black. The telescope had an entrance pupil diameter of 57 cm, and a central obscuration diameter of 24 cm, for an effective collecting area of 2019 cm**2. The measured system focal length was 545 cm and the system F/ number was 9.56. The measured plate scale at the focal plane was 1.585 mm/arcminute. Imaging was diffraction limited at 25, 60 and 100 microns, but not at 12 microns. The telescope was cooled by contact with the superfluid helium tank to temperatures ranging from 2 to 5 K. The operating temperature of the FPA was 2.6 K. The focal plane assembly contained the survey detectors, visible star sensors for position reconstruction, a Low Resolution Spectrometer (LRS) and a Chopped Photometric Channel (CPC). The LRS was a slitless spectrometer sensitive from 7.5 to 23 microns with a resolving power of about 20. The CPC operated during some pointed observations. It mapped sources simultaneously at 50 and 100 microns, and used a cold internal chopper for flux reference. However, the focal plane temperature was lower than expected, which resulted in CPC detector anomalies that rendered CPC data very difficult to use. The FPA survey array consisted of 62 infrared detectors divided into eight parallel modules, two for each color band. Each module contained either seven or eight detectors. The layout of the detectors is given in the IRAS Explanatory Supplement, Beichman et al. 1988 [BEICHMANETAL1988]. The detectors were rectangular with their narrow dimension arrayed parallel to the scan path of infrared sources. The design was such that the image of a real source would cross a detector in one module and subsequently cross another detector of the same band in a second module, providing a 'seconds-confirmation'. This allowed for discrimination between real source and spurious sources, such as cosmic rays. Most of the detectors in each band had standard size apertures (see table below), with one or two being half-sized. The infrared detectors were sampled at 16, 16, 8, and 4 Hz at 12, 25, 60, and 100 microns, respectively. The response functions of the four spectral bands used in the focal plane array are given in the IRAS Spectral Response dataset distributed through PDS and are listed in the IRAS Explanatory Supplement. The detectors exhibited a photon-induced responsivity enhancement (hysteresis), evidenced as they crossed bright sources. This was particularly noted when comparing ascending and descending scans of the galactic plane at 100 microns. Detector responsivity was also a function of frequency (dwell time). The ratio of the responsivity at nominal survey scan speed of point sources to that of a very extended background was adopted as 0.78, 0.82, 0.92 and 1.0 at 12, 25, 60, and 100 microns, respectively. To check the photometric calibration, the internal reference sources were flashed at the beginning and end of every scan, including scans broken by the South Atlantic Anomaly (SAA). The internal reference sources were themselves regularly checked against an astronomical reference source. For diffuse emission, the secondary calibration source was an area of the sky with a smoothly varying sky brightness, free from point sources and near the north ecliptic pole and NGC6543. The area was called the total flux photometric reference or TFPR. The calibration was boot-strapped from a model of the annual brightness variation of the TFPR and extended to the rest of the sky. This variation was caused by the changing viewing geometry through the zodiacal cloud over the course of the mission. The brightness model for the TFPR has varied with time, affecting the resultant calibration and data products generated using that calibration. The Medium Resolution Zodiacal History Files make use of an earlier TFPR brightness model that assumed the orbit of the Earth to be circular. This is referred to as Version 2. The Low Resolution Zodiacal History File uses an improved model, taking into account the eccentricity of the Earth's orbit and making other corrections, such as more accurate field of view measurements for the detectors. These are explained more fully in Oken 1988 [OKENETAL1988] and Boulanger 1988 [BOULANGER1988]. Output from FPA detectors is typically reported in units of flux density (Jansky, where 1 Jy = 1.e-26 Watt/(meter**2)Hz), calculated from in-band flux (Watts/meter**2) assuming the external source function to be proportional to 1/frequency. Jy (BD) is converted to Watt/(meter**2) (IB) by the relation BD*K=IB, where K is given in Moshir et al. 1989 [MOSHIRETAL1989] and is listed below: Wavelength K 12 1.348e-13 25 5.155e-14 60 2.577e-14 100 1.000e-14 Instrument Summary ------------------ In the table below, the center wavelengths are in microns. For the survey array, the FOV is determined by the rather large detector mask size and is roughly the native resolution of the data in that band. The resolution of the IRAS image data is not governed by the resolution of the telescope, which was diffraction limited longwards of 12 microns, but by the size of the detectors. ------------------------------------------------------------------ SURVEY ARRAY Center # working FOV Bandpass Detector Average 10-sigma Wavelength detectors (arcmin) (micron) Material Sensitivity (Jy) 12 16 .75 x 4.5 8.5- 15 Si:As 0.7 25 13 .75 x 4.6 19 - 30 Si:Sb 0.65 60 15 1.5 x 4.7 40 - 80 Ge:Ga 0.85 100 13 3.0 x 5.0 83 -120 Ge:Ga 3.0 ------------------------------------------------------------------ CPC Center # working FOV Bandpass Detector Average 10-sigma Wavelength detectors (arcmin) (micron) Material Sensitivity (Jy) 50 1 1.2 41 - 63 Ge:Ga 7.0 100 1 1.2 84 -114 Ge:Ga 7.0 ------------------------------------------------------------------ LRS Slit width Wavelength Range Detector Resolving (arcmin) (micron) Material Power 5.0 8 - 13 Si:Ga 14-35 7.5 11 - 23 Si:As 14-35 ------------------------------------------------------------------ For more information see Beichman et al. 1988 [BEICHMANETAL1988]." 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