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].