OBJECT                                       = SPACECRAFT_INSTRUMENT          
SPACECRAFT_ID                                = IRAS              
INSTRUMENT_ID                                = FPA             
OBJECT                                       = INSTRUMENT_INFORMATION          
INSTRUMENT_NAME                              = "FOCAL PLANE ARRAY"
INSTRUMENT_TYPE                              = "DETECTOR ARRAY"
INSTRUMENT_DESC                              = " The IRAS Focal Plane Array 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.
\n
\n
The FPA contains 62 infrared detectors, divided into eight parallel modules, 
two for each color band. Each module contains either seven or eight detectors. 
The layout of the detectors is given in the IRAS Explanatory Supplement. 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 the second module, providing a 
'seconds-confirmation'. This allows for the discrimination between real and 
spurious sources, such as cosmic rays.
\n
\n
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 4 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.
\n
\n
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.
\n
\n
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 (DC) was adopted as 0.78, 0.82, 0.92 and
1.0 at 12, 25, 60, and 100 microns, respectively. 
\n
\n
To check the photometric calibration the internal reference sources were
flashed at the beginning and end of every scan, including scans broken by
the 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 bootstrapped 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. 
\n
\n 
The brightness model for the TFPR has varied with time, effecting 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 assumes 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 and Boulanger
1988.
\n
\n
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 inband 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 and is listed below:

			Wavelength         K
                           12          1.348e-13
                           25          5.155e-14
                           60          2.577e-14
                          100          1.000e-14
\n
\n
for more information see Beichman etal 1988.  "
END_OBJECT                                   = INSTRUMENT_INFORMATION  
   
OBJECT                          = INSTRUMENT_REFERENCE_INFO
 REFERENCE_KEY_ID                = "BEICHMANETAL1988"
END_OBJECT                      = INSTRUMENT_REFERENCE_INFO


OBJECT                          = INSTRUMENT_REFERENCE_INFO
 REFERENCE_KEY_ID	        = "OKENETAL1988"
END_OBJECT                      = INSTRUMENT_REFERENCE_INFO
						
OBJECT                          = INSTRUMENT_REFERENCE_INFO
 REFERENCE_KEY_ID	        = "BOULANGER1988"
END_OBJECT                      = INSTRUMENT_REFERENCE_INFO

OBJECT                          = INSTRUMENT_REFERENCE_INFO
 REFERENCE_KEY_ID               = MOSHIRETAL1989
END_OBJECT                      = INSTRUMENT_REFERENCE_INFO

END_OBJECT                      = SPACECRAFT_INSTRUMENT

END