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Far-Infrared Photometer (FIRP)

The Far Infrared Pthotometer(FIRP) on-bord IRTS is a four-band photometer which is optimized to observe diffuse sky emission at sub-mm wave lengths. FIRP was capable of doin absolute photometry at 4 wavelengths (150, 250, 400, 700 μm) simultaneously with the beam size of 0.5 degrees. Ge composite bolometers were used with the AC-biased bridge circuit followd by cold J-FETs and low-noise lock-in amplifiers as the readout electronics. The bolometers were cooled down to 300 mK using the He closed cycle refrigerator that was specially designed to work at zero-gravity environmrent. The lifetime of Herium refrigerator per one cycle was approximately seven days. Absolute photometry was established with the use of internal cold shutter as well as 2K cooled telescope. In addition, the focal plane instruments were covered by an radiation shield which was also cooled down to around 2K to block the thermal radiation from the warmer part of the inside of cryostat. Relative responsivity change was able to be monitored by turning on the internal calibrator periodically.


SizeIrregular shape : 16 x 15 x 11 cm3 (overall)
Weight2185 g
Electrical Power DissipationCold part : 2-3 mW (typical), 17 mW (peak), Warm part : 2 W

Cryogenic Parameters

Tcoldplate< 1.8 K
Thousing1.92 K
Ttelescope1.91 K
Tstill293 mK
Tphot319 mK
Tpump2.1 K
3He lifetime10 days
3He cycle time15 hrs
Cooling power of still12 J
Heat load (on still)14.6 microW
Entrance aperture4 mm diameter (0.5 deg on sky) Winston Concentrator with flare entrance aperture
DetectorsComposite bolometers in AC bridge with dark reference detectors
Shutter frequency1/8 Hz(4 sec open / 4 sec closed - 8 sec / FOV)
Cold amplifiersModel J-1, Infrared Laboratories
Data reteStandard : 6 kb/s, Reduced : 3 kb/s


For more details, refer to the following documents.

FIRP Explanatory Supplement [PDF]

"Flight performance of the Far-Infrared Photometer"
T. Hirao, T. Matsumoto, S. Sato, and A. E. Lange
1996, in SPIE Proc.2817, Infrared Spaceborne Remote Sensing IV ed. M. S. Scholl and B. F. Andresen, p.276-285 [ SPIE ] [ ADS ]
"The Far-Infrared Photometer on the Infrared Telescope in Space"
A. E. Lange, M. M. Freund, S. Sato, T. Hirao, T. Matsumoto, and T. Watabe
1994, The Astrophysical Journal, vol.428, p.384-392 [ ADS ]

Last Modified: 20 July 2023