FIRP : Far-Infrared Photometer

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.


Size Irregular shape : 16 x 15 x 11 cm3 (overall)
Weight 2185 g
Electrical Power Dissipation Cold part : 2-3 mW (typical), 17 mW (peak), Warm part : 2 W
Cryogenic Parameters
Tcoldplate <1.8 K
Thousing 1.92 K
Ttelescope 1.91 K
Tstill 293 mK
Tphot 319 mK
Tpump 2.1 K
3He lifetime 10 days
3He cycle time 15 hrs
Cooling power of still 12 J
Heat load (on still) 14.6 microW
Entrance aperture 4 mm diameter (0.5 deg on sky) Winston Concentrator with flare entrance aperture
Detectors Composite bolometers in AC bridge with dark reference detectors
Shutter frequency 1/8 Hz(4 sec open / 4 sec closed - 8 sec / FOV)
Cold amplifiers Model J-1, Infrared Laboratories
Data rete Standard : 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
"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

Last Modified: 19 November 2018