The InfraRed Telescope in Space (IRTS)

The InfraRed Telescope in Space (IRTS) is the first Japanese orbiting mission for infrared astronomy.

The IRTS was equipped with a 15-cm cooled telescope and four scientific instruments; the Near-InfraRed Spectrometer (NIRS; 1.4-4.0 µm), the Mid-Infrared Spectrometer (MIRS; 4.5-11.7 µm), the Far-Infrared Line Mapper (FILM; 145, 155, 158, 160 µm), and the Far-InfraRed Photometer (FIRP; 150, 250, 400, 700 µm). Most wavelengths observed by IRTS are blocked by the Earth's atmosphere and cannot be observed from the ground-based telescopes. A summary of the instrumentation can be found here.

The telescope and the instruments were stored in a cryostat and cooled down to about -271 degree with liquid Helium. About 100 liter helium kept the system in a stable temperature at 1.9 K for 38 days.

The Near-InfraRed Spectrometer (NIRS) is a low-resolution grating spectrophotometer. A 12 x 2 channel InSb detector array covers the wavelength range of 1.4-4.0 µm with a spectral resolution of about 0.13 µm.

IRTS itself was not a selfstanding satellite. IRTS was installed into a multi-purpose spacecraft, the Space Flyer Unit (SFU). The SFU was launched on March 18, 1995 by an H-II rocket from Tanegashima Space Center in Japan.

The IRTS mission started on March 29th and terminated on April 26th due to boil-off of the liquid Helium. Unlike ESA's Infrared Space Observatory (ISO) launched in the same year, IRTS did not target individual objects. Instead, it continuously scanned the sky and retrieved the data along the path. In its 26 days mission, IRTS surveyed about 7 per cent of the whole sky in two stripes; one along the galactic plane and the other covering the high galactic latitude region. The SFU was retrieved by the Space Shuttle Endeavour on January 13, 1996, after carring out various experiments by the other instruments onboard.

The main scientific targets of IRTS were diffuse radiation sources: zodiacal light, interstellar dust and gas, extragalactic background radiation, and so on. The large aperture size of the instruments (e.g., NIRS has an 8'x 8' entrance aperture) allowed to efficiently observe those large scale radiation sources. In addition, the NIRS and the MIRS also detected many point sources, mostly stars, during the mission. An image map of the galactic plane was reconstructed from the scanned data, showing a number of point sources. These point sources were extracted from the scanned data and compiled in a catalogue of more than 14000 objects (NIRS catalogue). The MIRS point source catalogue contains about 600 sources. These catalogues provide a huge number of uniform spectra of stars in a wavelength range in which we had little information before.

Copyrights: ISAS


Hiroshi Murakami, Masahiro Tanaka, Issei Yamamura
E-mail : Hiroshi Murakami, Masahiro Tanaka, Issei Yamamura
Institute of Space and Astronautical Science
Sagamihara, Japan