Japanese Infrared Survey Mission IRIS (ASTRO-F)
Hiroshi Murakami
The Institute of Space and Astronautical Science, Kanagawa 229-8510, Japan
ABSTRACT
The Infrared Imaging Surveyor (IRIS) is the second infrared astronomy mission of the Institue of Space and Astronautical Science (ISAS). The IRIS is a 70 cm cooled telescope dedicated for infrared sky survey. This project has been approved as ISAS's 21st science mission "ASTRO-F", and prototype model development has been ongoing since 1997. The IRIS will be launched with ISAS's launch vehicle M-V, into a sun-synchronous polar orbit with an altitude of 750 km. The IRIS telescope has a 70 cm aperture and is cooled to 6K using Stirling-cycle coolers and liquid helium. The primary and secondary mirrors are light-weight mirrors made of silicon carbide. Two focal-plane instruments are installed. One is the Far-Infrared Surveyor (FIS) which will survey the entire sky in the wavelength range from 50 to 200 micron with angular resolutions of 30 - 50 arcsec. The other focal-plane instrument is the Infrared Camera (IRC). It employs large-format detector arrays and will take deep images of selected sky regions in the near and mid infrared range. The field of view of the IRC is 10 arcmin and the spatial resolution is approximately 2 arcsec. The IRIS has much higher sensitivity than that of the IRAS survey. The detection limits are 1 - 100 µJy in the near-mid infrared and 10 - 100 mJy in the far infrared. With the IRIS survey, great progress is expected in the research on evolution of galaxies, formation of stars and planets, dark matter and brown dwarfs. The IRIS is now scheduled to be launched in early 2003.
Keywords: Infrared Space Telescope, Infrared Sky Survey
1. BACKGROUND AND MISSION CONCEPT
The Infrared Imaging Surveyor (IRIS) is Japan's second satellite mission for infrared astronomy. The first mission, the Infrared Telescope in Space (IRTS)1, 2, was a small cooled telescope onboard the multi-purpose satellite Space Flyer Unit (SFU)3. The IRTS was launched in 1995 and successfully operated in space during its one-month mission. The new mission IRIS was planned on the basis of the achievements, especially in the space cryogenics, by the IRTS. In addition, the new launch vehicle of the Institute of Space and Astronautical Science (ISAS), M-V, had been developed. It has about three-times larger launch capability than that of our previous rockets. The first flight of the M-V rocket was successfully done at the beginning of 1997. This situation enabled us to plan a larger-aperture cooled telescope.
The IRIS was planned as a second-generation survey mission. The previous sky survey by the Infrared Astronomy Satellite (IRAS)4 brought a lot of new findings such as the infrared galaxies and Vega phenomenon, and provided huge catalogs of infrared sources. The investigations in various fields in astronomy have very much progressed with the IRAS results as a start. Now, a new survey beyond the sensitivity limits of the IRAS promises further progress. For example, a survey of the distant galaxies with the dust emission as a probe will unveil the formation and the evolution processes of galaxies. The new infrared survey will also provide a valuable guide map for the 10-m class ground-based telescopes which will become available soon.
The IRIS is a 70-cm cooled telescope. It has much more powerful capability for the survey work than the IRAS, owing to the advanced technologies now available:
*The IRIS covers wide wavelength range from K-band to 200 µm. The IRIS will perform the all-sky survey at wavelengths > 50 µm using high sensitivity Ge:Ga detector arrays. In the near- and mid-infrared ranges, large-format arrays are employed for a deep sky survey in the selected sky regions.
*The sensitivity of the IRIS is much higher than that of the IRAS. For example, the IRIS has 50 - 100 times higher sensitivity at 100 µm and more than 1000 times at mid-infrared wavelengths.
*The IRIS has a diffraction-limited angular resolution at wavelengths longer than 10 µm. The pixel sizes of the IRIS are less than 1 arcmin even at 100-200 µm, several times smaller beam than those of the IRAS.
*The IRIS has capabilities of low-resolution spectroscopy. In the near and mid infrared, slit-less spectroscopy using grisms are available. The Fourier-transform spectrometer is used for the wavelength range from 50 to 200 µm. This capability is very important for classification of sources detected in the deep survey, SED measurements, rough determination of the redshift and so on.
The IRIS project was approved by the ISAS as the 21st Science mission 'ASTRO-F' in 1995. The budget for the development of the prototype model has started in 1997 fiscal year. The launch of the IRIS is now planned to be at the beginning of 2003.
2. SCIENTIFIC OBJECTIVES
Since the IRIS mission is a non-biased sky survey, various objects, from the solar system objects to galaxies at cosmological distances, will be detected. The key science aspects are;
*Search for the primeval galaxies, and tracing the evolution of the luminous infrared galaxies and also normal galaxies to high redshifts z>3.
*Systematic investigation of the star formation process. The IRIS will detect protostars in the very early stage where the gas is still accreting onto the newly-born stars at 100 - 200 µm in the nearby star-formation regions. The IRIS will be able to detect the brown dwarfs and super planets in the nearby star-formation regions and also field brown dwarfs.
*Evolution of planetary system. The IRIS can trace the evolution of the protoplanetary disks beyond the weak-line T Tau stage, which the IRAS has missed. The debris of the planetary formation around normal stars will also be extensively surveyed.
3. SATELLITE SYSTEM
Figure 1 is a brief sketch of the IRIS spacecraft. The upper part is a liquid-helium cryostat which contains the telescope and focal-plane instruments. The bus module, which includes the reaction-control system, communication system and so on, is placed below the cryostat. The size of the bus module is 1.8 m in diameter and 1 m in height. The cryostat is about 2.5-m high. The total weight of the spacecraft is approximately 900 kg including the thruster fuel and the liquid helium. The size and the weight of the spacecraft fits the constraints of the M-V rocket. The thruster system is used for the orbit change and the coarse attitude maneuvour. The attitude control for the sky survey is performed with a 3-axis reaction wheel system. The downlink of the engineering house-keeping data and the command uplink are made through the S-band link. The scientific data downlink is performed through the X band at a high bit rate of 4 Mbps.
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Figure 1. IRIS overview
The IRIS cryostat is a light-weight liquid helium cryostat with a mechanical cooler. The cross-sectional view of the cryostat is shown in figure 2. The outer shell of the cryostat is thermally isolated from the bus module and cooled to below 200 K by radiation cooling. The cryostat has two vapor-cooled shields (VCS). The inner shield is cooled by two 2-stage Stirling-Cycle coolers in addition to the evaporated helium gas. This supplemental cooling by the coolers increases the life time of the liquid helium by about a factor of two and stabilize the temperature of the inner VCS. The temperature of the telescope and the focal-plane instruments, which are cooled by the evaporated helium gas, is approximately 6 K. Only the far-infrared arrays are thermally connected to the helium tank and cooled to 1.8 K. The life time of the liquid helium is approximately 440 days in space in the current design with 150-liter liquid helium. One of the advantages of using mechanical coolers is that the near-infrared observations can be continued even after the liquid helium runs out, as long as the cooler properly works. The life time of the cooler for the IRIS verified by laboratory tests is two years. Further extension of the life time is expected.
Figure 2. Cross-sectional view of the IRIS cryostat.
3.3.1. Telescope system
The IRIS telescope5 is a Ritchey-Chretien type telescope, whose effective aperture size is 70 cm and whose system F-number is 6. For the mirror material, silicon carbide (SiC) is adopted, because of its large Young's modulus and high thermal conductivity. The IRIS SiC mirrors consist of porous core and CVD coat. The porous SiC has a very low density (1.85 g/cm3) and is relatively easy to machine. The CVD coat is dense and can be polished very accurately. The current weight estimation for the 70-cm primary mirror is only 9 kg. The goal of the image quality is diffraction-limited performance at a wavelength of 5 µm, including the aberration of the camera optics at the focal plane.
3.3.2. Focal-plane instruments
The focal plane of the telescope is shared by two observing instruments, the Infrared Camera (IRC)6 and the Far-Infrared Surveyor (FIS)7. The IRC is a wide-field imaging instrument. It consists of three independent camera systems, each of which covers the near-infrared, 10 micron, and 20 micron region, respectively. The characteristics of the IRC are summarized in table 1. The filter bands are selected by rotating the filter wheels by commands. The IRC also has a capability of low-dispersion spectroscopy by replacing the filters with grisms which are also attached on the filter wheels. This spectroscopic capability is used to classify the detected sources, to get SED of the sources, and to roughly estimate the redshifts. The fields of view are 10'x10' for all three cameras. The near-infrared camera, however, has an additional field. The 412x412 area of the InSb array covers the 10'x10' field, and remaining detector area are used for a slit spectroscopy using a slit added to the entrance aperture (see figure 3).
Figure 3. Configuration of the focal plane.
The FIS covers the wavelength range from 50 to 200 µm. The primary purpose of the FIS is the all-sky survey. The characteristics are collected in table 2. The survey observations are performed in four filter bands at the same time. The FIS also has a Fourier-transform spectrometer with a resolution of 0.5 cm-1. Imaging spectroscopy is done for selected sources.
Figure 3 is the focal-plane configuration where the relative location of the entrance apertures are shown. The scan direction is the direction along the FIS all-sky survey trajectory on the celestial sphere. The three cameras of the IRC are operated simultaneously and observe different direction about 20' apart from each other. The aperture of the FIS is tilted from the scan direction to achieve the diffraction-limited resolution as difined by the Nyquist's sampling7.
The IRIS orbit is a sun-synchronous polar orbit along the twilight zone, which is similar to the orbit of the IRAS. This orbit is adequate for the all-sky survey with a cryogenically-cooled telescope, because the thermal environment is stable and looking at the zenith for the sky survey automatically results in avoiding the strong thermal emission from the earth. The third stage of the M-V rocket will bring the IRIS into an elliptical polar orbit with an apogee of 750 km. The IRIS then moves itself into a sun-synchronous orbit at 750 km altitude using a two-propellant reaction-control system.
The basic attitude control in the observing operation of the IRIS is similar to the IRAS-type survey. The spacecraft spins around the sun-pointed axis once every orbit, pointing the telescope toward the zenith. The FIS performs the all-sky survey in this attitude-control mode. As the interval of the successive scan is about 4 arcmin at the ecliptic plane, while the FIS field of views are more than 10 arcmin, one object is observed at least twice with an interval of approximately 100 min. The spacecraft is sometimes fixed in the inertia space for the imaging observation by the IRC, as shown in figure 4. The number of the pointing observation in one orbit revolution is three at maximum. The duration of one pointing observation is limited to approximately 10 minutes to avoid earth shine illuminating the inner surface of the telescope. In the current design, the allowed pointing direction is restricted within 1 degree from the orbital plane to simplify the control algorithm and some hardware in the attitude control system. The spectroscopic observation or imaging observation by the FIS is also made in this pointing mode. The IRIS observation is combination of these two attitude modes.
The IRIS observation period is separated into three phases. Phase 1 is the first 180 days where the FIS survey has a priority. The number of the pointing observation is limited to about 2000 times in this period. The second phase lasts until all the helium is exhausted. The duration of the phase 2 is about 200 days in the nominal case. We expect about 5000 pointing observations to be done in this period. The primary instrument in the second phase is the IRC. The FIS survey is also made to fill in the blanks of the phase-1 survey. The sky coverage of the FIS survey will be more than 90 % of the entire sky. The loss of 10 % is due to the Moon, the radiation effects by high-energy particles in the South Atlantic Anomaly and in the polar horn of the radiation belt. The third observing phase is the period after the helium runs out, where only near-infrared channel of the IRC can operate. The duration of this period is limited by the lifetime of the Stirling-cycle coolers.
The expected detection limits of the IRIS survey are shown in figure 4. The detection limits of the FIS are given for both the uniform survey and the pointing observation ( integration time = 500 s), while the IRC limits are given only for the pointing observation.
Figure 4. Attitude control operation for the IRIS survey.
Figure 5. Detection limits of the IRIS survey.
The basic design of the spacecraft has almost been fixed up to now. We are now in the detailed design phase of the prototype model and will move into the fabrication phase in autumn this year. The evaluation of the key elements, such as the focal-plane arrays, telescope mirrors, mechanical coolers, is under way.
The test of the prototype system is planned to be made in October and November, 1999. The design phase of the flight model will start at the same time. Fabrication of the flight model will be completed by the beginning of 2002 and launch operation at Kagoshima space center in Japan will start at the beginning of 2003 after one-year testing.
The definition of the IRIS mission and the basic design work was carried out at the ISAS, Nagoya University, Tokyo University, Communication Research Laboratory, Tokyo Metropolitan University and Tokai University. The author would like to thank all project members. The author would also like to acknowledge corporate contributions to this project under contract to the ISAS. The system design was performed by NEC Corporation. The design of the cryostat and the telescope was made by Sumitomo Heavy Industries and NIKON, respectively.
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7) M. Kawada, "FIS: Far-Infrared Surveyor onboard IRIS," in Infrared Astronomical Instrumentation, A. M. Fowler, ed., Proc. SPIE 3354, 1998.
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