Balloon-borne far-IR telescope

Observations in the far-IR spectral range (30–300μm) have become very important in contemporary astronomical research, particularly for studies of star and planetary-system formation, galaxy evolution (including ‘starburst’ phenomena, in which galaxies temporarily exhibit significantly enhanced star-formation rates), and interstellar physical and chemical processes. Astronomical bodies forming from or evolving (dying) into cold interstellar matter radiate predominantly at far-IR wavelengths. NASA's Spitzer Space Telescope, the Japanese AKARI satellite, and the European Space Agency (ESA)'s Herschel Space Telescope have delivered unique data of key importance for various fields of current astronomy, while in the next decade, the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) jointly planned by the Japan Aerospace Exploration Agency (JAXA) and ESA will open its far-IR eye to the sky with very high sensitivity.

However, improvements in spatial resolution proceed slowly, from one arcminute with the Infrared Astronomical Satellite (IRAS) in 1983 to one-tenth of an arcminute with Herschel in 2009. Spatial resolution is, in theory, diffraction limited by the ratio of the operating wavelength to the telescope's aperture size. This translates into a diffraction-limited resolution of seven arcminutes at 100μm for a 3.5m-aperture telescope. In contrast, radio, near- and mid-IR, and even visual waves can now be observed using interferometry at much higher angular resolutions (approximately one-hundredth of an arcsecond can be attained even in the near-IR range). The main difficulty associated with the astronomical far-IR regime is that it can only be observed from space because of prohibitive absorption by the Earth's atmosphere.

The goal of the Far-IR Interferometric Telescope Experiment (FITE) is to achieve (for the first time) interferometry in the far-IR spectral range and resolve, for instance, fine structures of protoplanetary and circumstellar disks, star-forming molecular cores, and nuclear starbursts in galaxies. We have developed a balloon-borne Fizeau-type interferometer with a baseline of 8m at first flight (with a maximum of 20m), corresponding to one arcsecond resolution at 100μm. The beam diameter of each aperture is 40cm.

The interferometer consists of two sets of collecting optics employing off-axis parabolic and plane mirrors (see Figure 1). The foci of the parabolic mirrors coincide in the input image plane of the cold optics where the two beams provide an interference fringe.¹ The cold-optics system is installed in a liquid-helium-cooled cryostat.² The latter contains a shutter, Lyot stop, and two dichroic beam splitters that split the incident light into far- and mid-IR, and visual beams. The mid-IR and visual beams focus onto individual detector arrays. The resulting images are used for monitoring the interference conditions in real time. The far-IR light is focused onto a linear array sensor specifically developed for FITE.

Figure 1. Concept of the Far-IR Interferometric Telescope Experiment (FITE).

The most important challenge for FITE is to maintain optical-system alignment during flight.³ Although precise adjustments are made on the ground before launch, temperature decreases to −50^°C or lower may introduce large deviations. Therefore, we developed a remotely controllable operating system to enable optical adjustments during flight. Another important challenge relates to attitude control. We adopted a three-axis control system (as for a satellite in free space) to minimize turbulence originating from the pendulum motion of the balloon-gondola system.⁴

Figure 2 shows the FITE payload. The telescope is supported at the elevation axis by the telescope gondola, which is fixed to the control gondola. The entire payload is attached at the center of mass by a train connected to the balloon through a parachute. During flight, telescope operation is conducted from the ground station through a bi-directional telemetry link (a downlink for telemetry and an uplink for commands).

Figure 2. FITE balloon payload.

Figure 3. Hanging test conducted in front of the assembly room of the launch base in Brazil.

FITE will be launched from the balloon base of the Brazilian National Institute of Space Science (INPE), some 150km from São Paulo. In 2008, we carried out flight preparations (see Figure 3) but did not finish in time to launch with good wind conditions both at high altitude and on the ground. Our next attempt is scheduled for November 2010. We selected Neptune as our first observational target because it is bright in visual, mid-, and far-IR bands, and several arcseconds in size. We will subsequently proceed to observe bright late-type stars (such as IR core IRC +10216), which are also bright in those same spectral regions. Once the FITE system has been validated, we will observe other scientifically interesting objects.

FITE was developed by a team composed of the IR astronomy groups at Osaka and Nagoya Universities (Japan), including more than 10 graduate students. Development was supported by grants in aid for scientific research (Specially Promoted Research, grant 15002006) of the Japan Society for the Promotion of Science. The flight campaign is conducted by JAXA (Japan) and INPE (Brazil) as a collaboration between universities/institutes in both countries.