We have developed an imaging Fourier transform spectrometer (iFTS) for space-based far-infrared astronomical
observations. The iFTS employs newly developed photoconductive detector arrays with a capacitive transimpedance
amplifier, which makes the iFTS a completely unique instrument. The iFTS was installed as a
function of the far-infrared instrument (FIS: Far-Infrared Surveyor) on the Japanese astronomical satellite,
AKARI, which was launched on February 21, 2006 (UT) from the Uchinoura Space Center. The iFTS had
worked properly in the space environment as well as in laboratory for more than one year before liquid helium
ran out on August 26, 2007. The iFTS was operated nearly six hundreds of pointed observations. More than
one hundred hours of astronomical observations and almost the same amount of time for calibrations have been
carried out in the mission life. Meanwhile, it becomes clear that the detector transient effect is a considerable
factor for FTSs with photoconductive detectors. In this paper, the instrumentation of the iFTS and interesting
phenomena related to FTSs using photoconductive detectors are described, and the calibration strategy of the
iFTS is discussed briefly.
We have developed the imaging Fourier Transform Spectrometer (FTS) for the FIS (Far-Infrared Surveyor) onboard the ASTRO-F satellite. A Martin-Puplett interferometer is adopted to achieve high optical efficiency in a wide wavelength range. The total optical efficiency of this spectrometer is achieved 40-80% of the ideal value which is 25% of the incident flux. The wavelength range of 50-200μm is covered with two kinds of detector; the monolithic Ge:Ga photoconductor array for short wavelength (50-110μm) and the stressed Ge:Ga photoconductor array for long wavelength (110-200μm). The spectral resolution expected from the maximum optical path difference is 0.18cm<sup>-1</sup>.
In order to evaluate the spectral resolution of the FTS, we measured absorption lines of H<sub>2</sub>O in atmosphere using the optics of the FTS with a bolometer at the room temperature. The measured line widths are consistent with the expected instrumental resolution of 0.18 cm-1. Some spectral measurements at the cryogenic temperature were carried out by using cold blackbody sources whose temperatures are controlled in a range from 20 to 50 K. The derived spectra considering with the spectral response of the system are consistent with expected ones.
Spectroscopic observations with the FTS will provide a lot of astronomical information; SED of galaxies detected in the all sky survey and the physical diagnostics of the interstellar matter by using the excited atomic or molecular lines.
Far-Infrared Surveyor (FIS) is one of the two focal plane instruments of ASTRO-F which is a Japanese infrared astronomical satellite and is planned to launch in 2004. The FIS has spectroscopic capability by a Fourier transform spectrometer (FTS) covering 50-200cm<sup>-1</sup> with spectral resolution of 0.2-0.33 cm<sup>-1</sup> in addition to the primary purpose of FIS (an all-sky photometric survey). The Martin-Puplett interferometer is adopted as the method for spectroscopy in order to achieve high optical efficiency in a wide wavelength range.
The most important issue of the FTS is the development of driving mechanism in order to scan a moving mirror with high optical performances. By the present we succeed to develop the driving mechanism satisfying a lot of limitations and requirements as a instrument onboard satellite. Furthermore the wire-grid polarizers are evaluated in optical performance, these are usable for polarized interferomter. We also measure FIR spectrum using Spectroscopy mode of FIS, and many absorption lines of H<sub>2</sub>O are detected on continuum spectrum of atmosphere. And the interferogram and spectrum are derived at low temperature (2K) that is practically used in space. The spectrum resembles expected one which are considered with optical components for flight model. The detection limit are estimated combining performances of optical components and detectors, the FISP has sufficient performance to archive objective sciences. FTS will provide a lot of astronomical information; determination of the SED in high-z objects detected by the survey observation of ASTRO-F, the redshift of such objects, and the physical conditions that are hard to be derived by optical/NIR-MIR observations, from FIR lines.