I-BEST

I-BEST is the Interferometer for Basic observation of Emitted Spectral Radiance of the Troposphere. It is an extension of the FTIR ABB-MR104 instrument in the far-infrared region. The FTIR ABB MR-100 instrument is operational at the laboratory of the DIFA and IMAA since 2001. In its basic version, the FIR-ABB MR104 (hereafter MR104) is a Fourier Transform Spectrometer (FTS) with a liquid nitrogen cooled sandwich MCT/InSb detector to cover the 500 to 5000 cm-1 spectral region. More precisely the detector provides sensitivity for 5 - 20 mm (MCT) and 2 - 5.5 mm (InSb) in channels 1 and 2, respectively. The spectral sampling is 0.4822 cm-1 for double-sided interferograms.

The absolute infrared spectral radiance of the sky is measured by using a two point calibration system where two stabilized infrared sources (black bodies), with an error of less than ± 0.01 K, filling the field of view, emit radiation at two temperature that differ by at least of 40 K from each other. These black bodies are essential to provide a well known stable hot and ambient temperature reference for calibration of the down welling sky view radiance. A typical measurement cycle consists of a sky dwell period followed by two dwell periods, one for each of the black bodies.

The MR104 instrument has a Field of View of ~40 mrad. In addition, it is not Michelson-type instrument, the optical path difference is obtained with a rotary scanning with a flex pivot, which means that there is no wavefront tilt and shear.

The I-BEST configuration is based on the the standard commercial-off-the-shelf MR series interferometer where the usual Zinc Selenide beamsplitter has been replaced by a Silicon window. The Silicon offers very good transmittance down to the 100 microns region. To avoid loss of signal due to multiphonon absorption, the material thickness has been kept as thin as possible (the current thickness is ~1 mm). To keep track of the optical path difference during the scan of the interferometer and to properly calibrate the spectral axis after the Fourier transform, a metrology laser is injected beside and parallel to the infrared beam. The metrology wavelength is around 1550 nm (solid state DFB laser diode) and the acquisition of one interferogram sample at every two fringes gives the appropriate free spectral range to meet the spectral coverage requirement. To cover the far infrared range, a new detector has been installed which is the standard DTGS offered in ABB's standard FTLA104 series spectrometers (ABB Assembly Number IOH1048). The instrument has the capability to be used either with DTGS (un-cooled, left panel below) or the MCT (cooled with liquid Nitrogen, right panel below) detector, which gains flexibility to the spectrometer. In addition, because of the new silicon beam-splitter, the MCT useful range has been moved down to 450 cm-1 against the previous limit of ~530 cm-1. Finally, because of the new metrology, the sampling rate has improved to the value of 0.3932 cm-1 against the old 0.4822 cm-1. More details can be found in the Applied Optics I-BEST paper.