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The REFIR Fourier transform spectroradiometer concept

Contact: Giovanni Bianchini, Istituto Nazionale di Ottica (INO-CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino (I) Tel. +39.055.522.6309, Fax. +39.055.522.6348

The REFIR Fourier transform spectroradiometer is based on a Mach-Zehnder optical scheme, with an optical path that is folded on two layers in order to keep the instrument compact and to achieve some degree of optical misalignment compensation and optimization.

The interferometer can operate both in a Martin-Puplett polarising configuration, in which all the 4 beamsplitter (BS) elements are used, or in an amplitude dividing scheme, in which only two of them are used.

The optical schematic, shown in the figure below, features a moving mirror assembly composed by two roof-top mirrors (RTMU - roof top mirror unit) that permits to obtain a maximum optical path difference in the interferometer that is 4 times the actual mirror displacement.

REFIR Fourier transform spectroradiometer optical schematic

The REFIR-PAD prototype has been built with the aim of operating both on a stratospheric balloon platform, and from ground-based measurement stations. For this reason the REFIR optical system has been enclosed in a compact and robust case that includes also all the control electronics, the reference black-body sources and a data storage system. A schematic of the REFIR-PAD instrument, in the balloonborne nadir-looking configuration is shown in the figure below. When operating from balloon several line of sight options are available, nadir, limb sounding and space view (30 degrees above horizontal). In the case of ground-based operation, the 0 degree limb view is used, through a folding mirror.

REFIR-PAD instrument layout (balloonborne configuration

The main specifications of the REFIR-PAD instrument are shown in the following table:

Parameter Value Units
Spectral range 100-1900 cm-1
Spectral sampling up to 0.25 cm-1
Optical throughput 0.02 cm2 sr
Radiometric accuracy < 0.5 K (@240K)
Size 600 dia. x 300 h. mm
Mass 55 kg
Power consumption 50 W
(computer) 40 W
(thermal control) 50 W

The REFIR-PAD instrument is installed in the Physics Shelter at Concordia station, the instrument is placed inside an enclosure that is connected directly to the outside through a viewport and an insulated chimney. A window cannot be used since there isn't a suitable material capable to be highly transparent across the wide operating spectral range of the REFIR-PAD instrument. The view port can be closed vith a motorized door in order to avoid exposing the instrument to low temperatures when it's not powered on.

View port and door REFIR-PAD and enclosure

The REFIR-PAD enclosure is thermally controlled within some fractions of a degree by means of an independent system that drives both some heaters and a fan that pumps cool air from outside. A flow regulating valve is used to stop the cool air flow when it is not needed, while other three fans, always on, help mixing the air inside the enclosure. The thermal control system can be monitored and controlled through a web interface:

Here can be found some literature on the topic of the REFIR spectroradiometer and its components, also a manual about the instrument electronics and a description of the instrument realignment procedure are available.