Radiation Explorer in the Far InfraRed-Prototype for Applications and Development

Razionale scientifico
REFIR-satellite - EU
REFIR-BB, ASI BreadBoard

The development of a prototype, called REFIR-BB, has been funded in 2001 by ASI with the project called “R&D bread-board for a FTS to remotely sense the rotational water vapour band (100-1000 cm-1)”. In the first year of activity the hardware of the prototype has been developed.

REFIR-BB has been developed at IFAC-CNR with the collaboration of IMAA-CNR. The chosen optical configuration of the interferometer follows the indications raised by EU study of the space mission. It is a polarising interferometer with a new optical scheme that is able to meet all the requirements pointed out by the space mission. This configuration is described in more details by B.Carli et al., Appl.Opt. 38, pp.3945-3950 (1999). It makes use of four polarising beam splitters (BS) and provides all the desired features including broad spectral coverage, two separated input ports and two output ports, optical compensation for tilt errors in the moving mirror unit, measurement of the overall input signal (both planes of polarization) on the same detector. This optical configuration maximizes the reliability of the spectrometer in particular for long lifetimepace applications and optimizes its performances for room temperature operations.

The most critical components of the instrument to test with the experiment are fou wire-grid photolithographic polarisers for FIR applications and two DLATGS room-temperature detectors. The polarisers are supplied by the University of Wales, UK, and by ESA (under the ESTEC contract N. 15612/01/NL/SF: ''Polarisers for REFIR Far-IR interferometry''). The two detectors are supplied by BAE Systems.

A summary of the main parameters that characterize REFIR-BE are reported in the following Table.

Interferometer type

Polarising interferometer with double input/output port

Spectral bandwidth

100-1100 cm-1

Spectral resolution (max)

0.25 cm-1 (double-sided interferogram), 0.1 cm-1(single-sided interferogram)

Optical throughput

0.0097 cm-1

IFOV (Field-of-view)

0.23 rad

Integration time

10-120 s.

About 1.3 mW/m2 sr cm-1 at 400 cm-1

62cm diameter X 26cm height
55 kg
Power consumption
75 W
Operational temperature
Room temperature

REFIR-BE can be divided into five main subsystems that can be seen in the following Figure; the interferometer, the pyroelectric detector unit (PDU), the radiometric calibration system (RCU and RBB), the reference laser system (RLS), and the data acquisition and control system (ACS), not shown in the figure.

Top-view of the opto-mechanical layout of the instrument.

REFIR-BB top view

The FIR interferometer is the main unit and includes the beam splitter unit (BSU), the roof-top-mirror unit (RTMU) on the flexural translation stage. The figure also shows the RLS sharing the same path as the FIR interferometer and the PDU with the two pyroelectric detectors assembled at the output of the collecting optics. The RCS is composed of three black body (BB) sources, a reference BB (RBB) in front of the first input port and a calibration unit (RCU) in front of the measurement channel. The RCU has an hot BB, a cold BB and a folding mirror that allows to switch between these two references sources and the unknown radiance emitted from the observed. The ACS includes electronical interfaces both for the acquisition of signals and for the control of the overall instrument operations. Signals are three high-resolution low-frequency signals for the two output FIR interferograms and the reference laser, and 32 low-resolution low-frequency signals for house-keeping (HK) informations. All the operations are controlled by PC-based electronics with LabView software. Acquisition cards are one ICS-610-4 24-bit resolution for the main signals and one NI-PCI-6033-E 16-bit resolutions for the HKs. The following Picture shows the spectrometer.

REFIR-BB spectrometer

REFIR-BB picture