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Instruments

PFS: Planetary Fourier Spectrometer

The Planetary Fourier Spectrometer (PFS) is an infrared spectrometer optimised for atmospheric studies and covering the wavelength range 0.9 to 45 microns in two channels with a boundary at 5 microns. The spectral resolution of the instrument is better than 2 cm-1 . The instrument field of view FOV is about 1.6 degrees FWHM for the Short Wavelength (SW) channel and 2.8 degrees for the Long Wavelength (LW) channel. These fields of view correspond to a spatial resolution of seven kilometres for the SW channel and 13 kilometres for the LW channel when Venus is observed from a height of 250 kilometres (nominal height of the pericentre).

PFS is equipped with a pointing device, which enables it to receive incoming radiation from the surface of Venus or to perform calibration measurements by pointing to a reference black body of known temperature or to deep space.

The incident radiation arrives from the pointing device and is divided into two beams by a dichroic mirror and then filtered before being directed into the two interferometers. The interferometers are of the double pendulum type and are positioned with their planes of operation one above the other so that a single motor can be used to move both pendulums. An optical reference channel controls the pendulum motion by passing light from a laser diode through the same optics as the radiation that is being analysed. The reference channel also generates the sampling signal for the analogue to digital converters that process the detector signals, triggering one sample for each 150 nm of retro-reflector motion. The interferometers are extremely sensitive to optomechanical distortions and the interferometer module must be very rigid and thermally stable to minimise these effects.

Summary of PFS Characteristics
Short Wavelength Channel Long Wavelength Channel
General
Spectral range (µm) 0.9 - 5.0 5.0 - 45
Spectral range (cm-1) 2000 - 11 100 222 - 2000
Spectral resolution (cm-1) 1.5 1.5
Field of view (rad) 0.035 0.07
Detectors
Type Photoconductor Pyroelectric
Material Lead selenide (PbSe)/ Lead sulphide (PbS) sandwich Lithium tantalate (LiTaO3)
Operating temperature (K) 200 - 220 290
Interferometer
Type Double pendulum
Reflecting elements Cubic corner reflectors
Beam splitter Calcium Fluoride (CaF2) Caesium Iodide (CaI)
Maximum optical path difference (mm) 5 5
Reference source Laser diode
Collecting optics
Type Parabolic mirror
Diameter (mm) 49 38
Focal length (mm) 20 20
Coating Gold
Channel separator Thallium bromide/iodide (KRS-5) crystal with multi-layered coating reflecting short wavelengths
Interferogram
Type Two sided
Sampling number 16 384 4096
Sampling step (nm) 608 2432
Dynamic range ± 215

The instrument is able to perform real time Fast Fourier Transform computations in order to select the spectral range of interest for data transmission to Earth.

The PFS instrument design is based on that flown on Mars Express, modified to optimise performance for the Venus Express mission.


Last Update: 18 July 2016

For further information please contact: SciTech.editorial@esa.int

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