content long 20-July-2019 02:43:40

Instrument Design

PFS: Planetary Fourier Spectrometer

Click for larger image.  Copyright - ESA/ASI/INAF (Formissano)

PFS Interferometer

The Planetary Fourier Spectrometer (PFS) is an infrared spectrometer optimised for atmospheric studies and covering the wavelength range 1.2 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 is about 2 degrees FWHM (Full Width, Half Maximum) for the Short Wavelength (SW) channel and 4 degrees FWHM for the Long Wavelength (LW) channel. These fields of view correspond to a spatial resolution of ten kilometres for the SW channel and 20 kilometres for the LW channel when Mars is observed from a height of 300 kilometres (the nominal height of the pericentre).

PFS is equipped with a pointing device, which enables it to receive incoming radiation from the surface of Mars 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
Spectral range (µm) 1.2 - 5.0 5.0 - 45
Spectral range (cm-1) 2000 - 8000 230 - 2000
Spectral resolution (cm-1) 1.5 1.5
Field of view (deg) 2 4
Type  Photoconductor Pyroelectric
Material Lead selenide (PbSe) Lithium tantalate (LiTaO3)
Shape / Size (mm) Square / 0.7 × 0.7 Circular / 1.4 (diameter)
Operating temperature (K) 220 290
Type Double pendulum
Reflecting elements Cubic corner reflectors
Beam splitter Calcium Fluoride (CaF2) Caesium Iodide (CsI)
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
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.

Last Update: 04 October 2017

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