content long 20-July-2019 03:35:37

Instrument Design

SPICAM: Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars

SPICAM (Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars) is an imaging spectrometer for ultraviolet and infrared radiation. SPICAM is equipped with two channels, one for ultraviolet wavelengths and one for infrared.

SPICAM Channels
  Ultraviolet (SUV) Infrared (SIR)
Spectral range (µm) 0.118 - 0.32 1.1 - 1.7
Spectral sampling (nm per pixel) 0.55 0.45 - 1.12

Operational Modes

The operational modes of SPICAM are Test mode (ground use only), Star mode, Sun mode, Limb mode and Nadir mode. The operational modes are derived from the scientific objectives and the related spacecraft attitudes.

SPICAM Operating Modes
Nadir mode Used during spacecraft nominal nadir observation mode. SUV and SIR observing.
Star mode Requires dedicated spacecraft attitude. SUV observing.
Limb mode Requires dedicated spacecraft attitude. SUV and SIR observing.
Sun mode Requires dedicated spacecraft attitude. SUV and SIR observing.

In Nadir mode, the instrument will point directly at the planet and will analyse solar radiation that has travelled through the atmosphere after being reflected from the planet surface. Nadir observations allow the measurement of total column abundance of atmospheric components. In star or Sun mode, the instrument will point tangentially through the atmosphere towards a star, or the Sun, which is observed through the atmosphere as it rises or sets. The instrument then analyses the light once components of it have been absorbed by the atmosphere, allowing derivation of vertical concentration profiles for atmospheric components. In limb pointing mode, the instrument will point across the atmosphere, as during Star mode, but without a target star, and the instrument will analyse the vertical profile of aeronomic emissions.

Ultraviolet Channel

The SPICAM ultraviolet channel (SUV) is based around a holographic diffraction grating.

Ultraviolet Channel Characteristics
Usable dimensions of primary mirror 40 × 40 mm
Slit width 0.05 and 0.5 mm
Slit length 6.6 mm
Wavelength range 118-320 nm
Spectral dispersion 0.55 nm/pixel
Transmission of optics (Telescope + grating) 30%
Pointing accuracy Better than 0.2°
Detector Intensified charge coupled device (CCD)
CCD dimensions 384 × 288 pixels
CCD pixel size 23 × 23 µm
Field of view of one pixel 40 × 40 arcseconds

The first optical element in the UV channel is an off-axis parabolic mirror, which collects the incident light entering through either the nadir or solar aperture and focuses it.

Ultraviolet Channel Mirror Characteristics
Off axis portion of parent with origin at centre of parent paraboloid x = 30 mm, y = 0 mm, z = 1.875 mm
Focal length 120 mm
Dimensions 44 × 52 mm
Entrance pupil dimensions 40 × 40 mm
Usable field of view 1° × 3.16°
Material Aluminium
Coating Magnesium Fluoride, MgF2

In the focal plane of the mirror, there is a slit, which can be moved in and out of the field of view by a mechanical actuator, providing two configurations:

  • Slit absent, for observation of stellar occultations with a field of view of 1° x 3.16°
  • Slit present, for the observation of extended sources

The slit has two parts, with two different widths, to give different flux resolutions.

The focal plane is the entrance of the spectrometer, a holographic concave grating, which collects the incoming light and directs it to the detector block.

Ultraviolet Channel Grating Characteristics
Type Holographic
Shape Toroidal
Coating Magnesium Fluoride, MgF2
Dimensions 50 × 50 mm
Radius of curvature 148.94 mm
Grooves per mm 280
Blaze wavelength 170 nm
Incident angle ~6.5°

The detection block consists of a CCD detector equipped with an image intensifier tube. The spectrum of a single source point in the focal plane is dispersed along the lines of the CCD. The usable spectral band is 118 to 320 nm, chosen so as to offer good resolution (~1 nm) for stellar observations and to cover the CO2 and O3 bands. The lower wavelength was selected to be just below the Lyman α wavelength and the upper wavelength was chosen to reject visible light. The quantum efficiency of the photocathode is zero beyond 320 nm and the detector is therefore solar blind. The detector has a large dynamic range - by varying the gain of the image intensifier, the spectrometer can perform individual photon counting and deal with very high input intensities.

To observe the Sun, a five-millimetre diameter mirror is positioned so as to reflect the light from the Sun via a dedicated entrance aperture onto the parabolic mirror.

Infrared Channel

The SPICAV infrared channel (SIR) is based around a scanning acousto-optical tuneable filter (AOTF).

Infrared Channel Characteristics
Diameter of primary lens 30 mm
Field of view 1° (3x10-4 steradians)
Slit width 1 mm
Wavelength range 1.1- 1.7 µm
Sampling per pixel 0.45 nm to 1.12 nm
Transmissivity of optics 25%
Detector InGaAs PIN photodiode
Resolution at nadir 5 × 5 km

The entrance optical system comprises a lens telescope with a diameter of thirty millimetres and a collimator lens, which collect the incoming radiation and direct it onto the AOTF.

The AOTF consists of a tellurium oxide (TeO2) crystal to which an acoustic wave is applied. The acoustic wave propagating in the crystal causes it to act in a similar way to a diffraction grating. A radio-frequency synthesizer drives a piezo-electric crystal attached to the TeO2 crystal to produce the wave. The frequency of excitation determines the wavelength of the acoustic waves and hence the select wavelength of the AOTF. The frequency range of the synthesizer corresponds to an AOTF passband of 1.1 - 1.7 µm

The two output beams from the AOTF are collimated by another lens and detected by two indium gallium arsenide PIN photodiodes.

Last Update: 04 October 2017

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