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 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.
The SPICAM ultraviolet channel (SUV) is based around a holographic diffraction grating.
|Ultraviolet Channel Characteristics|
|Usable dimensions of primary mirror|
|Slit width||0.05 and 0.5 mm|
|Slit length||6.6 mm|
|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|
|Field of view of one pixel|
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|
|Focal length||120 mm|
|Entrance pupil dimensions|
|Usable field of view|
|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|
|Coating||Magnesium Fluoride, MgF2|
|Radius of curvature|
|Grooves per mm||280|
|Blaze wavelength||170 nm|
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.
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|
|Sampling per pixel|
|Transmissivity of optics||25%|
|Detector||InGaAs PIN photodiode|
|Resolution at nadir|
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
The two output beams from the AOTF are collimated by another lens and detected by two indium gallium arsenide PIN photodiodes.
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