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.

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.

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.

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.

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.

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).

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.

PFS: Planetary Fourier Spectrometer