InstrumentsSPICAV: Spectroscopy for Investigation of Characteristics of the Atmosphere of VenusSPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) is an imaging spectrometer for ultraviolet and infrared radiation. SPICAV is derived from the SPICAM instrument flown on Mars Express, which was equipped with two channels, one for ultraviolet wavelengths and one for infrared. SPICAV retains these channels, SPICAV-UV (SUV) for ultraviolet and SPICAV-IR for infrared and adds an additional channel (SOIR, Solar Occultation at Infrared), to observe the Sun through Venus's atmosphere at longer infrared wavelengths (2.3 µm - 4.2 µm).
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| SPICAV 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, SIR and SOIR observing. |
In Nadir mode, the instrument points directly at the planet and analyses solar radiation that has travelled through the atmosphere after being reflected from the planet surface. In Star or Sun mode, the instrument points 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. In limb pointing mode, the instrument points across the atmosphere, as during Star mode, but without a target star, and the instrument can analyse the atmospheric glow.
Ultraviolet Channel
The SPICAV 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 (mirror + grating) | 30% |
| Pointing accuracy | better than 0.2° |
| Detector | Intensified CCD |
| CCD dimensions | 384×288 pixels |
| CCD pixel size | 23×23 μm |
| Field of view of one pixel | 40"×40" |
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 | 118.125 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° × 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 |
| Number of grooves per millimetre | 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 tunable filter (AOTF).
| Infrared Channel Characteristics | |
| Diameter of primary lens | 12 mm |
| Field of view | 2° (6×10-4 sr) |
| Slit width | 1 mm |
| Wavelength range | 0.25 - 1.7 μm |
| Sampling per pixel | 0.45 nm to 1.12 nm |
| Transmission of optics | 25% |
| Detector | Hybrid Si/InGaAs PIN photodiode (2.4x2.4 mm) |
| Resolution at nadir | 5×5 km |
The entrance optics comprise a lens telescope with a diameter of twelve 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 0.25 - 1.7 μm.
The two output beams from the AOTF are collimated by another lens and detected by two hybrid silicon/indium gallium arsenide PIN photodiodes.
When observing the Sun, light entering via the dedicated aperture in the instrument baseplate is directed by an optical fibre and a small mirror into the SIR channel main entrance.
SOIR Channel
The Solar Occultation at Infrared (SOIR) channel is also based around an acousto-optical tunable filter (AOTF).
| SOIR optical components | |
| Entry telescope | Newton type, focal length 180 mm, 35 mm × 50 mm parabolic mirror |
| Field of view diaphragm | 0.5 mm × 4 mm, nickel |
| Folding and polarizing prism | Tellurium oxide, 10 mm × 10 mm |
| Lens | Zinc selenide, focal length 35 mm, diameter 15 mm, R1 = 31.5 mm, R2 = 79.8 mm |
| AOTF | see table below |
| Lens | Zinc selenide , focal length 35 mm, diameter 15 mm, R1 = 79.8 mm, R2 = 31.5 mm |
| Folding and polarizing prism | Tellurium oxide, 10 mm × 10 mm |
| Slit | 0.06 mm × 3 mm, nickel |
| Main mirror | focal length 375 mm, 70 mm × 100 mm, off-axis 8° parabolic |
| Diffraction grating | blaze angle 63.42° |
| Folding mirror | |
| SOIR AOTF Characteristics | |||
| Wavelength (nm) | Excitation frequency (MHz) | Bandwidth (nm) | Angular aperture (°) |
| 2500 | 27.303 | 11.65 | 7 |
| 3172 | 21.971 | 18.87 | 8.2 |
| 4500 | 15.33 | 38.18 | 10.1 |
The SOIR optics pass the filtered and dispersed light to a photo-voltaic mercury cadmium telluride (HgCdTe) detector contained in an Integrated Detector Dewar Cooler Assembly (IDDCA). The detector is arranged as a 320×256 array of 30 μm square pixels. The IDDCA is equipped with a 0.4 Watt Stirling cycle rotary microcooler.
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PFS: Planetary Fourier Spectrometer |
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VeRa: Venus Radio Science |
