The instrument

The instrument integrates three independent hyperspectral channels to observe the limb in the ultraviolet (250–355 nm), the visible (440–675 nm) and the near-infrared (600–1020 nm) spectral range. Each channel is based on a reflective optical design and contains the following main optical elements:

• A front baffle to protect the instrument from out of field stray light
• A front periscope mirror that allows fine co-alignment of the different instrument lines of sites
• A front end optics module that collects the light and passes it to the spectral filter
• A spectral filter (acousto-optic tuneable filter in the visible and near-infrared and Fabry Perot in the ultraviolet) that allows the required wavelength band to be selected
• A back end optics module that forms an image of the selected spectral band in the observed scene onto the detector
• The focal plane assembly consisting of the detector and its proximity electronics

The instrument makes use of tuneable filters (as noted above), in particular:

• Acousto-optic tuneable filters for the visible and infrared channels
• Fabry-Perot interferometers for the ultraviolet channel

Acousto-optic tuneable filters are small birefringent crystals (typically a few cubic centimetres big) that serve as interaction medium between the incoming light and an acoustic wave propagating in the crystal. By carefully selecting the acoustic wave frequency, the acousto-optic interaction induces a Bragg diffraction regime which deflects light with a specific wavelength away from its incident direction. The diverted beam can then be collected by an off-axis optical detector.

In the ultraviolet region, the filter element consists of a cascade of Fabry-Perot interferometers, as the acousto-optic tuneable filter technology has not proven to be fully compatible with the Altius environment in this particular range of wavelengths.

The use of tuneable active spectral filters allows the Atius Instrument to perform observations with a spectral resolution between 2.5 nm and 10 nm in an extremely versatile operational concept.

The use of the acousto-optic tuneable filter and Fabry Perot technologies constitutes a novelty in limb-sounding missions, with development and performance having been proven by an extensive instrument pre-development phase.

The Altius instrument is designed to operate continuously over one orbit in various modes. It will perform acquisitions in a broad wavelength range coping with large differences in light intensity across the various observed scenes.

Play the Altius game

Light coming from a star is composed of photons of different wavelengths. The wavelength is associated to the photon's energy. Some photons are visible by the human eye (from 380 to 700 nm in wavelength). Our eye is able to differentiate between the different wavelengths and associates colors to them: 450 nm is blue and 700 nm is red, for example. 

During a star occultation, Altius observes a star rising or setting through the atmosphere and measures the light that is not absorbed by the atmosphere at different altitudes. The ozone present in the atmosphere absorbs photons differently depending on their wavelength. Therefore, by performing such measurements, the Altius team is able to retrieve the ozone concentration in the atmosphere at different altitudes by simulating the absorption profile corresponding to a given ozone vertical profile.

In the Altius game, as a player, you get to decide which wavelengths and which star to observe. You then replace the retrieval algorithm and try to pick the right ozone profile so that the simulations of light transmittance match the observations. 

Good luck!