The ALTIUS concept

Many spectrometers have to disperse light by means of a prism or by a diffraction grating. As a consequence, the use of a bi-dimensional detector (like a CCD) implies that one dimension will be reserved for wavelength whereas the other one is a spatial one, i.e. related to a particular direction in the field of view.

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The fundamental idea of ALTIUS is to use the entire detector as an imager of the atmospheric limb to solve the tangent altitude registration problem. Indeed, the capture of the observed scene allows for different techniques to be applied in order to assign to each detector pixel a well defined altitude with respect to the ground : observation of stars in the background, detection of geographical details, generalized « Knee » methods as well as the combined use of the satellite star tracker information.

Furthermore, full imaging of the limb is very interesting to measure horizontal variations of the ozone field, to identify cloudy environments and special clouds like the polar stratospheric clouds implied in the ozone destruction mechanism.

Of course, the wavelength selection of the detected light has to be performed by other means. It is possible to use a filter wheel system where a limited number of interference filters are moved in front of the system optical input. However, this not ideal for a space instrument where mechanical devices should be avoided as much as possible.

aotfprinciple.jpgRecently, major advances have been obtained in the use of acousto-optical tunable filters (AOTF) that can now be operated in the UV wavelength range. AOTF’s are crystals of birefringent material to which a travelling acoustic wave is applied through a tranducer coupled to the RF drive.The wave induces a periodical modulation of the refractive index responsible for a full concentration of the incident light into the first diffraction order, at a wavelength defined by the frequency of the acoustic wave. This interesting property allows to change very rapidly the selected wavelength by modifying the RF frequency (inversely proportional to the wavelength). A limitation of the technique is the fact that the tuning should not exceed one octave. Therefore, a full coverage of the UV-Vis-NIR (250-2000 nm) range requires 3 different AOTF’s. The standard crystal in the Vis-NIR range is made of TeO2 whereas the UV domain can only be addressed by KDP.

Hence, the basic ALTIUS configuration consists of a set of 3 « spectral cameras », each being a combination of an AOTF, a 2D detector and the appropriate optics.µ