A new method for determining the transfer function of an acousto optical tunable filter

Acousto-optic tunable filter spectrometers in space missions [Invited]

Spectrometers employing acousto-optic tunable filters (AOTFs) rapidly gain popularity in space, and in particular on interplanetary missions. They allow for reducing volume, mass, and complexity of the instrumentation. To date, space operations of 11 AOTF spectrometers are reported in the literature. They were used for analyzing ocean color, greenhouse gases, atmospheres of Mars and Venus, and for lunar mineralogy. More instruments for the Moon, Mars, and asteroid mineralogy are in flight, awaiting launch, or in the state of advanced development. The AOTFs are used in point (pencil-beam) spectrometers for selecting echelle diffraction orders, or in hyper-spectral imagers and microscopes. We review the AOTF-employing devices flown in space or ready to set off. The paper considers basic principles of the AOTF and science applications of the AOTF spectrometers, and describes developed instruments in some detail. We also address some advanced developments for future missions and plans. In addition, we discuss lessons learned during instrument design, build, calibration, and exploitation, and advantages and limitations in implementing the AOTF-based systems in space instrumentation.

Improved algorithm for the transmittance estimation of spectra obtained with SOIR/Venus Express

The Solar Occultation in the InfraRed (SOIR) instrument onboard the ESA Venus Express spacecraft, an infrared spectrometer sensitive from 2.2 to 4.3 μm, probed the atmosphere of Venus from June 2006 until December 2014. During this time, it performed more than 750 solar occultations of the Venus mesosphere and lower thermosphere. A new procedure has been developed for the estimation of the transmittance in order to decrease the number of rejected spectra, to check that the treated spectra are well calibrated, and to improve the quality of the calibrated spectra by reducing the noise and accurately normalizing it to the solar spectrum.

Improved calibration of SOIR/Venus Express spectra

The SOIR instrument on board the ESA Venus Express mission has been operational since the insertion of the satellite around Venus in April 2006. Since then, it has delivered high quality IR solar occultation spectra of the atmosphere of Venus. The different steps from raw spectra to archived data are described and explained in detail here. These consist of corrections for the dark current and for the non-linearity of the detector; removing bad pixels, as well as deriving noise. The spectral calibration procedure is described, along with all ancillary data necessary for the understanding and interpretation of the SOIR data. These include the full characterization of the AOTF filter, one of the major elements of the instrument. All these data can be found in the ESA PSA archive.

Deconvolution in acousto-optical tunable filter spectrometry

Spectrometers and spectral imaging systems based on the acousto-optical tunable filter (AOTF) are becoming commonly used in many different fields in which high spectral resolution is crucial, e.g., laser-induced breakdown spectroscopy, Raman spectroscopy, and absorption spectroscopy of gases. As AOTFs have many advantages over other spectroscopic instruments but lack spectral resolution, a procedure for resolution enhancement, composed of point spread function characterization and spectrum preprocessing and deconvolution, is proposed. Wiener, Fourier-wavelet regularized (ForWaRD), Richardson-Lucy, and Wavelet-Lucy deconvolution methods were tested and their performances assessed with two deconvolution quality measures: resolution enhancement and noise amplification. It was shown that the proposed spectral resolution enhancement is feasible and gives good results for line spectra and highly dynamic spectra.