Abstract
Acousto-optic tunable filter (AOTF) spectrometers are being criticized for spectral leakage, distant side lobes of their spectral response function (SRF), or the stray light. SPICAM-IR is the AOTF spectrometer in the range of 1000-1700 nm with a resolving power of 1800-2200 operating on the Mars Express interplanetary probe. It is primarily dedicated to measurements of water vapor in the Martian atmosphere. SPICAM H(2)O retrievals are generally lower than simultaneous measurements with other instruments, the stray light suggested as a likely explanation. We report the results of laboratory measurements of water vapor in quantity characteristic for the Mars atmosphere (2-15 precipitable microns) with the Flight Spare model of SPICAM-IR. We simulated the measured spectra with HITRAN-based synthetic model, varying the water abundance, and the level of the stray light, and compared the results to the known amount of water in the cell. The retrieved level of the stray light, assumed uniformly spread over the spectral range, is below 1-1.3·10(-4). The stray may be responsible for the underestimation of water abundance of up to 8%, or 0.6 pr. µm. The account for the stray light removes the bias completely; the overall accuracy to measure water vapor is ~0.2 pr. µm. We demonstrate that the AOTF spectrometer dependably measures the water abundance and can be employed as an atmospheric spectrometer.
Highlights
The stray light or spectral leakage of a spectrometer can be determined as a departure of a non-ideal spectral response function (SRF) from zero beyond its narrow peak corresponding to the spectrometer’s spectral resolution
Acousto-optic tunable filter (AOTF) spectrometers are being criticized for spectral leakage, distant side lobes of their spectral response (SRF) function, or the stray light
In order to characterize the stray light in the AOTF spectrometer, the Flight Spare Model of SPICAM-IR was used to measure the absorption in a cell filled with water vapor
Summary
The stray light or spectral leakage of a spectrometer can be determined as a departure of a non-ideal spectral response function (SRF) from zero beyond its narrow peak corresponding to the spectrometer’s spectral resolution. The spectral leakage hampers the possibility of the instrument to measure weak features in absorption, diffuse, or reflectance spectra. Acousto-optic tunable filter (AOTF) spectrometers are largely used in a variety of applications, from medicine to astronomy [1, 2]. The acoustic problems may result in amplification of the distant lobes, distortion of the SRF, or a continuous stray light. SPICAM-IR is operated in orbit around Mars since 2004, and it continuously delivers atmospheric information This high-resolution and broad-range instrument is especially susceptible to the stray light as explained in beginning of this section. In this paper we report the results of these measurements, demonstrating high fidelity of the AOTF spectra, and a low level of the stray light and distortions
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