The lunar surface consists of rocks of varying sizes and shapes, which are made of minerals, such as pyroxene, plagioclase, olivine, and ilmenite, that exhibit distinctive spectral characteristics in the visible and near-infrared (VIS–NIR) and short-wave infrared (SWIR) regions. To analyze the composition of the lunar surface minerals, several spectrometers based on acousto-optic tunable filters (AOTFs) have been developed to detect lunar surface objects and to obtain their reflectance spectra and geometric images. These spectrometers, including the VIS–NIR imaging spectrometer onboard China’s Chang’e 3/4 unmanned lunar rovers and the Lunar Mineralogical Spectrometer onboard the Chang’e 5/6 lunar landers, use AOTFs as dispersive components. Both are equipped with a VIS/NIR imaging spectrometer, one or several SWIR spectrometers, and a calibration unit with dust-proofing functionality. They are capable of synchronously acquiring the full spectra of the lunar surface objects and performing in-situ calibrations. We introduce these instruments and present a brief description of their working principle, implementation, operation, and major specifications, in addition to the initial scientific achievement of lunar surface exploration.
Minerals such as pyroxene, plagioclase, olivine, and ilmenite, which constitute most of the lunar surface rocks with varying size and shape, have distinctive spectral characteristics in the VNIR and SWIR regions. To analyze the composition of lunar surface minerals, several spectrometers based on AOTF was developed to detect lunar surface objects and to obtain their reflectance spectra and geometric images includes the Visible and Near-IR Imaging Spectrometer(VNIS) onboard China’s Chang'E 3 and Chang’E 4 lunar rover and Lunar Mineralogical Spectrometer(LMS) onboard Chang'E 5 and Chang'E 6 lunar lander. These spectrometers, which use acoustic-optic tunable filters as dispersive components, consist of a VIS/NIR imaging spectrometer, an SWIR spectrometer, and a calibration unit with dust-proofing functionality. They are capable of synchronously acquiring the full spectra of lunar surface objects and performing in-situ calibration. This paper introduces these instruments, including their working principle, implementation, operation, and major specifications, as well as the initial scientific achievement of lunar surface exploration.
The Visible and Near-Infrared Imaging Spectrometer (VNIS) onboard China’s Chang’E 3 lunar rover is capable of simultaneously in situ acquiring full reflectance spectra for objects on the lunar surface and performing calibrations. VNIS uses non-collinear acousto-optic tunable filters and consists of a VIS/NIR imaging spectrometer (0.45–0.95 μm), a shortwave IR spectrometer (0.9–2.4 μm), and a calibration unit with dust-proofing functionality. To been underwent a full program of pre-flight ground tests, calibrations, and environmental simulation tests, VNIS entered into orbit around the Moon on 6 December 2013 and landed on 14 December 2013 following Change’E 3. The first operations of VNIS were conducted on 23 December 2013, and include several explorations and calibrations to obtain several spectral images and spectral reflectance curves of the lunar soil in the Imbrium region. These measurements include the first in situ spectral imaging detections on the lunar surface. This paper describes the VNIS characteristics, lab calibration, in situ measurements and calibration on lunar surface.
As a new type of light dispersion device, Acousto-Optic Tunable Filter (AOTF) based on the acousto-optic interaction principle which can achieve diffractive spectral, has rapidly developed and been widely used in the technical fields of spectral analysis and remote sensing detection since it launched. The precise measurement of AOTF’s optical performance parameter is the precondition to ensure spectral radiometric calibration and data inversion in the process of quantitation for spectrometer based on AOTF. In this paper, a kind of AOTF performance analysis system in 450~3200nm wide spectrum was introduced, including the fundamental principle of the basic system and the test method of the key optical parameters of AOTF. The error sources and the influence of the magnitude of the error in the whole test system were analyzed and verified emphatically. The numerical simulation of the noise in detecting circuit and the instability of light source was carried out, and based on the simulation result, the method for improving the measuring accuracy of the system were proposed such as improving light source parameters, correcting and changing test method by using dual light path detecting, etc. Experimental results indicate that: the relative error can be reduced by 20%, and the stability of the test signal is better than 98%. Finally, this error analysis model and the potential applicability in other optoelectronic measuring system were also discussed in the paper.
Visible and Near-infrared Imaging Spectrometer (VNIS) is one of the scientific payloads mounted on “Yutu” rover in
Chang’e 3 lunar exploration project. The VNIS is composed with a visible and near-infrared (0.45-0.95 μm) spectral
imager and a short waveband (0.9-2.4 μm) spectrometer on basis of Acousto-Optic Tunable Filter. According to the
in-situ analysis, a calibration unit was also equipped for high precisely spectral radiance and reflectance inversion by
using solar as standard calibration source. The calibration unit was driven by lightweight ultrasonic motor, and it could
be located on three fixed position including detection (full-opened), calibration (horizontal) and dust-proof (closed). In
this paper, the principle of VNIS, especially calibration unit was described firstly. Then, radiometric correction
algorithms on lunar surface based on standard solar spectral irradiance were expounded. Through the analysis of VNIS
scientific data, the spectral radiance and reflectance curves of detection area were shown in the end.
In the field of lunar surface content analysis, spectral characteristics of the 2.2~3.2 μm waveband can provide a strong basis for analyzing and identifying specific compositions. For lunar soil, an object’s spectrum in this band includes not only reflective solar radiance, but also emissive radiance from the temperature characteristics of the object itself. These form a mixed spectrum, which complicates the spectral analysis for this band. Based on a mixed spectral model, considering both reflective and emissive radiance on the lunar surface, spectral characteristics for a 2.2~3.2 μm waveband are simulated and relationships are analyzed between reflective/emissive spectra and factors such as temperature, light, an object’s physical composition, etc. Moreover, a simulative platform (incorporating a spectrometer, light source, temperature controller, and simulative object) is developed according to lunar surface conditions, so as to further validate the mixed spectral model, which may provide a reference for lunar surface spectral analysis and the development of spectrometers.
Different objects require different imaging spectra, and a high-temporal-resolution imaging spectrometer has further
requirements for producing a large field of view(FOV). For these reasons, we present a new system based on a
staring/scanning area-array detector to allow for wide-FOV imaging, along with an acousto-optical tunable filter (AOTF)
to impart quick, programmable selection of spectra. Using AOTF as a dispersive component and a turntable as the
motion platform, we designed a staring, spectral-programmable imaging prototype with a working band of 400~1000
nm. Besides, we could obtain a large FOV by image mosaic. We performed imaging experiments and image processing
for an initial verification of this technique. This technical solution can be used not only for VNIR, but also in SWIR,
MWIR, and other bands; it adapts to spectral and FOV requirements for varying subjects, and provides an advanced
technical means for further application of high-resolution spectral imaging.
To analyze the composition of lunar surface minerals, one of the scientific payloads of the Chang’E 3 Yutu rover, the
Visible and Near-infrared Imaging Spectrometer (VNIS), was developed to detect lunar surface objects and to obtain
their reflectance spectra and geometric images. The VNIS, which uses acousto-optic tunable filters as dispersive
components, consists of a VIS/NIR imaging spectrometer (0.45-0.95 μm), a shortwave IR spectrometer (0.9-2.4 μm),
and a calibration unit with dust-proofing functionality. It is capable of synchronously acquiring the full spectra of lunar
surface objects and performing in-situ calibration. After landing successfully on the Moon, the VNIS performed several
explorations and calibrations, and obtained several spectral images and spectral reflectance curves of the lunar soil in the
Imbrium region. This paper introduces the VNIS, including its working principle, implementation, operation, and major
specifications, as well as the initial scientific achievement of lunar surface exploration.
The Acousto-Optic Tunable Filter (AOTF) is an electronically tunable optical filter based on Acousto-optic effect and
has its own special compared with other dispersive parts. Imaging spectrometer based on acousto-optic tunable filter
(AOTF) is a useful high-spectral technology, especially in deep space exploration applications because its characteristics of staring imaging, electronic tunable spectral selection and simple structure. Because the diffraction of light in AOTF filters is dependent on both wavelength and angle of incidence, the Spectral and geometrical calibration must therefore
be performed over the entire spectral range of AOTF hyper-spectral imaging systems. In this paper, the dispersive principle of AOTF is introduced firstly and its application predominance in space-based spectral detection is analyzed.
Then, a method for calibration of acousto-optic tunable filter (AOTF) hyper-spectral imaging systems is proposed and
evaluated. This paper introduces the calibration of a VIS-NIR Imaging Spectrometer (VNIS) by the method. The VNIS is
a payload instrument for lunar detection and provides programmable spectral selection from 0.45 to 0.95μm. The
results indicate that the method is accurate and efficient. Therefore, the proposed method is suitable for spectral and geometrical calibration of imaging spectrometers based on AOTF.