2.1.

Visible and Near-Infrared Imaging Spectrometer

The VNIS is one of the main scientific payloads on the lunar rovers for Chang’e 3 and Chang’e 4. It uses AOTFs as the dispersive components.^10,11 The VNIS mostly addresses the lunar surface material composition and available resource exploration for determining the lunar surface mineral composition and performing a comprehensive analysis of the chemical compositions. The VNIS is mounted on the platform of the lunar rover and detects the spectra and images of the lunar objects in the roving area to provide scientific data.⁹ As a passive optical instrument, the VNIS measures the radiance diffusely reflected from solar illumination of the Moon’s surface. It includes two detection channels: a VIS/NIR channel (0.45 to $0.95\mu \mathrm{m}$) with a CMOS area array detector and a SWIR channel (0.9 to $2.4\mu \mathrm{m}$) with an InGaAs single-element detector. Each channel consists of image-forming lens, field stop, collimating lens, convergent lens, and the AOTF. Furthermore, a calibration unit with dust-proofing functionality, motor drive circuit, radio frequency (RF) drive circuits, and main control circuits has been integrated in this payload as the public component, as shown in Fig. 2. The structure, optical module, and design of the VNIS are detailed in Figs. 3 and 4.

Fig. 2

Schematic diagram of the optical, electronic, and calibration modules of the VNIS.

Fig. 3

(a) Assembly diagram of the VNIS, (b) the optical modules, (c) AOTFs used in VNIS; the aperture of visible and near-infrared (VIS–NIR) AOTF is $8\times 8{\mathrm{mm}}^2$, and the SWIR one is $12\times 12{\mathrm{mm}}^2$.

Fig. 4

Diagram of the optical design of the VNIS.

As shown in Fig. 4, two types of AOTFs have been used in the optical system. Each AOTF has two transducers with separate ports to optimize the bandpass (FWHM) and diffraction efficiency. Furthermore, an optical wedge has been designed at the exit port to eliminate chromatic aberration in both AOTFs. The main characteristics of the VNIS AOTFs are shown in Table 1. The performance of AOTFs such as acoustic frequency range and diffraction efficiency is shown in Fig. 5.

Table 1

Main characteristics of the VNIS AOTFs.

Fig. 5

The performance of AOTFs equipped in VNIS. (a) The relationship between wavelength and driving frequency and (b) the diffraction efficiency of the VIS–NIR AOTF and SWIR AOTF.

After its soft landing on the Moon, Chang’e 3 carried out a lunar survey followed by scientific exploration activities. The VNIS mostly analyzed the lunar surface material composition and explored the available resources. The VNIS operated in two modes: detection and calibration. In the detection mode, the VNIS acquired scientific data from the lunar surface objects; the angle between the framework of the calibration unit and the horizontal mounting face of VNIS was about 55 deg. In the calibration mode, using solar radiation as the calibration source, the diffusing calibration plate of the calibration unit was set to a position parallel to the mounting plane to allow calibration of the instrument. The calibration unit of the VNIS at the light entrance consisted of an ultrasonic motor (USM), harmonic reducer, framework, and an internal diffuser panel. The operating modes and dust-proofing function were realized by the self-locking characteristics of the USM.¹⁰ The structure and function of the calibration unit are shown in Fig. 6.

Fig. 6

Schematic view of the structure and functioning of the calibration unit.

The VNIS was mounted in front of the lunar rover to detect lunar surface objects at a 45-deg viewing angle and to obtain the spectra and geometric data at a height of 0.695 m, as shown in Fig. 7.

Fig. 7

(a) Structural dimensions and mounting position of the VNIS. (b) The photograph of the Yutu lunar rover taken by the terrain camera (TCAM) at point A of the soft landing site.

2.2.

Lunar Mineralogical Spectrometer

The LMS is one of the main scientific payloads onboard the lunar landers for Chang’e 5 and Chang’e 6. The LMS primarily follows the technology of the VNIS but expands its spectral range from 0.48 to $3.2\mu \mathrm{m}$. It has the following functions: (1) to obtain the spectral image data of the specified objects in the VIS–NIR and spectral data in the SWIR channels, (2) to perform in-situ calibration, and (3) spectral imaging and spectral detection with multiband in full scanning region.

The LMS includes four detection channels: one is a hyperspectral imager with a spectral range of 0.48 to $0.95\mu \mathrm{m}$, and the others are three spectrographs and their total spectral range being 0.9 to $3.2\mu \mathrm{m}$. The LMS includes a two-dimensional (2-D) scanning mechanism, image-forming lens, collimating lens, AOTF, convergent lens, the detector components, motor drive, RF drive, and a main control circuit, as shown in Fig. 8. It is a kind of scanning, spectral programmable imaging spectrometer and includes a scanning mechanism and a programmable spectral selection spectrometer.

Fig. 8

System diagram of LMS.

As shown in Fig. 9, two AOTFs have been used in the optical system. They also have ports to optimize the bandpass (FWHM) and diffraction efficiency. The $-1$ order diffraction light of the VIS–NIR AOTF was used for the VIS spectral imaging and NIR spectra acquirement, where an optical wedge was designed for the AOTF to eliminate chromatic aberration. The $\pm 1$ order diffraction lights of the IR AOTF were used for the SWIR and MWIR channels, respectively. The main characteristics of these AOTFs are shown in Table 2.

Fig. 9

Diagram of the optical design of LMS.

Table 2

Main characteristics of the VNIS AOTFs.

After spectral calibration, the spectral resolution was 2.4 to 9.4 nm in the VIS–NIR and 7.6 to 24.9 nm in the IR channels, as shown in Fig. 10.

Fig. 10

Spectral resolution of LMS, separated into four channels by AOTF ports.

Images of the lander and LMS are shown in Fig. 11. The LMS is mounted on the $+Z-Y$ panel of the lander to detect the lunar surface objects and obtain the spectra and geometric data at a height of about 1.55 m. Because the platform on which the LMS is mounted is unmovable, a pointing component is required for expanding the observable areas. The 2-D scanning mechanism includes USMs, harmonic reducer, rotary potentiometers, and an aluminum-based scanning mirror, which ensure that the system is light and minimized and the system can adapt to a high surrounding temperature of up to 94°C.

Fig. 11

Schematic diagram of Chang’e 5 mission: (a) mounting position of the LMS on the lander of Chang’e 5 probe and (b) the flight product of the LMS.

3.1.

Visible and Near-Infrared Imaging Spectrometer

The VNIS can simultaneously obtain a spectral image in the VIS/NIR channel and spectral data in the SWIR channel. The optical axis of the VIS/NIR and SWIR channels are parallel to each other at a distance of 18 mm. The fields of view (FOVs) in the VIS/NIR and SWIR channels are $8.5\mathrm{deg}\times 8.5\mathrm{deg}$ and $\mathrm{\varphi }3.6\mathrm{deg}$, respectively. According to the spectral calibration, the spectral ranges of the VNIS are 449 to 950 nm with 2- to 7-nm resolution in the VIS/NIR channel and 899 to 2402 nm with 3- to 12-resolution in the SWIR channel, which are shown in Fig. 12.

Fig. 12

Spectral resolution of VNIS/Yutu, separated into four channels by AOTF ports.

The signal-to-noise ratio (SNR) of the VNIS is tested and analyzed by ground radiometric calibration. The SNR changes with the input signal, and it is an important indicator of the radiometric response characteristics of the VNIS. The SNR is $>31\mathrm{dB}$ in the VIS–NIR band when the albedo is 0.09 and the solar incident angle is 45 deg; and the SNR is $>32\mathrm{dB}$ in the SWIR channel when the albedo is 0.09 and the solar incident angle is 75 deg.¹⁰ The main performance specifications of the VNIS are shown in Table 3.

Table 3

Main performance specifications of VNIS/Yutu.10

After the Chang’e 3 mission began its scientific exploration stage, the VNIS instrument successfully completed the first lunar surface spectral acquisition at BTC 10:10 on December 23, 2013. During the first two lunar days, the VNIS performed measurements at four locations and obtained data in detection mode (four times) and calibration mode (three times). The total size of data was 350 MB.¹¹ The topographical images of N205 and the REFF of all detection nodes are shown in Fig. 13.

Fig. 13

(a) Data from VNIS/Yutu at N205 note, false color picture (500, 550, and 645 nm, in the right) and (b) REFF curves of four lunar exploration nodes during two lunar days by VNIS/Yutu.

During the Chang’e-4 mission, VNIS was carried on Yutu-2 lunar rover. Since the successful soft landing in Von Kármán crater on the far side of the Moon on January 3 2019,¹² VNIS had been working for five lunar days and performed 27 detections. The selected targets such as Yutu-2’s wheel track and rock, obtained in the detection node A and LE00303, and their corresponding REFF features are shown in the Fig. 14.

Fig. 14

(a) A 635-nm image from VNIS/Yutu-2 at node A, (b) 635-nm image from VNIS/Yutu-2 at node LE00303, and (c) REFF curves of nodes A and LE00303.

3.2.

Lunar Mineralogical Spectrometer

In the detection mode, the LMS obtained spectral images in the VIS/NIR channel and spectral data in three SWIR channels in sequence. The main performance specifications of the LMS are shown in Table 4.

Table 4

Main performance specifications of the LMS.

Performance parameters such as FOV, spectral range, spectral resolution, radiance, and geometric imaging features were calibrated in the ground experiment. The FOV of the LMS was $4.17\mathrm{deg}\times 4.17\mathrm{deg}$ in all channels.

The SNR is $>34\mathrm{dB}$ in the VIS/NIR band, and $>39\mathrm{dB}$ in the IR band, as shown in Fig. 15.

Fig. 15

SNR of LMS. The performance of LMS is calculated by the radiance calibration at the typical temperature (20°C).