Characteristics of Staring Imaging Spectrometer and Its Application Prospect in Asteroid Exploration

HE Zhiping,ZHAO Haiting,LI Jinning

Shanghai Institute of Technical Physics of the CAS,Shanghai 200083

Abstract:Compared with traditional ground asteroid observations,deep space exploration is an important way to explore and comprehensively understand the characteristics of asteroids.Imaging spectrometer integrates morphological measurement and spectral measurement,and has the ability to acquire image and spectral data simultaneously.By combining morphometry and spectrometry,it is possible to achieve efficient identification and quantitative analysis of the chemical components of the exploration target,and has the strong advantage in the field of asteroid exploration.This paper analyzes the principle of the staring imaging spectrometer and the technological progress in various countries.Based on the requirements of light,small payloads and the space characteristics of spectroscopic devices,the application of staring imaging spectrometer is discussed.Then,this paper introduces the conceptual design of an acousto-optic staring imaging spectrometer,combined with the technical characteristics of its area array stare frame imaging and fast electronic control spectrum selection.An experimental verification is carried out,which provides a reference for the feasibility of this type of instrument in asteroid exploration.

Key words:imaging spectrometer,staring imaging,asteroid exploration,AOTF,space applications

1 INTRODUCTION

Finding extraterrestrial life or evidence of its existence,discovering extraterrestrial civilization and space migration,and revealing the origin of life are one of the main purposes of human exploration of the universe.Ground-based asteroid observation usually obtains some basic orbital parameters and physical properties,but the resolution of the classification and shape of asteroid material components is relatively limited.In particular,the measurement of the asteroid's volume,material composition,internal structure,gravitational field,magnetic field and other parameters is almost absent.Compared with the limitations of ground observation,deep space exploration has become an important way to explore and comprehensively understand the characteristics of asteroids[1].

In the past two decades,asteroid exploration missions have been gradually increasing.In 1991,the“Galileo”Jupiter Orbiter Probe,launched by the United States,made a leap forward flying by the 951 Gaspra asteroid,which was the first human observation of the asteroid.In 2000,NASA’s“NEAR”probe successfully entered an orbit of 35 km around the 433 Eros asteroid,and as part of its mission carried out multiple observations,including multi-spectral detection.In 2003,Japan’s“Hayabusa 1”probe was successfully launched and returned to Earth in 2010,becoming the world’s first probe to conduct an asteroid sampling return mission[2].In 2014,“Hayabusa 2”was launched at the Tanegashima Space Center in Kagoshima Prefecture.After nearly four years of flight,it rendezvoused with asteroid 162173 Ryugu in 2018 and entered the scheduled orbit 20 km from Ryugu.Spectral detection and finally sampling on Ryugu were achieved.It is expected that it will bring the asteroid samples back to Earth by the end of 2020,helping to answer puzzles such as the formation of the solar system and the origin of life[3].

Morphological and spectral measurements are the main methods for studying target components.The imaging spectrometer can obtain the geometric and spectral characteristics of the target simultaneously.It can apply the combination of morphometry and spectrometry to achieve efficient identification and quantitative analysis of chemical components in exploration targets.It is widely used in deep space exploration,mineral exploration,agricultural and forestry resources survey,environmental monitoring,and urban planning[4-6].

The imaging spectrometer operation is divided into three types：scanning,push-broom and staring,according to the different imaging methods.Staring imaging spectroscopy is a way to measure the geometric and spectral information of an object to be observed without scanning through the platform or by self-motion.Compared with the traditional push-broom and scanning imaging techniques based on grating or prism,it has the characteristics of frame imaging.

So far,remote sensing exploration missions for asteroids include ground-based exploration,fly-by exploration,orbiter exploration,and unmanned landing exploration.Considering the size of asteroids,adopting the staring imaging spectral instrument is unique in the simultaneous exploration of asteroid images and spectra due to its feature of frame imaging.

2 ANALYSIS OF MAIN SPECTROSCOPIC DEVICES AND THEIR SPACE APPLICATION CHARACTERISTICS

The core component of the imaging spectrometer is the spectroscopic device,which determines the spectral resoluion,spectral range,operating mode,operating temperature range,weight,volume and other performance parameters of the spectrometer.The staring imaging spectrometer mainly uses an acousto-optic tunable filter (AOTF),a liquid crystal tunable filter(LCTF),and a filter wheel as the spectroscopic components.

2.1 Acousto-optic Tunable Filter

Acousto-optic Tunable Filter (AOTF) is an electro-optic modulation spectroscopic device.It uses solid-state crystal materials to achieve wavelength selection based on the principle of acousto-optic effect[7-8].Practical AOTF crystal operating spectrum range is from visible light to infrared spectrum (0.35-5.5 μm).At present,AOTF devices produced in China can achieve functional performance in the visible to short-wave infrared spectrum (up to 3200 nm),and have the ability to expand to about 5000 nm.The AOTF device and the spectroscopic process are shown in Figure 1[7].When a beam of polychromatic light passes through a high-frequency vibrating optical elasticity crystal,monochromatic light of a certain wavelength will be diffracted inside the crystal and transmitted from the crystal at an angle,so that image information of this wavelength can be obtained.When the crystal vibration frequency changes,the wavelength of the transmissive monochromatic light also changes accordingly.By scanning the frequency,a set of image information and image cubes of different wavelengths are obtained.

2.2 Liquid Crystal Tunable Filter

Liquid Crystal Tunable Filter (LCTF) is a new type of spectroscopic device based on the field-induced birefringence effect of liquid crystal.It has functions of light modulation,deflection and filtering[9].The practical LCTF has a working spectrum ranging from visible light to infrared (0.40-2.4 μm).The LCTF is a tunable filter formed by combining a liquid crystal material with a Lyot and a Solc type birefringent filter based on a field-induced birefringence effect of liquid crystal material.It can selectively output light waves of different spectral bands by changing the polarization offset of the birefringent wave plate by adjusting the voltage applied to the LCTF,and obtain image information and image cubes of different wavelengths of the target[10].

2.3 Filter Wheel

The filter is a precision optical filter device based on the principle of optical film interference.The spectral properties are obtained by designing and changing the structure of the optical film system and the optical properties of the film layer.In order to switch between multiple wavelengths,multiple filters of different transmission wavelengths must be mounted on the rotating wheel,and the image wheel of different wavelengths can be obtained by rotating the filter wheel[11].

Figure 2 LCTF device and spectroscopic process

Figure 3 Filter wheel

2.4 Variable Filter

A variable filter is an optical device whose spectral characteristics vary with position.It is formed by a process such as ion assisting or ion beam sputtering by plating a range of film systems having varying thicknesses on the surface of the substrate as shown in Figure 4.Compared with the traditional narrow-band filter,it has a nearly continuous spectral channel,so the use of the variable filter for splitting can achieve higher spectral resolution[12].

2.5 Feature Comparison and Space Application Analysis

In general,the filter wheel space application is relatively old,mainly used for wavelength selection of multi-spectral imagers.However,its imaging channel is limited and cannot meet the needs of spectroscopic devices for hyperspectral imagers.

To his surprise, Diana treated his proposal as a joke, She actually giggled7. But soon she could see that Prince Charles was serious. Despite an insistent8（，） voice inside her head that told her she would never be Queen, she accepted his proposal.

Figure 4 Schematic diagram of structure VF

As a new type of spectroscopic device,the AOTF and the LCTF have gradually become the mainstream spectroscopic devices for staring spectrometers.Imaging spectrometers based on LCTF have the advantage of adopting a simple optical path,but limited by the characteristics of liquid crystal materials,hence space environment adaptability is relatively poor,especially in terms of mechanical properties and temperature adaptability.In addition,limited by the technical level,current liquid crystal materials and polarizing materials have poor performance in the infrared spectrum,which limits the useful range of the LCTF.

The AOTF is a tunable filter,solid state and no movable parts.It has good mechanical,temperature and radiation characteristics and strong adaptability to the space environment.A imaging spectrometer based on the AOTF is easy to realize in terms of small size,light weight and high reliability,and it is easy to meet the spectral requirements for light and small,highly reliable staring imaging spectrometers for space applications[13].

3 SEVERAL STARING IMAGING SPECTROMETERS FOR DEEP-SPACE EXPLORATION

Here we briefly introduce several kinds of deep-space application staring imaging spectrometers,and provide a basis for the technical approach of imaging spectrometers based on asteroid exploration.

3.1 Surface Stereo Imager

The Surface Stereo Imager (SSI) is the eye of the NASA-launched Phoenix Rover.Its spectroscopic components and instruments are shown in Figure 5.It provides high-resolution,stereoscopic,and full-color images of the Martian geographical surface features and indicates Mars mining potential via the detector along with atmospheric dust and cloud detection.Multi-spectral imaging of 12 bands is achieved using a filter wheel that can hold 12 filters[14].

3.2 Visible And Near-Infrared Imaging Spectrometer For Lunar Surface [15-17]

The Visible and Near-Infrared Imaging Spectrometer (VNIS)is a typical staring-type acousto-optic spectrum imaging instrument.It is one of the payloads on the Chang’e 3 and Chang’e 4 lunar payloads for China.It has visible detection near the spectral imaging of the infrared spectrum (0.45-0.95 μm) and the spectral exploration of the short-wave infrared spectrum(0.9-2.4 μm) providing scientific exploration data for the mineral composition analysis of the lunar surface.The infrared imaging spectrometer is based on the lunar static platform.When the roving is stopped,the in-situ spectral imaging of the roaming area target is detected at a height of 0.69 m,giving the mineral composition and distribution analysis for the roving area,thus enabling comprehensive research on energy and mineral resources with the scientific data.VNIS and exploration data are shown in Figure 6.

Figure 5 SSI color full-frame image and instrument physical

Figure 6 Visible and near-infrared imaging spectrometer and exploration data

4 CONCEPTUAL DESIGN OF STARING IMAGING SPECTROMETER FOR ASTEROID EXPLORATION

The conceptual design of the asteroid imaging spectrometer uses the AOTF spectral device to split the light.When the broad spectrum of the solar radiation is reflected on the surface of the asteroid and then reflected,after entering the detector's exploration window,the corresponding surface component spectra are collected.Corresponding curves which are characterized by spectral absorption and reflection characteristics,can be used to analyze the nature of the surface composition of asteroids through spectral features.

Deep space asteroid exploration usually includes a fly-by exploration phase,local flight exploration phase,attachment phase and return exploration phase.The preferred target for asteroid exploration is a near-Earth asteroid,which is usually less than 2 km in diameter.It is selected for overflying and local exploration at an orbital altitude of 20 km.The corresponding field of view is less than 6°.The principle of detection using a staring imaging spectroscopy instrument is shown in Figure 7 and Figure 8.

Asteroids are usually made up of minerals and require sufficiently fine spectral resolution to distinguish mineral components,so high-spectral resolved spectrometers are needed for asteroid exploration.Based on the above analysis,the main parameters for the preliminary design of the asteroid exploration imaging spectrometer are shown in Table 1.

Figure 7 Staring imaging spectrometer for asteroid exploration

A prototype of a staring imaging spectrometer was developed to verify the imaging and spectral exploration capabilities of different sizes of exploration targets.The system block diagram is shown in Figure 9.

Table 1 The main indicators of the asteroid exploration imaging spectrometer

Figure 8 Exploration principle and data from asteroid imaging spectrometer

Figure 9 Block diagram of the staring imaging spectrometer [18]

The imaging lens uses a commercial lens (photo lens,telescope head),and the core spectral imaging module is connected through the C-mount bayonet.The core beam splitting component uses AOTF to obtain the spectrum of the ground object being imaged.The ground imaging test device is shown in Figure 10 and 700 nm lunar spectral image is shown in Figure 11[19].In order to adapt to the long-distance lunar imaging test,the photographic lens is replaced by an astronomical telescope,and the moon is imaged at night.The lunar spectral imaging data is obtained,and the preliminary configuration of the asteroid imaging spectrometer is obtained.The program was verified.

Figure 10 Schematic diagram of the test device of the staring imaging spectrometer

5 CONCLUSIONS

In this paper,the principle of the staring imaging spectrometer and the main spectroscopic device is introduced,and the space adaptability and typical application are discussed.Through the detailed analysis of asteroid exploration requirements and domestic and foreign technological advancement,based on the requirements of light,small and reliable payloads,a conceptual design of a staring-type imaging spectrometer for asteroid is provided,which provides a reference for future asteroid exploration.

Figure 11 700 nm lunar spectral image