Liyin Yuan

Operating principles and detection characteristics of the Visible and Near-Infrared Imaging Spectrometer in the Chang’e-3

The Visible and Near-Infrared Imaging Spectrometer (VNIS), using two 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. The VNIS was utilized to detect the spectrum of the lunar surface and achieve in-orbit calibration, which satisfied the requirements for scientific detection. Mounted at the front of the Yutu rover, lunar objects that are detected with the VNIS with a 45° visual angle to obtain spectra and geometrical data in order to analyze the mineral composition of the lunar surface. 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 region of Mare Imbrium. This paper describes the working principle and detection characteristics of the VNIS and provides a reference for data processing and scientific applications.

Optical design of wide swath hyperspectral imager

This paper describes a design concept for wide swath hyperspectral imager. The challenge is to meet the requirement of good image quality and high precision registration from 400nm to 2500nm. A new type spherical prism imaging spectrometer is presented in the paper. The swath of system can reach 60 kilometer from a 600km sun-synchronous orbit with 30 meter ground sample distance (GSD). The optical system consists of a TMA objective and 2 30mm-slit spherical prism spectrometer operating both VNIR and SWIR. Key features of the design include (1) high signal to noise ratio for high efficiency of F-silica prism; (2) high precision band registration for same spectrometer operating from 400nm to 2500nm.

Optical design, laboratory test, and calibration of airborne long wave infrared imaging spectrometer

We discuss and evaluate a long wave infrared imaging spectrometer in terms of its opto-mechanical design and analysis, alignment, testing, and calibration. The instrument is a practical airborne sensor achieving high spectral resolution and sensitive noise equivalent delta temperature. The instrument operates in the 8 to 12.5 μm spectral region with 28.85 nm spectral sampling, 1 mrad instantaneous field of view, and >40° cross track field. The instrument comprises three uniform sub-modules with identical design parameters and performances. The sub-module design is based on a refractive foreoptics feeding an all-reflective spectrometer. The optical form of the spectrometer is a double-pass reflective triplet with a flat grating, which has a fast f/2 and high optical throughput. Cryogenic optics of 100 K is implemented only for the spectrometer. Assembly and thermal deformation and focusing adjustment design are particularly considered for this low temperature. All the mirrors of the spectrometer are opto-mechanical-integrated designed and manufactured by single-point diamond turning technology. We consider the center sub-module as an example, and we present its laboratory testing results and calibration; the results indicate the instruments potential value in airborne sensing.

Recent progress of push-broom infrared hyper-spectral imager in SITP

In the past decades, hyper-spectral imaging technologies were well developed in SITP, CAS. Many innovations for system design and key parts of hyper-spectral imager were finished. First airborne hyper-spectral imager operating from VNIR to TIR in the world was emerged in SITP. It is well known as OMIS(Operational Modular Imaging Spectrometer). Some new technologies were introduced to improve the performance of hyper-spectral imaging system in these years. A high spatial space-borne hyper-spectral imager aboard Tiangong-1 spacecraft was launched on Sep.29, 2011. Thanks for ground motion compensation and high optical efficiency prismatic spectrometer, a large amount of hyper-spectral imagery with high sensitivity and good quality were acquired in the past years. Some important phenomena were observed. To diminish spectral distortion and expand field of view, new type of prismatic imaging spectrometer based curved prism were proposed by SITP. A prototype of hyper-spectral imager based spherical fused silica prism were manufactured, which can operate from 400nm~2500nm. We also made progress in the development of LWIR hyper-spectral imaging technology. Compact and low F number LWIR imaging spectrometer was designed, manufactured and integrated. The spectrometer operated in a cryogenically-cooled vacuum box for background radiation restraint. The system performed well during flight experiment in an airborne platform. Thanks high sensitivity FPA and high performance optics, spatial resolution and spectral resolution and SNR of system are improved enormously. However, more work should be done for high radiometric accuracy in the future.

High spectral resolution airborne short wave infrared hyperspectral imager

Short Wave InfraRed(SWIR) spectral imager is good at detecting difference between materials and penetrating fog and mist. High spectral resolution SWIR hyperspectral imager plays a key role in developing earth observing technology. Hyperspectral data cube can help band selections that is very important for multispectral imager design. Up to now, the spectral resolution of many SWIR hyperspectral imagers is about 10nm. A high sensitivity airborne SWIR hyperspectral imager with narrower spectral band will be presented. The system consists of TMA telescope, slit, spectrometer with planar blazed grating and high sensitivity MCT FPA. The spectral sampling interval is about 3nm. The IFOV is 0.5mrad. To eliminate the influence of the thermal background, a cold shield is designed in the dewar. The pixel number of spatial dimension is 640. Performance measurement in laboratory and image analysis for flight test will also be presented.

New generation VNIR/SWIR/TIR airborne imaging spectrometer

Imaging spectrometer plays an important role in the remote sensing application. Imaging spectrometer can collects and provides a unique spectral signature of many materials. The spectral signature may be absorbing, reflecting, and emitting. Generally, optical spectral bands for earth observing consist of VNIR, SWIR, TIR/LWIR. VNIR band imaging spectrometer is well-known in vegetation remote sensing and ocean detection. SWIR band imaging spectrometer is widely applied in mineralogy investigation. For its uniquely capability of spectral radiance measurement, TIR/LWIR imaging spectrometer attracts much attention these years. This paper will present a new generation VNIR/SWIR/TIR imaging spectrometer. The preliminary result of its first flight will also be shared. The spectral sampling intervals of VNIR/SWIR/TIR are 2.4nm/3nm/30nm, respectively. The spatial pixel numbers are 2800/1400/700,respectively. It’s a push-broom imaging spectrometer.

Optical design of MWIR imaging spectrometer with a cold slit

MWIR imaging spectrometer is promising in detecting spectral signature of high temperature object such as jet steam, guided missile and explosive gas. This paper introduces an optical design of a MWIR imaging spectrometer with a cold slit sharply reducing the stray radiation from exterior environment and interior structure. The spectrometer is composed of a slit, a spherical prism as disperser, two concentric spheres and a correction lens. It has a real entrance pupil to match the objective and for setting the infrared cold shield near the slit and a real exit pupil to match the cold shield of the focal plane array (FPA). There are two cooled parts, one includes the aperture stop and slit, and the other is the exit pupil and the FPA with two specially positioned cooled shields. A detailed stray radiation analysis is represented which demonstrates the outstanding effect of this system in background radiation restraint.

Manufacture, alignment and measurement for a reflective triplet optics in imaging spectrometer

Reflective triplet (RT) optics is an optical form with decenters and tilts of all the three mirrors. It can be used in spectrometer as collimator and reimager to get fine optical and spectral performances. To alleviate thermal and assembly stress deformation, opto-mechanical integrated design suggests that as with all the machine elements and the mainframe, the mirrors substrates are aluminum. All the mirrors are manufactured by single-point diamond turning technology and measured by interferometer or profilometer. Because of retro-reflection by grating or prism and reimaging away from the object field, solo three mirrors optical path of RT has some aberrations. So its alignment and measurement needs an aberration corrected measuring optical system with auxiliary plane and sphere mirrors and in which the RT optics used in four pass. Manufacture, alignment and measurement for a RT optics used in long wave infrared grating spectrometer is discussed here. We realized the manufacture, alignment and test for the RT optics of a longwave infrared spectromter by CMM and interferometer. Wavefront error test by interferometer and surface profiles measured by profilometer indicate that performances of the manufactured mirrors exceed the requirements. Interferogram of the assembled RT optics shows that wavefront error rms is less than 0.0493λ@10.6μm vs design result 0.0207λ.

Visible and near-infrared imaging spectrometer (VNIS) for in-situ lunar surface measurements

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.

In situ lunar surface visible and near-IR measurements

Chang’E 3 is an unmanned lunar exploration mission operated by the China National Space Administration (CNSA). The mission consists of a lunar surface lander and a lunar rover, which is known as Yutu (meaning ‘jade rabbit’). Both parts of Chang’E 3 carry a number of scientific payload instruments.1–3 This mission is the third in the Chang’E program, and is seen as a precursor to future robotic and manned lunar missions. The main mission objectives were to achieve the first soft landing of space hardware by the CNSA and to conduct the first Chinese lunar exploration with a rover. In addition, this mission has led to the development and demonstration of key technologies that will be used for future missions. Change’E 3 entered into orbit around the Moon on 6 December 2013 and landed on 14 December 2013. The landing site was at 44.12N and 19.51W, within relatively young titaniumrich lava flows in the northern part of Mare Imbrium. One of the main scientific instruments on Yutu is the Visible and Near-IR Imaging Spectrometer (VNIS). This instrument has been designed to obtain mineralogical and compositional information for the lunar soil as the rover traverses the surface. The VNIS underwent a full program of pre-flight ground tests, calibrations, and environmental simulation tests. In our VNIS instrument we use non-collinear acousto-optic tunable filters (AOTFs) as dispersive components within the visible/near-IR spectrometer (operating in the 0.45–0.95 m range) and shortwave IR (SWIR) spectrometer (operating in the 0.9–2.4 m range), as well as in the calibration module that has dust-proofing functionality.4, 5 The VNIS is mounted on the front of the Yutu rover (see Figure 1) so that it can detect objects on the lunar surface within a 45 viewing angle. Spectra and geometric data are obtained with the VNIS from a height of 0.69m. The major technical specifications of the VNIS are given in Table 1.5 Figure 1. Photograph of the Chinese lunar rover Yutu. The red box indicates the position of the Visible and Near-IR Imaging Spectrometer (VNIS) that is mounted on the front of the rover (shown in bottom right inset). The top right inset illustrates how reflectance spectra are measured by the instrument.