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.
Field of view is a principle parameter of the push-broom imaging spectrometer which mainly affects its working efficiency. Integration of multiple sub-modules with smaller field of view is an optional solution to gain wider field of view. An approach for field registration and test for multi-module imaging spectrometer is presented. A single beam is reflected by multiple angle-tunable mirrors into multiple beams with different directions, which then enter the multimodule imaging spectrometer. Thus, one sub-module responses to lights with multiple incident angles simultaneously. A field registration platform is designed, which includes a collimating beam simulator, a multi-field beam generator, and an instrument support frame. The collimating beam simulator is composed of a collimator, an illuminant source, and a twodimensional shift pinhole target placed in the collimator focal plane. The multi-field beam generator is composed of a rotary table and multiple angle-tunable mirrors. The postures and the numbers of the mirrors are determined by the field registration requirement of the multi-module imaging spectrometer. Field registration is operated by monitoring the pixel response of the sub-module detector, and is tested by the two-dimensional shift of the pinhole target. Field registration was implemented on the platform for an airborne multi-module imaging spectrometer. Test result showed that the field registration is less than 0.2 pixels. Flight data of the imaging spectrometer demonstrated its good field registration alignment, which verified the engineering feasibility and value of the field registration approach.
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.
In the past decades, hyperspectral imaging technologies was well developed in the whole world. Visible and Near Infrared hyperspectral imagers play an important role in agriculture, land use, forestry, etc. Higher performance airborne hyperspectral imagery is strongly expected these years. Wider Field of View and higher resolution instrument can acquire data more efficiently. A VNIR PHI with 40 degree FOV, 0.125mrad IFOV, 256bands was integrated last year. The system can adapt to the Velocity to Height Ratio lower than 0.04. The system consists of 3 subsystems. Every subsystem consists of TMA fore optics, spectrometer with planar blazed grating and electronics. The 3 subsystems work for left, middle, right FOV, respectively. Thanks to CCD’s pixel binning function, the system can operate in high spectral resolution mode, high spatial resolution mode, and high sensitivity mode for different applications. The integration was finished, and airborne flight validation experiments were conducted.
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.
As a key component of the hyperspectral imager, the use of high frame frequency CCD was more and more widely. CCD working principle was analyzed. Methods to realize high frame frequency and image programmable online were presented. Hardware driver design was designed. The image with high visibility, whose frame frequency was more than 320 Hz, noise was below 75e-, dynamic range was superior to 70dB, was acquired by the CCD. Based on the operating characteristic of the CCD, the temperature experiment was designed. Through the experiment, the relation curve of dark level and noise with the CCD detector temperature was given, which was helpful for the later image correction.
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.
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.
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.
In the infrared system, cooling down the optic components' temperature is a
better choice to decrease the background radiation and maximize the sensitivity. This
paper presented a 100K cryogenic optical system, for which an integrated designation
of mechanical cooler, flexible thermal link and optical bench was developed. The
whole infrared optic components which were assembled in a vacuum box were cooled
down to 100K by two mechanical coolers. Low thermal conductivity supports and
low emissivity multi-layers were used to reduce the cryogenic optical system's heat
loss. The experiment results showed that in about eight hours, the temperature of the
optical components reached 100K from room temperature, and the vibration from the
mechanical coolers nearly have no affection to the imaging process by using of
thermal links. Some experimental results of this cryogenic system will be discussed in
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.
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.
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.
As an optical remote sensing equipment, the thermal infrared hyperspectral imager operates in the thermal infrared
spectral band and acquires about 180 wavebands in range of 8.0~12.5μm. The field of view of this imager is 13° and the
spatial resolution is better than 1mrad. Its noise equivalent temperature difference (NETD) is less than
0.2K@300K(average). 1 The influence of background radiation of the thermal infrared hyperspectral imager,and a
simulation model of simplified background radiation is builded. 2 The design and implementationof the Cryogenic
Optics. 3 Thermal infrared focal plane array (FPA) and special dewar component for the thermal infrared hyperspectral
imager. 4 Parts of test results of the thermal infrared hyperspectral imager.The hyperspectral imaging system is
China’s first success in developing this type of instrument, whose flight validation experiments have already been
embarked on. The thermal infrared hyperspectral data acquired will play an important role in fields such as geological
exploration and air pollutant identification.
The application of high solution thermal infrared imaging system highly relies on the signal to noise ratio (SNR) of instruments to acquire images. To enhance the imaging quality of the thermal infrared imaging system, the noise model for images of the thermal infrared imaging system has been analyzed. Spatial noise elimination method is proposed, including the results of the comparison between different methods. With combination of the testing data of the thermal infrared imaging system, it is showed that the method mentioned in this paper can really enhance the spatial-domain NETD and spatial resolution of images effectively.
We present the design of a compact, wide-angle pushbroom hyperspectral imager with a 42-deg wide field of view and a broadband response that covers the spectral range 450 to 2500 nm and provides a spectral sampling of 10 nm on SWIR and 5nm on VISNIR. The hyperspectral imager has finished the remote sensing experiment by emplaning on the YUN-12,and its performance meets the design parameters. The design is the high level technology and serves as an example for illustrating the design principles specific to this type of system.
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
Our group designed a thermal IR hyper-spectral imaging system in this paper mounted in a vacuum encapsulated cavity
with temperature controlling equipments. The spectral resolution is 80 nm; the spatial resolution is 1.0 mrad; the spectral
channels are 32. By comparing and verifying the theoretical simulated calculation and experimental results for this
system, we obtained the precise relationship between the temperature and background irradiation of optical and
mechanical structures, and found the most significant components in the optic path for improving imaging quality that
should be traded especially, also we had a conclusion that it should cool the imaging optics and structures to about 100K
if we need utilize the full dynamic range and capture high quality of imagery.
A ground-based long-wave hyperspectral imaging spectrometer (LWHIS) is designed and simulated. The spectrometer is
based on a focal plane array detector with a spectral response that covers the range 7700 to 9300 nm. Optical system of
this instrument is all-reflective and provides up to 30 continuous spectral channels with 54 nm of dispersion per pixel.
The entrance aperture is 20 mm and feeds an F/2 telescope front end. The telescope has a 11-deg field of view with 256
spatially resolved elements (detector pixel size is 30 μm). To get high enough signal noise rate (SNR), no concern about
the electronic part, first, the cool stop of the detector is used as soon as possible, and second, background thermal
radiance of the opto-mechanical system seen by the focal plane must be suppressed. Thus, the entire instrument is set in
a vacuum chamber and the opto-mechanical subsystem is cooled by liquid nitrogen. The background thermal radiance
verse different cases is discussed. Based on the radiation simulation and analysis, if the opto-mechanical subsystem of
the spectrometer within the vacuum chamber is cooled blew 100 Kelvin, significant performance gains can be realized.
The design and simulation provides an example for illustrating the design principles specific and radiation simulation to
this type of system.
The null compensation-test method with zero power corrector for concave aspheric mirrors with deep relative aperture and wide clear aperture, including oblate ellipsoids which are very difficult to be tested, is investigated in detail. Based on the third-order aberration theory, both two-lens zero-power corrector and the three-lens zero-power corrector are introduced. This method is simple and free of chromatic aberrations. As for wide clear aperture and deep relative aperture concave aspheric mirrors, it is difficult to use a two-lens zero power corrector to do compensation test, so a three-lens zero-power corrector is used instead to solve the problem well. The design method of the corrector and specific design examples with clear apertures 800mm and relative apertures 1/2, 1/3, 1/4 of paraboloids are given and their design results are analyzed and compared.
As for laser beam expander with high magnification, wide aperture and fast relative aperture, the off-axis aberrations of Galilean and Keplerian structures are difficult to be corrected. Based on third-order aberration theory, a novel field flattening laser beam expander optical system is presented. An aspheric objective is introduced, and a combined eyepiece of Galilean and Keplerian is put forward. As an example, a laser beam expander with magnification 30, field of view 2mrad, clear aperture of objective 300mm, focal length of objective 800mm, and primary working wavelength 0.6328μm is discussed,. Final test results by laser interferometer show that the PV value of the objective is better than λ/10 on axis and rms λ/90, PV value of optical system is better than λ/8 on axis and rms λ/55.
Schmidt system is a famous optical system. The corrector equation based on the third-order aberration theory has been acknowledged all along. When the previous equation is confirmed in characteristic of the aspheric surface equation and optical design program ZEMAX, it is found that the equation of the corrector has some errors. Analyses of this problem are given. A new corrector equation is established. The new equation is confirmed seriously by the optical design program ZEMAX again, it can be deduced that the coefficient α=1/2r02, and the spherical aberration coefficient ΣS1=0. This improvement is very useful for the optical design of Schmidt system, which quickens the optimization and easily reaches the optimal design data.
Maksutov made a good deal of study in the field that the reflective mirror was used to compensate and test the conicoid mirror, its principle is that normal aberration of conicoid mirror is compensated by reflective mirror. Based on the third-order aberration theory, the initial configuration parameters of the reflective mirror compensator are described in the paper. Assuming spherical aberration coefficient ΣS1=0, the relations between compensating mirror and the mirror under test are obtained, the surface of compensator may be sphere or ellipsoid. Setting e12 = 0,0.1,0.2,0.3,0.4. based on the equations of the graph of e22/α ~ β, αr01/r02 ~ β is drew, the relations of β ∝ α, e12, e22, r01, r02, d12 and the initial configuration parameters of compensating mirror are also obtained with the equations and graph. There are two cases about the location of compensator, one is α>0, α>1, the compensator is set in front of curvature center of tested mirror; the other is α<0, the compensator is set in back of curvature center of tested mirror, each case includes β>0, β=0, β<0. By analyzed in detail, all cases of compensator can be got, some cases have been discussed before, others haven't been discovered and discussed. In this way people can understand the mirror compensator comprehensively, which is much good for compensating test.
Based on the third-order aberration theory, the equation expressing the relationship between the conic constant and unaberration conjugate points is obtained. The corresponding graphs illuminating the relationship between e2 and β are given. According to the equation and graphs, the analyses of convex and concave surfaces are given in detail. We not only consider the unification of all kinds of reflective testing methods, such as Hindle test, concave elliptical mirror test and Ritchey-Common test etc, but also apply the unaberrational conjugate points test to refractive surface. The auto-collimating and interferometer diagnostic methods are powerful for the manufacture of lens. Kinds of convex and concave aspheric lenses have been manufactured with the detecting methods as mentioned above.