Analysis and design of an ultra-wide temperature difference very long wave infrared multifunctional imaging spectrometer.

The detection of various cryogenic targets, including the polar cryosphere, high-altitude clouds, and cosmic galaxies through spectral analysis, is a highly valuable area of research. Nevertheless, creating a very long wave infrared (VLWIR) imaging spectrometer capable of detecting these targets presents a significant challenge. In this paper, we introduce a design concept for an ultra-wide temperature difference athermalization VLWIR multifunctional imaging spectrometer. Initially, we analyze the multifunctional characteristics of an imaging spectrometer that utilizes a coaxial optical layout. Subsequently, we delve into the constraints associated with smile aberration correction and coaxial optical layout of the imaging spectrometer, which utilizes a grism as the dispersion component. Finally, we construct a computational model to determine the parameters of the grism. In the study, we provide evidence that imaging spectrometers with symmetrical structural forms can effectively minimize the impact of temperature variations on the system. Building on these findings, we developed the ultra-wide temperature difference athermalization VLWIR multifunctional imaging spectrometer, which boasts a temperature variation range over 200 K. This versatile instrument features a multifunctional mode that can be easily tuned to meet a range of observation missions. The spectrometer has a spectral range of 12µm to 16µm, a field of view (FoV) of 16.8m m×6m m, a numerical aperture (NA) of 0.334, an alignment temperature of 293.15 K, and an operating temperature of 60 K. The analysis results demonstrate the many working modes and high imaging quality of the designed imaging spectrometer. This paper's research offers a fresh approach for low-temperature VLWIR imaging spectrometer systems.

Large numerical aperture off-axis reflective telescope design with a freeform mirror based on aperture expansion strategy.

Currently, the emission of greenhouse gases is one of humanity's leading threats. To accurately and efficiently measure greenhouse gas levels in the atmosphere, we must develop imaging spectrometer systems with larger numerical apertures (NAs). However, designing a telescope with a large NA is difficult in this system. This paper presents a design strategy for aperture expansion to create a freeform telescope with a large NA. We compared different off-axis reflective telescopes and chose the Korsch structure, which has obvious advantages because of its wide field of view (FoV), large NA, and low stray light. Moreover, based on the influence of the position of the freeform surface in the aberration correction, we propose to use a single freeform surface to reduce the cost and increase manufacturability. A freeform telescope with an effective focal length of 84 mm, a large NA of 0.25, and a wide FoV of 20° is successfully designed. The modulation transfer function of the system is better than 0.62, the maximum distortion is controlled to be less than 0.486%, and the incident angle of the beam on the image plane is less than 10°. The design result shows that the instrument has wide FoV, large NA, low stray light, and high performance. At the same time, the design strategy in this paper provides an effective method for the telescope design of the imaging spectrometer with a large NA.

Analysis and design of a wide-field and large-numerical-aperture compact imaging spectrometer with a freeform surface.

Wide-field-of-view (FoV) Offner imaging spectrometers with freeform surfaces have been studied extensively in recent years. However, a design result with a large numerical aperture (NA) cannot be simultaneously obtained with this layout. We present the concept of a limited system in the tangential direction. Based on this insight, we present a new design method, to the best of our knowledge, based on the decenter anamorphic stop, which can achieve large NA in compact wide-FoV Offner imaging spectrometers with freeform surfaces. Compared to conventional imaging spectrometers with the same parameters, the light-gathering capacity of the decenter anamorphic stop-based imaging spectrometer is increased by more than 40%. In addition, based on the presented method, we design a compact imaging spectrometer with a wide FoV and large NA. The designed imaging spectrometer with a freeform surface has excellent performance. Finally, we fabricate and measure the freeform mirror. The surface irregularity of the freeform mirror is better than 1/30λ (λ=632.8n m). The result shows that the Offner imaging spectrometer with a freeform surface can be fabricated and will play a significant role in the fields of aeronautical and astronautical remote sensing.

Conceptual Design and Image Motion Compensation Rate Analysis of Two-Axis Fast Steering Mirror for Dynamic Scan and Stare Imaging System.

In order to enable the aerial photoelectric equipment to realize wide-area reconnaissance and target surveillance at the same time, a dual-band dynamic scan and stare imaging system is proposed in this paper. The imaging system performs scanning and pointing through a two-axis gimbal, compensating the image motion caused by the aircraft and gimbal angular velocity and the aircraft liner velocity using two two-axis fast steering mirrors (FSMs). The composition and working principle of the dynamic scan and stare imaging system, the detailed scheme of the two-axis FSM and the image motion compensation (IMC) algorithm are introduced. Both the structure and the mirror of the FSM adopt aluminum alloys, and the flexible support structure is designed based on four cross-axis flexural hinges. The Root-Mean-Square (RMS) error of the mirror reaches 15.8 nm and the total weight of the FSM assembly is 510 g. The IMC rate equations of the two-axis FSM are established based on the coordinate transformation method. The effectiveness of the FSM and IMC algorithm is verified by the dynamic imaging test in the laboratory and flight test.

Analysis method of the Offner hyperspectral imaging spectrometer based on vector aberration theory.

Compact hyperspectral imaging spectrometers with a wide field of view (FoV) have significant application value. However, the aberration field of such imaging spectrometers is sensitive, and varies for different wavelengths when reducing the spectrometer volume. It is difficult to explain the variation in the aberration field using traditional aberration theory. In this study, we extend the vector aberration theory (VAT) to the Offner imaging spectrometer. We deduce the expression of the aberration field decenter vector of the Offner spectrometer based on the real ray-trace method. Furthermore, we derive the expression of the third-order vector aberration of the system. Subsequently, we explain some common phenomena in the Offner imaging spectrometer. This new analysis method can provide useful guidance for designing a compact wide FoV Offner imaging spectrometer. With this new insight, we designed a compact wide FoV Offner imaging spectrometer with a freeform tertiary mirror. Compared to conventional spectrometers with the same specifications, the total length of the spectrometer decreased by 37%, and the volume by 75%. After the tolerance analysis, the freeform optics satisfied the existing machining technology. The analysis method presented in this paper furthers the designer's understanding of the aberration field of the Offner imaging spectrometer. The method is significant in the design of a compact wide FoV Offner imaging spectrometer with freeform optics.

Optomechanical design and simulation of a cryogenic infrared spectrometer.

To continuously monitor, identify, and classify a wide range of areas all day and satisfy the hyperspectral remote sensing requirements in disaster reduction, environment, agriculture, forestry, marine, and resource areas, the authors participated in a pre-research program for full spectrum hyperspectral detection in geostationary orbit. As part of the program, the authors designed a cryogenic infrared spectrometer working at the diffraction limit. Such spectrometer complied with prism dispersion, exhibiting a 120 mm long slit, 2.5-5 µm band range, and 50 nm minimum spectral resolution. The spectrometer should overtake a temperature variation of 143 K for its assembly temperature at 293 K and the working temperature at 150 K. Low-temperature invar and carbon fiber were adopted as the framework material. The spectrometer was composed of two reflective Zerodur mirrors and one CaF2 Fery prism. Compensation mounts were developed for the reflective mirrors, while a spring-loaded autocentering cryogenic lens mount was designed for a CaF2 prism. CaF2 material exhibits a large linear expansion coefficient, making its mount difficult to design. The alignment requirements of the system were described, and the calculations that ensure the lenses undergo both appropriate stresses and temperature differences were presented. Structural thermal optical performance analysis was also conducted to assess the degradation in optical performance caused by temperature variation to verify the overall optomechanical design.

Design of a compact hyperspectral imaging spectrometer with a freeform surface based on anastigmatism.

Hyperspectral imaging spectrometers with a wide field of view (FoV) have significant application values. However, enhancing the FoV will increase the volume of the imaging spectrometer and reduce the imaging quality, so a wide-FoV spectrometer system is difficult to design. Based on the theory of off-axis astigmatism, we present a method that includes a "prism box," "partial anastigmatism," and a partial differential equation to solve the parameters of a freeform surface. In this method, a compact wide-FoV imaging spectrometer with a freeform surface is designed. The spectrometer is an Offner structure with two curved prisms as the dispersion elements. The primary mirror and tertiary mirror of the Offner spectrometer are an aspheric surface and a freeform surface, respectively, to correct the off-axis aberration of a wide FoV. The ratio of the slit length to the total length of the spectrometer is close to 0.4. In comparison to conventional spectrometers of the same specifications, the total length of the spectrometer is reduced by 40% and the volume by 70%. The compact imaging spectrometer has potential application in the field of space remote sensing. In addition, the design method of the spectrometer provides a reference for the design of other optical systems with freeform surfaces.

Optical design of double-grating and double wave band spectrometers using a common CCD.

A new type of optical system comprising double-grating and double wave band spectrometers is designed for atmospheric detection. The optical system can bring oxygen A band (758-778 nm) and water vapor absorption band (758-880 nm) on a charge-coupled device (CCD) at the same time for ultrahigh resolution spectrum measurement. Each absorbed band with three observation directions of atmospheric radiation is imaged in different positions of a common CCD. The spectral resolution is less than 0.07 nm in oxygen A band (758-778 nm), and the spectral resolution is less than 0.28 nm in water vapor absorption band (758-880 nm). Three end faces of the optical fiber are on the slit plane for each wave band, and each end face corresponds to an observation angle. The optical fiber core diameter is 600 µm, the slit width is 25 µm, and the slit length is 18.4 mm. The principle of smile correction is analyzed. The smile of the Czerny-Turner double-grating spectrometer can be compensated by using the tilt field lens in front of the focal plane. The design results corroborate that the maximum smile of the double-grating spectrometer is 5 µm and that the approach of correcting smile is effective. The stray light is analyzed, and the approaches of suppressing the stray light are proposed.

Absolute detector-based spectrally tunable radiant source using digital micromirror device and supercontinuum fiber laser.

High-accuracy absolute detector-based spectroradiometric calibration techniques traceable to cryogenic absolute radiometers have made progress rapidly in recent decades under the impetus of atmospheric quantitative spectral remote sensing. A high brightness spectrally tunable radiant source using a supercontinuum fiber laser and a digital micromirror device (DMD) has been developed to meet demands of spectroradiometric calibrations for ground-based, aeronautics-based, and aerospace-based remote sensing instruments and spectral simulations of natural scenes such as the sun and atmosphere. Using a supercontinuum fiber laser as a radiant source, the spectral radiance of the spectrally tunable radiant source is 20 times higher than the spectrally tunable radiant source using conventional radiant sources such as tungsten halogen lamps, xenon lamps, or LED lamps, and the stability is better than ±0.3%/h. Using a DMD, the spectrally tunable radiant source possesses two working modes. In narrow-band modes, it is calibrated by an absolute detector, and in broad-band modes, it can calibrate for remote sensing instrument. The uncertainty of the spectral radiance of the spectrally tunable radiant source is estimated at less than 1.87% at 350 nm to 0.85% at 750 nm, and compared to only standard lamp-based calibration, a greater improvement is gained.

[Laser-based radiometric calibration].

Increasingly higher demands are put forward to spectral radiometric calibration accuracy and the development of new tunable laser based spectral radiometric calibration technology is promoted, along with the development of studies of terrestrial remote sensing, aeronautical and astronautical remote sensing, plasma physics, quantitative spectroscopy, etc. Internationally a number of national metrology scientific research institutes have built tunable laser based spectral radiometric calibration facilities in succession, which are traceable to cryogenic radiometers and have low uncertainties for spectral responsivity calibration and characterization of detectors and remote sensing instruments in the UK, the USA, Germany, etc. Among them, the facility for spectral irradiance and radiance responsivity calibrations using uniform sources (SIRCCUS) at the National Institute of Standards and Technology (NIST) in the USA and the Tunable Lasers in Photometry (TULIP) facility at the Physikalisch-Technische Bundesanstalt (PTB) in Germany have more representatives. Compared with lamp-monochromator systems, laser based spectral radiometric calibrations have many advantages, such as narrow spectral bandwidth, high wavelength accuracy, low calibration uncertainty and so on for radiometric calibration applications. In this paper, the development of laser-based spectral radiometric calibration and structures and performances of laser-based radiometric calibration facilities represented by the National Physical Laboratory (NPL) in the UK, NIST and PTB are presented, technical advantages of laser-based spectral radiometric calibration are analyzed, and applications of this technology are further discussed. Laser-based spectral radiometric calibration facilities can be widely used in important system-level radiometric calibration measurements with high accuracy, including radiance temperature, radiance and irradiance calibrations for space remote sensing instruments, and promote the development of aerospace, atmospheric physics, spectroscopy, biological science and so on in the fields of research and industry.

[Spectral line shift property of prism dispersive imaging spectrometer].

In order to study the spectral line shift property of prism-dispersive imaging spectrometer, the influencing factors and mechanisms of spectral line shift were presented, and the mathematical model based on linear optics model was established to describe the spectral line shift property. Code V API functions was used, in Matlab environment, to verify the validity of mathematical model, and the sensitivity coefficient of spectral line shift was analyzed. Results indicate that rigid body motion of optical mirror surface generated by environmental variation is the key causation of spectral line shift. When the decenter of mirror surface is no more than 0.2 mm and the tilt is less than 0.02 degrees, the value of spectral line shift of different wavelengths at different fields is equivalent, and the error is less than 0.1 pixel. Spectral line shift due to mirror rigid body motion is linear and independent, and the total shift of the spectral line is the algebraic sum of values produced by the single freedom of motion (DOF) of single mirror surface. The mathematical model based on linear optics model can be used to study the spectral line shift property of the prism-dispersive imaging spectrometer. It will provide some guidance for spectral calibration and spectral property analysis under complex work condition.