Design and Modelization of Aconvex Grating for an Hyperspectral Imager of the Chandrayaan 2 Instrument for the Moon Probe in the Infrared (original) (raw)

Compact hyperspectral instruments

International Conference on Space Optics — ICSO 2020

This presented study is initiated in the frame of CNES advanced studies. It aims at providing a better understanding of driving parameters for this type of instruments, and exploring suitable, very compact hyperspectral instruments based on grating spectrometers. First a scalar model for multiblazed gratings is developed, and confronted with more detailed diffraction models. The spectral band is in the range of 400-2500 nm, with a spectral resolution of about 10 nm width. The Ground Sampling Distance (GSD) shall be between 10 and 15m, the Field of View (FOV) shall be larger than 8km, the orbit Height is 550km. Three types of instruments are proposed, a common spectrometer with a unique detector, a common spectrometer working in diffraction order 1 for the SWIR, and 2 for the VIS spectral band, and 2 spectrometers within field separa tion. For each instrument the grating blazed profile is optimized, the key instruments parameters computed, and an optical configuration is proposed. The study shows compactness optimization with these three instrument concepts, thanks to an entrance pupil diameter reduction along with better grating diffraction efficiency

Imaging Infrared Spectrometer onboard Chandrayaan-2 Orbiter

Current Science, 2020

Imaging Infrared Spectrometer (IIRS) is an imaging hyperspectral instrument for mineralogy of the lunar surface (including the hydroxyl signature). IIRS operates in the 0.8-5 μm spectral range with about 250 contiguous bands. It has 80 m ground sampling distance and 20 km swath at nadir from 100 km orbit altitude. Optical design is based on fore-optics and Offner (convex multi-blazed grating)-type spectrometer. Focal plane array is HgCdTe (mercury-cadmium-telluride)based actively cooled to 90 K, having 500 × 256 pixels format with 30 μm pixel size. Electronics comprises proximity, logic and control, power supply and cooler drive electronics. Mechanical system is realized to house various subsystems, namely optics, detector, electronics and thermal components meeting the structural, opto-mechanical thermal component and alignment requirements. Thermal system is designed such that the instrument is cooled and maintained at fixed temperature to reduce and control instrument background. Aluminum-based mirror, grating and housing are developed to maintain structural as well as opto-mechanical and thermal requirements. This article presents IIRS realization and spectroradoimetric performance.

Binary blazed reflection grating for UV/VIS/NIR/SWIR spectral range

International Conference on Space Optics — ICSO 2018, 2019

We report on the design and fabrication of a reflection grating for hyperspectral applications operating in the range from 340 nm to 1040 nm wavelength. The blazed grating is based on an effective medium approach, where the desired functionality is realized using a binary surface relief structure. For each period, a gradient in size of the local grating features mimics an interface which adds a linear phase profile to the illuminating beamthus introducing diffraction. The surface relief structure is composed of 2D structures-pillars with diameters from 200 nm to 350 nm to voids with diameters from 300nm to 120 nm. Overall, an entire number of ~50 such features are arranged to establish an overall unit cell of the grating over a length of 30 µm. By purposeful design of size, shape and arrangement of the sub-wavelength features such gratings offer novel opportunities in tailoring the spectral response, i.e. particular broadband efficiency or the enhancement of the efficiency in specific sub-domains of the spectrum. We will present measured performance results of a grating covering a circular area of 80mm in diameter manufactured on a 4inch-wafer. Finally, we will give an outlook on how such structures can be applied to curved surfaces and even ultra-broadband operation.

Modeling the Chandra high-energy transmission gratings below 2 keV

X-Ray and Gamma-Ray Instrumentation for Astronomy XI, 2000

The High Energy Transmission Grating Spectrometer of the Chandra X-Ray Observatory is a high spectral resolution instrument utilizing gold X-ray transmission gratings. The gratings have been subjected to a rigorous program of calibration, including testing at synchrotron facilities for the purpose of refining and testing the grating model. Here we conclude our investigation of the optical constants of gold, extending it below 2 keV to complete the coverage over the Chandra energy range. We investigate the carbon, nitrogen, oxygen and chromium edge structures introduced by the grating support membrane. Finally, we summarize the state of the grating model, identifying those energy regions where the residuals are most significant and suggesting where the model might be improved.

Freeform Grating-Based Hyperspectral Instruments: When SmallSat Solutions Benefit to Big Missions

2019

Hyperspectral Earth Observation is a fast-growing field requiring high performing imaging spectrometers. Since 2010, the European Space Agency has initiated a series of developments demonstrating the feasibility of miniaturized hyperspectral instruments on mini-and nano-satellites [1]. Among them, ELOIS and CHIMA are two innovative full Aluminum instruments based on diffraction gratings ruled on a freeform surface (FFG : Free-Form Grating). That solution offers a reduction of about a factor of 4 in volume with respect to a Offner-Chrisp spectrometers with equivalent performances. The Spectrometers combines three promising new technologies for future hyperspectral instruments: complex blazed grating, freeform optics and backside-illuminated hyperspectral CMOS sensor. With an image space F-number of 2.1, ELOIS is also one of the fastest instrument of this type. The ratio between Swath and Ground Sampling Distance is about twice as big as currently planned hyperspectral missions. Breadboards of these spectro-imagers, limited to the visible and NIR spectra, has been manufactured and tested. This breadboard program confirmed the achievement of the challenging design specifications. Based on these demonstrations, a complete payload is now developed to cover the VNIR and SWIR spectral ranges (400nm to 2450 nm) with a spectral resolution of 10 nm. The proposed technologies are now studied in the context of the "Copernicus Space Component Expansion" program. Six candidate missions have been identified by the European Commission (EC) as priorities for implementation in the coming years. Among them, the CHIME mission (Copernicus Hyperspectral Imaging Mission for Environment) aims to provide precise spectroscopic measurements in the VNIR/SWIR spectral range. Those data will be used to derive quantitative surface characteristics supporting the monitoring, implementation and improvement of a range of policies in the domain of raw materials, agriculture, soils, food security, biodiversity, environmental degradation and hazards, inland and coastal waters, snow, forestry and the urban environment.

The Point Spread Function of the Reflection Grating Spectrometer

1998

X-ray calibrations of the individual re ection grating elements in the Re ection Grating Array at Columbia Nevis Labs and the full arrays at MPE Panter have yielded both the e ciency and spectral resolution information necessary for a detailed, comprehensive understanding of the Re ection Grating Spectrometer.

Spectrometers consisting of a diffractive lens and a concave diffraction grating

Journal of Modern Optics, 2004

The concave diffraction grating is used in many spectrometers and monochromators. Since it has both dispersion and focusing properties, one does not need a collimator or camera. In a nearly symmetric setup with a small deviation angle, the second-and third-order aberrations of the spherical concave diffraction grating are less then the respective aberrations of the setup with a plane diffraction grating and spherical mirrors. However, the residual defocusing limits the applicability of concave gratings in high resolution devices. This aberration can only be partially compensated by a grating with nonequidistant grooves. The focal distance of the diffractive lens has a linear dependence on the wavelength. This property is used to design hybrid diffractive-refractive lens systems for a wide spectral region. It is both tempting and promising to compensate the residual defocusing of a concave diffraction grating by means of a diffractive lens. The present paper analyses, both analytically and numerically, the effect of such compensation. The use of the diffractive lenses in the concave grating spectrometers is shown for the flat-field spectrometer and the double monochromator.

Compact Spectrometer based on Concave Grating

In this paper, the compact spectrometer has been designed and implemented with concave grating. By using the holographic corrected concave grating, the compact spectrometer without movable parts, with a fixed grating and an array detector, could obtain a relative high spectral resolution in a wide spectral range. Then, the spectral resolution has been estimated by the slit function. The spectral resolution (〖Δλ〗_FWHM) is smaller than 5nm from 300nm to 1100nm. It is very suitable for photometry, colorimetry and radiometry.

A 30-cm objective grating for far-UV astronomy: theoretical study and laboratory tests

Applied Optics, 1989

We have performed theoretical determination and experimental calibrations of an objective grating designed for high resolution spectroscopy of astronomical faint sources in the EUV and far-UV wavelength ranges (500-1400 A). First through theoretical calculations we show the feasibility of the concept with an aspheric shape for the grating blank and determine its geometrical parameters. A grating of this large size has been manufactured and tested, associated with a photon counting detector, in a vacuum environment. Finally we demonstrate that a resolving power of 3 X 104, a total equivalent effective area of -5-10 cm 2 can be achieved, together with a very low scattered light level (10-4-10-5 of the peak value).