Over the past twenty years, a very high number of different space hyperspectral spectrometers have been designed, developed and launched. Recently, several new compact designs based on freeform optics have been proposed in the literature. We thus considered that it was interesting to make a survey of the already existing hyperspectral imagers and innovative freeform designs to identify the most promising solutions to increase the compactness and the performance. Some surveys on hyperspectral imagers have already been done in the past. For example, the survey by Herring et al, in 1993 which proposed some hyperspectral spectrometer concepts. However, these studies are now outdated and cannot be used to identify state of art concepts. The review proposed by Kumar et al., in 2015 presented the advantages to browse the different hyperspectral imager concepts but provided only superficial technical details that cannot be used to identify the specific designs that improve both the compactness and the image and spectral qualities. We thus decided to make a bibliographic survey to analyze the most recent designs of different space hyperspectral imagers with the objective to select the concepts that will provide the best compromise between the volume and the performance. We have categorized the different systems into four groups: prism based, Offner, Three Mirror Anastigmat (TMA) and Dyson spectrometers. We analyzed these different concepts and we identified and the advantages and the drawbacks. We concluded that the most interesting state-of-art designs are the Freeform Offner, double-pass TMA and Dyson spectrometers. These designs present the most interesting performance and combine with a compact volume. We thus established a preliminary trade-off that summarizes the advantages and the drawback of these concepts. This trade-off analysis could be used as a starting point for any future study aiming at designing compact hyperspectral imager. Finally, we present the progress of some important and recent technological developments related to the manufacturing of freeform and convex gratings.
We explore the possibility of using transition energies of a doubly excited helium atom to describe the absorption lines of interstellar medium. We follow the method of approximated separation of variables in hypersperical coordinate in solving the energy levels of doubly excited helium. Although the results do not exactly match with the absorption spectrum of the diffuse interstellar bands, they show some correlation. We have also shown that the distribution of the difference between observed spectrum and the calculated result can be fit by normal distribution with the mean value close to zero. Furthermore, we argued that the distribution obtained from randomly generated lines are significantly different from the calculated result.
The observations of the solar system Jovian planets performed by ground-based medium size telescopes can provide an efficient support to the space missions by performing observations of the planet atmospheres. In particular, ground-based medium size telescopes are able to provide high resolution images close to the diffraction limit of giant planets while observing through the Earth atmosphere by using some Lucky Imaging processing. These observations of the Jovian planet atmospheres ideally require i) an instrument with a high angular resolution close to the diffraction limit and ii) a high contrast, especially for the low and medium spatial frequencies that corresponds to the turbulence areas inside the atmosphere clouds bands. We thus decided to design one telescope that shall provide diffraction limited images (without the contribution of the atmosphere) over a circular Field of View (FOV) of diameter equal to 2 arcminutes. This, over the photometric spectral band V, R and I of the Johnson-Cousin photometric. In this paper, we present the design and the performance of a Ritchey-Chretien telescope dedicated to solar system planet imagery with a linear central obscuration lower than 0.15 and an active system to correct the low frequency distortions of the wavefront before each observation. First, we describe the optical design, then we establish the image quality budget. Finally we show that the stray light signal induced by the moon light scattering is negligible during the observations of Jupiter.
The National Astronomical Research Institute of Thailand (NARIT) is currently developing a five lenses prime focus camera in order to enlarge the field of view of the 2.3 m Thai National Telescope to a one degree diameter circle. The instrument shall operate in the spectral bands B, V, R and I of the Johnson-Cousins photometric system with an angular resolution better than 2 arcsecond. In this paper, we describe the camera design, we estimate the theoretical performance in the V-band and we show that the theoretical angular resolution after tolerancing is better that 1.3 arcsecond. Then, we present the results of the stray light analysis and we show that the system is free of critical ghost images.
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