Any high spatial resolution space-borne electro-optical sensing system operating in long wavelengths, like Earth-observation facilities operating in the longwave infrared are subjected to an inherent design and implementation challenge of deploying large monolithic primary aperture mirrors, to achieve a ground resolution distance of a few tens of cm. To outflank this issue, many present-date missions design and commission lightweight segmented mirrors, mostly with equal sized sub-apertures. One step ahead, these sub-apertures could be of particular non-uniform size distributions (One-by-Three, Taylor-ln and Taylor-invtan), thereby ensuring a smaller and even lighter primary and with marginal compromise in imaging quality due to significant sidelobe suppression. This is also confirmed by the fact that these particular non-uniform sized mirrors have very less loss of spatial frequencies with respect to that of equal-sized segmented mirrors. Therefore, under lossless conditions, there is hardly any degradation in imaging performance of these two configurations. However, in the presence of gaussian, impulse and shot noise, the situation worsens because of the compromised collecting area as well as noise contribution. A simple deconvolution technique for image restoration in presence of noise is no longer possible because of the lack of convergence. This calls upon for the use of iterative reconstruction algorithms with denoisers like Total Variation (TV), Block Matching and 3D Filtering (BM3D) or Convolutional Neural Networks (CNN) in the post-processing step to ensure better output images with high Peak Signal-to-Noise Ratio (PSNR) and Structural Similarity Index Measure (SSIM) along with good edge and texture preservation of the features. A comparison of these three kinds of denoisers, TV, BM3D and DnCNN implemented as a part of the Alternating Directions Method of Multipliers (ADMM) reconstruction technique is presented in this work. It is seen that in presence of some shot noise, random gaussian noise with σ= 0.03 and some impulse noise, the best performance is achieved for ADMM-BM3D technique with comparable performance from the ADMM-DnCNN method (except for Taylor-ln design). On the contrary, denoising with TV can perform well only in presence of shot noise. Additionally, this technique is nearly rejected for use in case of the Taylor-invtan model because of extremely low SSIM when all three noise types are incorporated.
The solar ultraviolet imaging telescope (SUIT) is an imaging telescope on-board the Aditya-L1 satellite, which is India’s maiden space mission dedicated solely to solar observations. The spatially resolved, high cadence observations are designed to be taken in eleven science filters with full width half maxima ranging between 0.1–58 nm and spread over the near-ultraviolet (NUV) domain of the solar spectrum (200–400 nm). The huge incoming solar flux, limited by the linearity regime performance of the charge coupled device (CCD) as well as the thermal operational constraints, mandate the use of an entrance aperture filter, the thermal filter (TF), for SUIT. The design of this filter is, further, constrained by exposure time and enhanced emission of the sun during eruptive events. From performance perspective, the TF reflects ∼50% of the incident radiation and allows only 0.1–0.45% of the incoming flux to pass within 200–400 nm. The transmission on either side of the operational range is satisfactorily reduced, so as to ensure minimum unwanted light leaking into the imaging system. Therefore, the TF plays a significant role in increasing the photometric efficiency as well as maintaining the operational temperature of the telescope. To the best of our knowledge, this is the first time any attempt of designing and manufacturing any such rejection filter aiming optimized performance in the NUV range is being done for a space-based imaging solar telescope. The choice of materials for substrate and coating for the filter poses several challenges in terms of contamination, corrosion/ oxidation, durability during manufacturing process, long-term exposure to harsh space environment as well as formation of pinholes. The transmission and reflection profiles of the fabricated TF is satisfactory to meet our design and technical constraints. The TF is also qualified for various environmental and radiation conditions. The transmission of the TF is seen to be well within our allowed margins (±10% of the design value) even after being exposed to these qualification tests.
KEYWORDS: Space operations, Ultraviolet radiation, Space telescopes, Telescopes, Space telescopes, Solar processes, Sensors, X-ray imaging, Plasma, Ions, Magnetosphere
The Solar Ultraviolet Imaging Telescope (SUIT) is an instrument onboard the Aditya-L1 spacecraft, the first dedicated solar mission of the Indian Space Research Organization (ISRO), which will be put in a halo orbit at the Sun-Earth Langrage point (L1). SUIT has an off-axis Ritchey–Chrétien configuration with a combination of 11 narrow and broad bandpass filters which will be used for full-disk solar imaging in the Ultravoilet (UV) wavelength range 200-400 nm. It will provide near simultaneous observations of lower and middle layers of the solar atmosphere, namely the Photosphere and Chromosphere. These observations will help to improve our understanding of coupling and dynamics of various layers of the solar atmosphere, mechanisms responsible for stability, dynamics and eruption of solar prominences and Coronal Mass ejections, and possible causes of solar irradiance variability in the Near and Middle UV regions, which is of central interest for assessing the Sun’s influence on climate.
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