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B.R. Brandl
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1
The Mid-infrared ELT Imager and Spectrograph (METIS), which will be one of the first-light instruments on the Extremely Large Telescope (ELT), will be equipped with an Adaptive Optics (AO) system that will use a natural guide star as reference for the correction of the wavefront errors, introduced by the atmospheric turbulence. Even though this system will suffice for the achievement of the instrument’s primary science goals, it will not allow for observations of additional objects of interest, due to the limited sky coverage it offers. In order to increase the sky coverage of the instrument, a supplementary AO system, which will use a single laser guide star as reference, is planned to be added at a later stage. In the present thesis, a possible solution for the (opto)mechanical design of this laser-assisted AO system is provided, which ensures compliance with the predetermined mass, volume and budget limits.
...
The Mid-infrared ELT Imager and Spectrograph (METIS), which will be one of the first-light instruments on the Extremely Large Telescope (ELT), will be equipped with an Adaptive Optics (AO) system that will use a natural guide star as reference for the correction of the wavefront errors, introduced by the atmospheric turbulence. Even though this system will suffice for the achievement of the instrument’s primary science goals, it will not allow for observations of additional objects of interest, due to the limited sky coverage it offers. In order to increase the sky coverage of the instrument, a supplementary AO system, which will use a single laser guide star as reference, is planned to be added at a later stage. In the present thesis, a possible solution for the (opto)mechanical design of this laser-assisted AO system is provided, which ensures compliance with the predetermined mass, volume and budget limits.
On the Atoms of Robustness
Robust Matrix Decomposition for Spectral Imaging
Modern imaging modalities across many application domains increasingly acquire a large number of very high-dimensional measurements, commonly collecting hundreds to millions of variables per spatial resolution element. That high-dimensional nature can severely challenge traditional (often Euclidean distance based) approaches to noise and dimensionality reduction. Furthermore, statistical analysis of such data is often hampered by the curse of dimensionality, concomitant with large numbers of pixels and channels, by the growing abundance of low signal-to-noise ratio (SNR) measurements, and by the detrimental effects of noise accumulation. It is therefore necessary to find efficient means of reducing high-dimensional (and large data size) measurement sets to a lower-dimensional representation while incurring minimal information loss. Addressing this challenge is essential (a) to enable processing of the massively multivariate measurement sets acquired by several promising imaging technologies today (commonly hundreds of gigabytes to terabytes per experiment), (b) to avoid that computational analysis becomes a bottleneck for the development of new instrumental capabilities, with hardware setups theoretically capable of yielding terabyte to petabyte imaging but currently considered impractical, and (c) to enable archiving massively multivariate measurement sets, often required to be stored for several years and containing information suitable for additional science projects. This thesis focuses on this challenge specifically. First, it explores structured and regularized matrix decomposition methods based on the l1-norm, e.g. building on principal component pursuit (PCP), to address this challenge. Second, it delivers custom implementations of these methods. Third, it develops and implements an application-driven framework for automatic setting of hyperparameters. Finally, it applies and compares these methods using several spectral imaging case studies spanning both very small scales, in molecular imaging of organic tissue, as well as very large scales, in the spectroscopic imaging of Outer space.
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Modern imaging modalities across many application domains increasingly acquire a large number of very high-dimensional measurements, commonly collecting hundreds to millions of variables per spatial resolution element. That high-dimensional nature can severely challenge traditional (often Euclidean distance based) approaches to noise and dimensionality reduction. Furthermore, statistical analysis of such data is often hampered by the curse of dimensionality, concomitant with large numbers of pixels and channels, by the growing abundance of low signal-to-noise ratio (SNR) measurements, and by the detrimental effects of noise accumulation. It is therefore necessary to find efficient means of reducing high-dimensional (and large data size) measurement sets to a lower-dimensional representation while incurring minimal information loss. Addressing this challenge is essential (a) to enable processing of the massively multivariate measurement sets acquired by several promising imaging technologies today (commonly hundreds of gigabytes to terabytes per experiment), (b) to avoid that computational analysis becomes a bottleneck for the development of new instrumental capabilities, with hardware setups theoretically capable of yielding terabyte to petabyte imaging but currently considered impractical, and (c) to enable archiving massively multivariate measurement sets, often required to be stored for several years and containing information suitable for additional science projects. This thesis focuses on this challenge specifically. First, it explores structured and regularized matrix decomposition methods based on the l1-norm, e.g. building on principal component pursuit (PCP), to address this challenge. Second, it delivers custom implementations of these methods. Third, it develops and implements an application-driven framework for automatic setting of hyperparameters. Finally, it applies and compares these methods using several spectral imaging case studies spanning both very small scales, in molecular imaging of organic tissue, as well as very large scales, in the spectroscopic imaging of Outer space.
As part of the New Earths in the Alpha Cen Region (NEAR) experiment, VISIR, an imager and spectrometer for the mid-infrared at the Very Large Telescope (VLT) of the European Southern Observatory, will be soon upgraded to improve its performance in terms of high contrast and sensitivity for the detection of Earth-like planets in the Alpha Centauri star system, the closest to the Sun. Part of the VISIR upgrade consists in a novel chopper system based on the Dicke-switch concept to suppress the excess low frequency noise (ELFN) suffered by the detector array. The ELFN is a form of temporally correlated noise and can be mitigated by modulating the incident light at high enough frequency. The other main challenge of the NEAR experiment is the need of high contrast to image the circumstellar environment, and it will be achieved with the implementation of an annular groove phase mask (AGPM) coronagraph for the suppression of the starlight. The main goals of the thesis project were the validation of the Dicke-switch chopper systems to assess its ability to mitigate background level and ELFN, and the characterisation of the performance of the AGPM coronagraph in terms of null depth and contrast level. The first part of the laboratory characterisation of the Dicke-switch chopper system focused on the verification of the radiometric flux level achievable by its internal blackbody unit and by the so called artificial sky, a cold blackbody needed to reproduce the background level typical at the VLT, where the combined sky and telescope thermal radiation is the same as the one of a greybody at 280K and 10% emissivity. After the completion of the radiometric tests and mitigation of parasitic light effects caused by reflection of internal and external thermal radiation, the tests to validate the ability of the Dicke-switch chopper system to remove background level and suppress the ELFN started. The obtained results confirm that the Dicke-switch chopper system mitigate the ELFN as done also with pseudo-chopping. This was verified also for large offsets between the flux level of internal blackbody unit and artificial sky. Similarly, the characterisation of the AGPM coronagraph started with the preparation of the laboratory setup, in particular design of the optical setup, alignment of all the optical components and measurement of the optical quality of the system to limit residual aberrations and vignetting. The results of the coronagraphic tests revealed that the two candidate AGPMs for the NEAR campaign, both optimised for wavelengths between 10 and 12.5 μm, are not able to provide the required contrast level and null depth. On the contrary, an older AGPM, spare part of the one currently mounted in VISIR, and thus optimised for wavelengths between 11 and 13.1 μm, provided better coronagraphic performance, and even though it is still not compliant with the requirements, at present it represents the baseline for the NEAR experiment.
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As part of the New Earths in the Alpha Cen Region (NEAR) experiment, VISIR, an imager and spectrometer for the mid-infrared at the Very Large Telescope (VLT) of the European Southern Observatory, will be soon upgraded to improve its performance in terms of high contrast and sensitivity for the detection of Earth-like planets in the Alpha Centauri star system, the closest to the Sun. Part of the VISIR upgrade consists in a novel chopper system based on the Dicke-switch concept to suppress the excess low frequency noise (ELFN) suffered by the detector array. The ELFN is a form of temporally correlated noise and can be mitigated by modulating the incident light at high enough frequency. The other main challenge of the NEAR experiment is the need of high contrast to image the circumstellar environment, and it will be achieved with the implementation of an annular groove phase mask (AGPM) coronagraph for the suppression of the starlight. The main goals of the thesis project were the validation of the Dicke-switch chopper systems to assess its ability to mitigate background level and ELFN, and the characterisation of the performance of the AGPM coronagraph in terms of null depth and contrast level. The first part of the laboratory characterisation of the Dicke-switch chopper system focused on the verification of the radiometric flux level achievable by its internal blackbody unit and by the so called artificial sky, a cold blackbody needed to reproduce the background level typical at the VLT, where the combined sky and telescope thermal radiation is the same as the one of a greybody at 280K and 10% emissivity. After the completion of the radiometric tests and mitigation of parasitic light effects caused by reflection of internal and external thermal radiation, the tests to validate the ability of the Dicke-switch chopper system to remove background level and suppress the ELFN started. The obtained results confirm that the Dicke-switch chopper system mitigate the ELFN as done also with pseudo-chopping. This was verified also for large offsets between the flux level of internal blackbody unit and artificial sky. Similarly, the characterisation of the AGPM coronagraph started with the preparation of the laboratory setup, in particular design of the optical setup, alignment of all the optical components and measurement of the optical quality of the system to limit residual aberrations and vignetting. The results of the coronagraphic tests revealed that the two candidate AGPMs for the NEAR campaign, both optimised for wavelengths between 10 and 12.5 μm, are not able to provide the required contrast level and null depth. On the contrary, an older AGPM, spare part of the one currently mounted in VISIR, and thus optimised for wavelengths between 11 and 13.1 μm, provided better coronagraphic performance, and even though it is still not compliant with the requirements, at present it represents the baseline for the NEAR experiment.
Single Laser Adaptive Optics system for the ELT/METIS
Feasibility and Performance study
Demonstrated by the James-Webb Space Telescope launch, the scientific community shows more and more interest in mid-infrared observations. The diffraction limit of an instrument is inversely proportional to the diameter of the telescope: the larger the primary mirror, the higher the resolution. Therefore, the instrument METIS (Mid-Infrared ELT-based Imager and Spectrograph), based on the 37-meter aperture Extremely Large Telescope, will take advantage of the giant mirror to achieve an unrivaled resolution in mid-infrared. However, to reach this limit, the perturbations created by the temperature fluctuations in the atmosphere need to be removed by Adaptive Optics (AO). METIS will already have an AO system, called Single Conjugate Adaptive Optics (SCAO). However, this system will require a Natural Guide Star (NGS) with a K-band magnitude ofmK Æ 11. This limits considerably the sky coverage where the atmospheric compensation (thus high-resolution observations) could be done. The idea is to use an artificial guide star, called Laser Guide Star (LGS), to get rid of this constraint. This is performed by shooting a laser at the 90-km sodium layer, tuned so that it excites the atoms there that re-emits toward the ground. For a telescope with the size of the ELT, several lasers are recommended in a configuration called Laser Tomography AO or Multi-Conjugate AO. The plurality of the lasers allows removing errors such as the focus anisoplanatism. However, because of the complexity and cost of such a solution, the only available answer for the ELT/METIS is a Single Laser AO system (SLAO). This will be less complex and expensive at the expense of a decrease in the performances. The question is: what are the performances of such a SLAO system, and how could it be made? The Yorick Adaptive Optics (YAO) simulator has been used to estimate the performances of the SLAO system for the ELT/METIS. The elongation of the LGS beam, created by the finite thickness of the sodiumlayer, plays a crucial role in the process and has direct consequences on the performances. A parameter analysis has been performed to define the most suitable Field of View, number of sub-apertures, pixels size to tackle the problem. An algorithmhas also been developed inMATLAB, coupledwith the results fromthe YAOalgorithm. The goal of this algorithm was to estimate the correction that could be brought by the so-called truth sensing on the features left by the LGS correction because of the beam elongation. This sensing will require a natural guide star coupled with the existing SCAO sensor of the METIS instrument and will work in addition to the SLAO sensor. Once the parameters settled, it has been possible to create an optical design with ZEMAX. Because the distance between the telescope and the sodium layer varies relatively to the zenith angle, the focal point of the LGS beamchanges accordingly: the optical system needs to tackle this. It was also required to find a solution to separate the LGS beam from the science beam: two solutions have been described, with their advantages and limitations detailed. Finally, a mechanical implementation of the systemhas been made to explain how such a system could be hanged. The work performed has allowed estimating the Strehl ratio obtained for the SLAOsystemwith the ELT/METIS, which would be around 60%. These results have been confirmed by the European Southern Observatory and their AO experts. The study made on the truth sensing of the system has allowed putting constraints on the natural guide star brightness required (mK Ç 16.5), and the sky coverage of the SLAO system which would be almost 100%. The optical design made with ZEMAX has shown great performances so that the perturbations induced by the system are small enough to be able to measure correctly the ones created by the atmosphere. The solution proposed with the mechanical design could pave theway for further study in the domain. The Adaptive Optics with a single laser guide star is complicated, especially for a telescope with the size of the ELT. Using a laser guide star instead of a natural one creates additional complications that need to be solved to achieve good performances, especially if only one LGS can be used. The solution proposed would allow to considerably increase the sky coverage of the system, which is currently limited to few percents with the SCAO system. However, further studies need to be done, such as the evaluation of the real impact of the NGS magnitude on the system performances, or the design of the complete and realistic mechanical design.
...
Demonstrated by the James-Webb Space Telescope launch, the scientific community shows more and more interest in mid-infrared observations. The diffraction limit of an instrument is inversely proportional to the diameter of the telescope: the larger the primary mirror, the higher the resolution. Therefore, the instrument METIS (Mid-Infrared ELT-based Imager and Spectrograph), based on the 37-meter aperture Extremely Large Telescope, will take advantage of the giant mirror to achieve an unrivaled resolution in mid-infrared. However, to reach this limit, the perturbations created by the temperature fluctuations in the atmosphere need to be removed by Adaptive Optics (AO). METIS will already have an AO system, called Single Conjugate Adaptive Optics (SCAO). However, this system will require a Natural Guide Star (NGS) with a K-band magnitude ofmK Æ 11. This limits considerably the sky coverage where the atmospheric compensation (thus high-resolution observations) could be done. The idea is to use an artificial guide star, called Laser Guide Star (LGS), to get rid of this constraint. This is performed by shooting a laser at the 90-km sodium layer, tuned so that it excites the atoms there that re-emits toward the ground. For a telescope with the size of the ELT, several lasers are recommended in a configuration called Laser Tomography AO or Multi-Conjugate AO. The plurality of the lasers allows removing errors such as the focus anisoplanatism. However, because of the complexity and cost of such a solution, the only available answer for the ELT/METIS is a Single Laser AO system (SLAO). This will be less complex and expensive at the expense of a decrease in the performances. The question is: what are the performances of such a SLAO system, and how could it be made? The Yorick Adaptive Optics (YAO) simulator has been used to estimate the performances of the SLAO system for the ELT/METIS. The elongation of the LGS beam, created by the finite thickness of the sodiumlayer, plays a crucial role in the process and has direct consequences on the performances. A parameter analysis has been performed to define the most suitable Field of View, number of sub-apertures, pixels size to tackle the problem. An algorithmhas also been developed inMATLAB, coupledwith the results fromthe YAOalgorithm. The goal of this algorithm was to estimate the correction that could be brought by the so-called truth sensing on the features left by the LGS correction because of the beam elongation. This sensing will require a natural guide star coupled with the existing SCAO sensor of the METIS instrument and will work in addition to the SLAO sensor. Once the parameters settled, it has been possible to create an optical design with ZEMAX. Because the distance between the telescope and the sodium layer varies relatively to the zenith angle, the focal point of the LGS beamchanges accordingly: the optical system needs to tackle this. It was also required to find a solution to separate the LGS beam from the science beam: two solutions have been described, with their advantages and limitations detailed. Finally, a mechanical implementation of the systemhas been made to explain how such a system could be hanged. The work performed has allowed estimating the Strehl ratio obtained for the SLAOsystemwith the ELT/METIS, which would be around 60%. These results have been confirmed by the European Southern Observatory and their AO experts. The study made on the truth sensing of the system has allowed putting constraints on the natural guide star brightness required (mK Ç 16.5), and the sky coverage of the SLAO system which would be almost 100%. The optical design made with ZEMAX has shown great performances so that the perturbations induced by the system are small enough to be able to measure correctly the ones created by the atmosphere. The solution proposed with the mechanical design could pave theway for further study in the domain. The Adaptive Optics with a single laser guide star is complicated, especially for a telescope with the size of the ELT. Using a laser guide star instead of a natural one creates additional complications that need to be solved to achieve good performances, especially if only one LGS can be used. The solution proposed would allow to considerably increase the sky coverage of the system, which is currently limited to few percents with the SCAO system. However, further studies need to be done, such as the evaluation of the real impact of the NGS magnitude on the system performances, or the design of the complete and realistic mechanical design.
Master thesis
(2018)
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Lenka Husárová, Bernhard Brandl, M Mueller, Stephanie Cazaux, Alessandra Menicucci
The interest in asteroids is increasing, due to their promising clues on the origin of the Solar System, e.g. their albedos and sizes can provide insight into the protosolar nebula. The new scientific mission scheduled to launch in 2021, the James Webb Space Telescope (JWST), provides an opportunity for observation of asteroids. In this thesis, the feasibility of using the Mid-Infrared Instrument's (MIRI) imager on the JWST for serendipitous characterization of asteroids is evaluated. The imager and the medium resolution spectrometer (MRS) run simultaneously. When MRS is running, imager data could be used for detection of asteroids in infrared, where the error in size is only 10%, comparing to 100% in optical, because the thermal radiation is independent of the albedo. Combining the optical and infrared, the albedo can be determined. This prospective use is researched by simulating the sensitivity of the imager. A tool, created during this thesis, simulates realistic cases using proposal (GTO 1282) and determines the known asteroids in the field of view (FOV) of the imager. MIRISim, created by the MIRI European Consortium, simulates their signatures. The results show a 4.1% chance of an asteroid appearing in the FOV and a 96.9% detection of such an asteroid. In a typical MIRI observation, exposure time of 488.4 s, with preferred filter F1280W, the imager can detect asteroids bigger than 250 m in diameter and closer than 3 AU. This could lead to a detection of 733 256 yet undetected asteroids and 183 314 currently known asteroids in the lifetime of JWST.
...
The interest in asteroids is increasing, due to their promising clues on the origin of the Solar System, e.g. their albedos and sizes can provide insight into the protosolar nebula. The new scientific mission scheduled to launch in 2021, the James Webb Space Telescope (JWST), provides an opportunity for observation of asteroids. In this thesis, the feasibility of using the Mid-Infrared Instrument's (MIRI) imager on the JWST for serendipitous characterization of asteroids is evaluated. The imager and the medium resolution spectrometer (MRS) run simultaneously. When MRS is running, imager data could be used for detection of asteroids in infrared, where the error in size is only 10%, comparing to 100% in optical, because the thermal radiation is independent of the albedo. Combining the optical and infrared, the albedo can be determined. This prospective use is researched by simulating the sensitivity of the imager. A tool, created during this thesis, simulates realistic cases using proposal (GTO 1282) and determines the known asteroids in the field of view (FOV) of the imager. MIRISim, created by the MIRI European Consortium, simulates their signatures. The results show a 4.1% chance of an asteroid appearing in the FOV and a 96.9% detection of such an asteroid. In a typical MIRI observation, exposure time of 488.4 s, with preferred filter F1280W, the imager can detect asteroids bigger than 250 m in diameter and closer than 3 AU. This could lead to a detection of 733 256 yet undetected asteroids and 183 314 currently known asteroids in the lifetime of JWST.
The following report aims to provide an initial approach to the user for quantifying the diverse methods for correcting the fringes within astronomical spectra. This study is developed as part of the Master thesis conducted at the Aerospace Engineering Faculty of Delft University of Technology. Determining the composition of the environment in space has always been a key feature for understanding it. Perceiving what cannot be seen by the human eye on images was realized by means of spectrometers, combined with focal plane detectors. Spectral features depicted on the delivered spectra provide scientists with vital information on the observed entities. However, further analysis showed that multi-beam interference of the light spectrumoccurred within this instrumentation, misleading the results. This physical principle, also called as the Fabry-Pérot principle, generates fringes, that appeared to be present in many space telescope instruments such as the STIS, SWS/LWS, Spitzer IS, HIFI and the MRS. Therefore, the need to find a reliable solution for correcting these fringes has been a key requirement within the astronomy, especially for the future spectrometers and dedicated detectors to be sent in space on board the JamesWebb Space Telescope in 2019...
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The following report aims to provide an initial approach to the user for quantifying the diverse methods for correcting the fringes within astronomical spectra. This study is developed as part of the Master thesis conducted at the Aerospace Engineering Faculty of Delft University of Technology. Determining the composition of the environment in space has always been a key feature for understanding it. Perceiving what cannot be seen by the human eye on images was realized by means of spectrometers, combined with focal plane detectors. Spectral features depicted on the delivered spectra provide scientists with vital information on the observed entities. However, further analysis showed that multi-beam interference of the light spectrumoccurred within this instrumentation, misleading the results. This physical principle, also called as the Fabry-Pérot principle, generates fringes, that appeared to be present in many space telescope instruments such as the STIS, SWS/LWS, Spitzer IS, HIFI and the MRS. Therefore, the need to find a reliable solution for correcting these fringes has been a key requirement within the astronomy, especially for the future spectrometers and dedicated detectors to be sent in space on board the JamesWebb Space Telescope in 2019...