Using spatial light modulators to investigate the limitations of adaptive optics performance for the European Extremely Large Telescope
Malone, Deborah Catherina
Malone, Deborah Catherina
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Publication Date
2024-02-02
Type
Thesis
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Abstract
The European Extremely Large Telescope (E-ELT) will be the world’s largest ground based optical / near-infrared telescope ever built. Its primary mirror, M1, will be 39 m in diameter and consist of 798 individual hexagonal segments, each with a diam eter of 1.2 m. The telescope will host various cutting-edge instruments, including a Multi-Conjugate Adaptive Optics (MCAO) instrument, spectrographs, and imagers, all of which are being built alongside the telescope, with some of them planned to be operational for first light. In particular, the Multiconjugate adaptive Optics Relay For ELT Observations (MORFEO) instrument is planned to be one of the first instruments installed, along with the High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI), Multi-AO Imaging Camera for Deep Observations (MICADO), and Mid-infrared ELT Imager and Spectrograph (METIS) instruments. This means that MORFEO needs to be calibrated and tested before integration into the ELT infrastructure. For this, the Calibration / Test Unit (CU/TU) is being developed as a method to ensure MORFEO is working as expected before installation. The CU/TU will be required to provide six Laser Guide Star (LGS), three Natural Guide Star (NGS), science sources, emulated turbulence and dynamic aberrations, as well as wavefront correction which emulates the functionality of the M4 mirror, and the segmentation effects of the M1 mirror. This thesis is focused on the Spatial Light Modulator (SLM), a device which consists of an array of Liquid Crystal (LC) pixels which have a controllable index of refraction. This allows for the spatial modification of the phase of any wavefront that passes through the liquid crystals. Here, their ability to emulate atmospheric turbulence and segmented mirrors will be tested, with the intention of using them in a calibration unit for an Adaptive Optics (AO) system. Specifically, this work will use off-the-shelf SLMs for the emulation of atmospheric turbulence with different Fried parameters, and for the emulation of segmented mirrors with different numbers of segments and phase discontinuities. This will test whether they can; i) reproduce the sharp discontinuities one would expect by using small segmented mirrors, and; ii) have these edges detected by a pyramid wavefront sensor and use the measurements to produce a calibration curve which can be used to estimate the phase discontinuity across a misaligned edge. Additionally, the devices will be used to emulate a digital, reconfigurable pyramid wavefront sensor as an alternative method to sensing phase discontinuities. The output of this research will be to determine whether these easily available SLMs could be used to accurately test and calibrate an adaptive optic system, and whether the specific requirements of the MORFEO system could be met by them.
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NUI Galway