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Proceedings Paper

Progress on the prevention of stray light and diffraction effects on the Thai National Telescope
Author(s): Christophe Buisset; Apirat Prasit; Apichat Leckngam; Thierry Lépine; Saran Poshyajinda; Boonrucksar Soonthornthum; Puji Irawati; Andrea Richichi; Utane Sawangwit; Vik Dhillon; Liam K. Hardy
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Paper Abstract

The 2.4-m Thai National Telescope (TNT) is the main facility of the Thai National Observatory located on the Doi Inthanon, Thailand's highest mountain. The first astronomical images obtained at the TNT suffered from diffraction and stray light problems: bright spikes spread from bright stellar images over few arcminutes in the focal plane, and the images taken during observations in bright moon conditions were contaminated by high levels of stray light. We performed targeted investigations to identify the origin of these problems. In a first time, these investigations consisted of analyzing the irradiance distribution of defocused stellar images and of identifying the contributors. We concluded that these bright spikes around the bright stellar images were due to the chamfer and the wavefront error at the mirror edge. We thus installed an annular mask along the edge of the primary mirror that fully suppressed these spikes and we quantified the improvement by observing the double star Sirius. In a second time, we identified the contributors to the stray light by placing a pinhole camera at the TNT focal plane. Then, we designed a new baffle to improve the stray light rejection. The final design of the baffle comprises 21 diaphragms, is painted with an ordinary black paint and was designed, developed and installed on the TNT in less than 8 months. We assessed the improvement on the performance by measuring the variation of the stray light signal before and after installing the baffle in the telescope structure. These steps significantly improved the image quality and enhanced the rejection of the stray light at the focal plane level. In this paper, we present our investigations, we describe the method used to design the TNT baffle, and we present the improvement in quantitative terms.

Paper Details

Date Published: 23 September 2015
PDF: 11 pages
Proc. SPIE 9626, Optical Systems Design 2015: Optical Design and Engineering VI, 96262E (23 September 2015); doi: 10.1117/12.2190411
Show Author Affiliations
Christophe Buisset, National Astronomical Research Institute of Thailand (Thailand)
Apirat Prasit, National Astronomical Research Institute of Thailand (Thailand)
Apichat Leckngam, National Astronomical Research Institute of Thailand (Thailand)
Thierry Lépine, Lab. Hubert Curien (France)
Saran Poshyajinda, National Astronomical Research Institute of Thailand (Thailand)
Boonrucksar Soonthornthum, National Astronomical Research Institute of Thailand (Thailand)
Puji Irawati, National Astronomical Research Institute of Thailand (Thailand)
Andrea Richichi, National Astronomical Research Institute of Thailand (Thailand)
Utane Sawangwit, National Astronomical Research Institute of Thailand (Thailand)
Vik Dhillon, The Univ. of Sheffield (United Kingdom)
Instituto de Astrofisica de Canarias (Spain)
Liam K. Hardy, The Univ. of Sheffield (United Kingdom)


Published in SPIE Proceedings Vol. 9626:
Optical Systems Design 2015: Optical Design and Engineering VI
Laurent Mazuray; Rolf Wartmann; Andrew P. Wood, Editor(s)

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