Low-resolution spectroscopy mode for CASTLE telescope with a composite grism

Eduard Muslimov; Jean-Francois Sauvage; Simona Lombardo; Damir Akhmetov; Danila Kharitonov

doi:10.1117/12.2628688

29 August 2022 Low-resolution spectroscopy mode for CASTLE telescope with a composite grism

Eduard Muslimov, Jean-Francois Sauvage, Simona Lombardo, Damir Akhmetov, Danila Kharitonov

Proceedings Volume 12184, Ground-based and Airborne Instrumentation for Astronomy IX; 121848G (2022) https://doi.org/10.1117/12.2628688
Event: SPIE Astronomical Telescopes + Instrumentation, 2022, Montréal, Québec, Canada

Conference Poster

Abstract

The CASTLE is a small robotic telescope designed for the observations of ultra-low surface brightness objects and the search and detection of transient objects. The telescope is designed and built at Laboratoire d’Astrophysique de Marseille and will be installed at the Calar Alto Observatory. In the present work we consider a possibility to implement a low-resolution slitless spectroscopy mode for CASTLE in order to provide a fast spectrum of the transient immediately after the first detection. Such an additional observation mode could be realized by introduction of a grism in the pre-focal position at the filter wheel. The grism can create spectral images with a minimal decentering and a small defocusing, so the same detector and focusing mechanism can be used. Use of a volume-phase holographic grating as the main dispersing part of the grism allows to reach a high diffraction efficiency together with the low level of scattered light. The value of the spectral resolving power is R=163- 698 over the extended spectral range of 350-950 nm, while telescopes nominal f/#=2.5 is maintained and an extended field of view of 9’x14’ is covered. Due to the limitations set by the telescope optical design the grism will be placed in a converging beam far from the pupil, so that its aberrations and diffraction efficiency will vary notably across the clear aperture. In order to compensate for these variations we consider using a composite holographic component, which represents a mosaic of zones. In each of them such parameters of the hologram as the photosensitive layer thickness, refraction index modulation depth and aberrations of the recording wavefronts can be optimized separately. We estimate the performances of such an innovative element and compare it with that of an ordinary grism. The comparison demonstrates gain up to a factor of 3 in the image quality and up to a factor of 4.2 in the diffraction efficiency.

Citation Download Citation

Eduard Muslimov, Jean-Francois Sauvage, Simona Lombardo, Damir Akhmetov, and Danila Kharitonov "Low-resolution spectroscopy mode for CASTLE telescope with a composite grism", Proc. SPIE 12184, Ground-based and Airborne Instrumentation for Astronomy IX, 121848G (29 August 2022); https://doi.org/10.1117/12.2628688

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