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Towards precision radial velocity science with SALT’s High-Resolution Spectrograph

Crause, Lisa A. and Kniazev, Alexei and Butler, R. Paul and Kuhn, Rudi and Roy, Arpita and Browne, Keith and Christians, Alrin and Simon, Etienne and Koeslag, Anthony and Love, Jonathan and Julie, Roufurd (2020) Towards precision radial velocity science with SALT’s High-Resolution Spectrograph. In: Ground-based and Airborne Instrumentation for Astronomy VIII. Proceedings of SPIE. No.11447. Society of Photo-Optical Instrumentation Engineers (SPIE) , Bellingham, WA, Art. No. 1144746. ISBN 9781510636811.

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We describe efforts to equip the Southern African Large Telescope (SALT) for precision radial velocity (PRV) work. Our current focus is on commissioning the high-stability (HS) mode of the High-Resolution Spectrograph (HRS), the mode intended to support exoplanet science. After replacing the original commercial iodine cell with a custom-built, precisely characterised one and following established best practice in terms of observing strategy and data reduction, this system now delivers 3-4 m/s radial velocity stability on 5th and 6th magnitude stars. Unfortunately, the throughput is compromised by the HRS dichroic split being at 555 nm (i.e. roughly midway through the 100 nm span of the iodine absorption spectrum). Furthermore, SALT’s fixed elevation axis limits the exposure time available for a given target and hence the depth and/or precision achievable with the iodine cell. The HS mode’s simultaneous ThAr option uses the full 370–890 nm passband of the HRS and does not suffer gas cell absorption losses, so it may be more suitable for exoplanet work. The first step was to quantify the internal stability of the spectrograph, which requires simultaneously injecting arc light into the object and calibration fibres. The HS mode’s optical feed was modified accordingly, stability test runs were conducted and the necessary analysis tools were developed. The initial stability test yielded encouraging results and though more testing is still to be done, SAL a laser frequency comb to support the development of HRS PRV capability.

Item Type:Book Section
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Butler, R. Paul0000-0003-1305-3761
Roy, Arpita0000-0001-8127-5775
Additional Information:© 2020 Society of Photo-Optical Instrumentation Engineers (SPIE). We acknowledge the National Research Foundation of South Africa and the Southern African Large Telescope Foundation for support for this work. The observations reported in this paper were obtained with the Southern African Large Telescope (SALT). We gratefully acknowledge the detailed discussions with Richard McCracken (of the UltraFast Optics group at Heriot-Watt University in Edinburgh) regarding laser frequency combs and options that may be suitable for SALT HRS. Most significantly, we wish to acknowledge the superb effort by the team at Durham University’s Centre for Advanced Instrumentation that produced the HRS. They designed and built a general-purpose high-resolution spectrograph, most of a decade ago, that successfully incorporated many of the tricks of the PRV trade that are still in use in instruments being built today. Not that we doubted you, but it just seemed too good to be true given the demands of this particular application!
Funding AgencyGrant Number
National Research Foundation (South Africa)UNSPECIFIED
Southern African Large Telescope FoundationUNSPECIFIED
Subject Keywords:Southern African Large Telescope (SALT), High Resolution Spectrograph (HRS), precision radial velocity (PRV), iodine cell, simultaneous ThAr, calibration, exoplanets, laser frequency comb
Series Name:Proceedings of SPIE
Issue or Number:11447
Record Number:CaltechAUTHORS:20201218-081014760
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Official Citation:Lisa A. Crause, Alexei Kniazev, R. Paul Butler, Rudi Kuhn, Arpita Roy, Keith Browne, Alrin Christians, Etienne Simon, Anthony Koeslag, Jonathan Love, and Roufurd Julie "Towards precision radial velocity science with SALT’s High-Resolution Spectrograph", Proc. SPIE 11447, Ground-based and Airborne Instrumentation for Astronomy VIII, 1144746 (13 December 2020);
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107184
Deposited By: Tony Diaz
Deposited On:18 Dec 2020 16:30
Last Modified:16 Nov 2021 19:00

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