PROCEEDINGS ARTICLE | July 10, 2018
Proc. SPIE. 10703, Adaptive Optics Systems VI
KEYWORDS: Telescopes, Spectrographs, Gemini Observatory, Spectroscopy, Imaging spectroscopy, Multiplexing, Adaptive optics, Near infrared spectroscopy, Astronomical imaging, Iterated function systems
The GIRMOS instrument will bring a world-class capability to Gemini that does not currently exist or is planned for at 8-10-meter observatories. By combining the latest innovations in multi-object adaptive optics (MOAO) with quad, high-throughput integral-field spectrographs (IFS), GIRMOS’ sensitivity will be comparable to that of JWST’s IFS while offering significant multiplexing. GIRMOS will usher in a new era of large, near-infrared spectroscopic surveys that will take advantage of the superior image quality of an 8-10 meter-class telescope fed with AO. The expected gain with the inclusion of multiplexing and superior AO and spectroscopic performance is ten times other existing single-object AO-fed spectrographs. The capability to perform such observations in multi-object mode means GIRMOS will revolutionize studies in many fields in astrophysics, from observations of galaxies in the distant universe, stars in the Milky Way, the process of star formation, to the characterization of planetary atmospheres. The expected commissioning date is early 2023.
In this contribution, we will present a conceptual design for the GIRMOS MOAO, simulation results, and the open loop control testing we have done to date, aimed at achieving diffraction limited performance on an 8m-class telescope in the presence of realistic turbulence. The GIRMOS spectrograph project is striving firstly for an operational instrument on Gemini that is best-in-class, and secondly, to develop and demonstrate an MOAO system as a TMT pathfinder. Central to this will be the MOAO system, which will use an object selection mechanism to patrol the 2' field of regard of the GeMS Multi-Conjugate Adaptive Optics system, correct for higher order atmospheric turbulence, and then feed the 4 IFU spectrographs. GIRMOS-MOAO will utilize deformable mirrors (DMs) with ~16x16 actuators to yield diffraction limited performance at a minimum of 50% Strehl at H-band.
The instrument requires GeMS for the laser constellation and to do the wavefront sensing, as the scope and budget of GIRMOS relies on this. The MOAO system consists of deformable mirrors in each of the four spectrographs where the GeMS AO system acts as a woofer to keep the WFSs in the linear regime, while the GIRMOS MOAO system acts as a tweeter that corrects for the field specific to each IFS. Our AO simulations show that we are able to reach close to the AO performance target set by our scientific requirements with the current incarnation of GeMS (2-mirror system), and an upgrade path for GeMS in the coming years will further enhance the performance. In addition to improvements in Strehl ratios across the full GeMS field, our MOAO system will increase the overall sensitivity of the instrument ~3 times for an unresolved point source over GeMS AO alone.
In the GIRMOS MOAO system behind GeMS, the DMs are used to tune the adaptive correction for a particular direction on the sky. The greatest challenge of implementing the MOAO is in tuning the open loop (OL) control, developing a system that be calibrated and run without large static errors that degrade performance. We are part-way into a development program to overcome these open loop control challenges, making use of the RAVEN test bed (Lardiere et al., 2014) at the University of Victoria, a previous MOAO pathfinder experiment that was commissioned on the Subaru telescope in 2014. To allow this significant infrastructure to be reused for this purpose, we require a new larger lenslet array and DM than currently implemented in RAVEN. These changes will allow us to improve and develop MOAO calibration techniques that we can then build into GIRMOS. GIRMOS is a Canadian instrument recently funded by the Canada Foundation for Innovation (CFI) and provincial governments.
