Sky coverage assessment for the European ELT: a joint evaluation for MAORY/MICADO and HARMONI
Résumé
The instruments developed for the upcoming Extremely Large Telescopes (ELTs) will need efficient Adaptive Optics (AO) systems to correct the effects of the atmospheric turbulence and allow imaging at the highest angular resolution. One of the most important requirement for ELT AO-assisted instruments will be to deliver diffractionlimited images in a significant part of the sky. For that, the instruments will be equipped with Laser Guide Stars (LGSs) providing most of the information required by AO instruments. But even with LGSs, AO systems still require the use of Natural Guide Stars (NGSs) to compensate for image motion (jitter) and some low order aberrations. These NGSs are eventually limiting the fraction of the sky that can be achieved by AO systems, the so-called Sky Coverage. In this paper, we first present the sky coverage assessment methods used for HARMONI (High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph) and MAORY/MICADO (Multi-conjugate Adaptive Optics RelaY/Multi-AO Imaging Camera for Deep Observations), that are both instruments for the Extremely Large Telescope of the European Southern Observatory (ESO). They are based on a semi-analytical description of the main contributors in the AO error budget, allowing for a fast estimation of the residual jitter. As such, these methods are well suited for statistical estimation of the sky coverage on multiple science fields, and/or to efficiently explore the system parameter space. We then compute the sky coverage of the two instruments in cosmological fields from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) catalog. The goal is to provide an insight on the possibilities given by two different types of tomographic AO systems, i. e. Laser Tomography AO (LTAO) with HARMONI and Multi-Conjugate AO (MCAO) with MAORY, on the same telescope. In particular, we show that HARMONI and MAORY/MICADO are complementary, meaning that the overall sky coverage of ESO's ELT is much improved for applications common to both systems.
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