Proceedings Article | 10 July 2018
KEYWORDS: Adaptive optics, Diffraction, Tomography, Laser optics, Calibration, Sensors, Telescopes, Visible radiation, Spectrographs, Near infrared spectroscopy
HARMONI is a visible and near-infrared integral field spectrograph, providing the ELT’s core spectroscopic capability. It will exploit the ELT’s scientific niche in its early years, starting at first light. To get the full sensitivity and spatial resolution gain, HARMONI will work at diffraction limited scales. This will be possible thanks to two adaptive optics systems, complementary to each other. The first one is a simple but efficient Single Conjugate AO system (good performance, low sky coverage), fully integrated in HARMONI itself. The second one is a Laser Tomographic AO system, providing a very high sky-coverage to the instrument. Both AO modes for HARMONI has gone through the Preliminary Design Review at the end of 2017, and will enter in Final Design phase from early 2018 to early 2020.
In this paper, we present the results of the PDR and in particular the system choices that have been made for these SCAO and LTAO modules. We describe the strategy developed for the different Wave-Front Sensors: pyramid for SCAO, the LGSWFS concept, the NGSWFS path, and the truth sensor capabilities. We also detail a set of key items that have been addressed during the PDR, namely:
The “Island effect” produced by the ELT segmented pupil, and how we plan to tackle it.
The calibration strategy, including optical gain tracking, for the pyramid WFS
The strategy for Non-Common Path Aberration (NCPA) calibration, and control
The calibration strategy for the interaction matrices acquisition, and update during observations
The impact of spot truncation for the LGSWFS, and the strategy developed to minimize the effect
The jitter and telescope windshake strategy, and the computation of the corresponding sky coverage.
Finally, we present the overall integration of the AO modules within the HARMONI instrument, and a full end-to-end simulation of the expected performance once at the Telescope.
