AOF upgrade for VLT UT4: an 8m class HST from ground

Abstract

In this paper we present numerical simulations and an initial design for a visible MCAO system for the VLT-UT4 telescope. The proposed concept takes great advantage of the existing HW developed for the Adaptive Optics Facility (AOF) at the VLT-UT4, in particular the 4x20W Toptica lasers and the adaptive secondary mirror with 1170 actuators. The mentioned units makes the VLT-AOF a unique facility to develop a second generation AO system aiming to provide corrected FoV at short wavelength. In particular the flux provided by the four lasers steerable on sky and the high density of actuators (20cm equivalent on M1) provides the temporal bandwidth and the spatial sampling to push the correction down to the visible wavelengths. In addition to this the request of a reasonable size corrected FoV with uniform performance calls for an MCAO system. For such reason here we propose to complement the AOF with post-focal DMs that together with the VLT DSM can provide a corrected FoV of roughly 20/30 arcsec diameter size. An additional challenge for the system is the provided a large sky coverage. Such condition comes from the efficiency of LO wavefront sensors that use field NGS. The presented simulations give some first results for (a) the achieved performance at visible wavelength 0.4-0.9 um as a function of DMs and tip tilt NGSs characteristics (b) the achieved system sky coverage after. Pushing performance toward visible wavelengths calls for embedded and efficient post-processing methods. Being able to capture short-exposure science images (with the trade-off on noise and overheads), would allow retrieving the ultimate performance by compensating the residual turbulence aberrations left over by the AO system. Considerations about advanced analysis tools that may potentially relax the system constraints are discussed. Finally the paper presents a conceptual arrangement for the opto-mechanics of the considered AO module including the additional DMs and wavefront sensors.