Adaptive optics at short wavelengths |
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| Authors: | Guido Agapito Carmelo Arcidiacono F Quirós-Pacheco Simone Esposito |
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| Institution: | 1. INAF-Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi, 5, 50125, Firenze, Italy
2. INAF-Osservatorio Astronomico di Bologna, Via Ranzani, 1, 40127, Bologna, Italy
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| Abstract: | The First Light Adaptive Optics (FLAO) system has been successfully commissioned at the Large Binocular Telescope. It delivers extreme adaptive optics performance using bright natural guide stars reaching 90 % Strehl Ratios in H-band. Observations with current adaptive optics systems are limited to the near infrared wavelengths, in these bands the diffraction limited resolution of the largest ground-based telescopes (8–10 meter class) is comparable to the one of the much smaller Hubble Space Telescope that observes in the visible bands. This study aims to demonstrate the feasibility of an adaptive optics system designed to achieve very high order correction at visible wavelengths (0.5 to 0.8 μ m) with significant sky coverage. Upgrading the FLAO design with a low noise CCD relaxes the reference magnitude limit needed to achieve greater performance. In particular, we demonstrate that a gain of 1–2 magnitudes is possible by upgrading the wavefront sensor with a very low read out noise CCD. For future AO systems, in addition to low noise CCDs, deformable (secondary) mirrors with a higher actuator density will be able to move the high order correction capability from the near infrared to the visible wavelengths (Strehl Ratio of 80 % in R (0.7 μ m), 60 % in V (0.5 μ m)). We investigate, by means of numerical simulation, the gain in imaging performance obtained at Near Infrared, Visible, and UV wavelengths. The results of these simulations have been used to derive the empirical relation between Strehl Ratio and magnitude of the reference star and we then use this relationship to perform a detailed sky coverage analysis based on astronomical catalog data. The detailed simulations of the Point Spread Functions allow us to compute Ensquared Energy and Strehl Ratio for the magnitude working range of such an Adaptive Optics system. We present the results of the instrumental isoplanatic angle determination. We then used these values to compute the relationship between correction level and the off-axis angle from the reference star. The Strehl Ratio relationship with the reference magnitude and the angular distance provides the information needed to perform the sky-coverage analysis, which demonstrates that the designed system is able to provide V and R bands correction on a not negligible few percent of the sky. |
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