Solar multi-conjugate adaptive optics system for the Big Bear Observatory

Solar observations are performed over an extended field of view and the isoplanatic patch over which conventional adaptive optics (AO) provides diffraction limited resolution is a severe limitation. The development of multi-conjugate adaptive optics (MCAO) for the new large aperture solar telescopes ranging from 1.6 to 4 meters-diameter is extremely important to achieve many solar physics scientific goals. The Sun is an ideal object for the development of MCAO since solar structure provides multiple “guide stars” in any desired configuration. We propose a concept for an MCAO system for the New Solar Telescope at Big Bear Observatory. This MCAO system uses three deformable mirrors conjugated to the telescope entrance pupil and to two layers in the upper atmosphere. We present the result of numerical analysis of the performance of this system for large range of elevations as required in solar observations. Introduction Multi-conjugate adaptive optics (MCAO) is a tool used to extend the field of view of astronomical adaptive optics (AO) systems. For solar observations at visible wavelengths, angular anisoplanatism, limits the field of view of AO systems to about ~ 10 arcseconds. Solar multi-conjugate adaptive optics has been proven to increase the field of view [1,2] and now is been built for many existing solar telescopes [3,4]. Two of the « extremely » large solar telescopes foreseen for a near future: the Advanced Technology Solar Telescope (ATST) [5] and The European Solar Telescope (EST) [6] will be equipped with MCAO as a baseline. The solar physics requirements of high Strehl at visible wavelengths and over a wide field-of-view (FOV) with these 4-meter diameter telescopes lead to the design of adaptive optics of very high complexity due to the large number of degrees of freedom. We present in this paper results from the numerical simulation of the MCAO system performances for the 1.6m New Solar Telescope at Big Bear Observatory using Shack-Hartman wavefront sensing on multiple solar sunspots and granulation. We use a wide field of view Shack-Hartmann WFS with cross correlation applied to selected guiding regions. In this case, using extended images of the Sun's granulations and sunspots allows more flexibility than the night-time experiment because it is possible a e-mail : maud.langlois@obs.univ-lyon1,fr Third AO4ELT Conference Adaptive Optics for Extremely Large Telescopes Florence, Italy. May 2013 ISBN: 978-88-908876-0-4 DOI: 10.12839/AO4ELT3.13316 Adaptive Optics for Extremely Large Telescopes III to select the separation, the size, the geometry and the number of guiding sub-images. The main limitation comes from the contrast of the observed solar scenes. These simulations were performed using with the Fractal Iterative Method (FrIM3D) [8,9], chosen for its speed and its low-requirements in precomputations when changing the AO system configurations. These assets of the FRiM-3D are due to its iterative nature, its use of sparse and fast operators and its way of modeling based on geometric optics. Figure 1: The 1.6meter off-axis New Solar Telescope at Big Bear Solar Observatory. 1. Solar MCAO concept Solar MCAO uses a wide field of view Shack-Hartmann wavefront sensor with selected guiding regions in order to estimate the local gradients over the telescope aperture. The cross-correlation centroiding algorithm is currently used on each sub-field subapertures large enough to sample the granulation involved in the correlation procedure or to cover the field of interest when using sunspots. On granulation the minimum field is 5′′ × 5′′ and the field could be as large as 8′′ × 8′′ to ensure more robust AO correction [3]. 2. MCAO System Details The MCAO system uses three DMs at conjugate heights of 0 km (ground layer DM) and 2 and 6 km, as shown (see Figure 2A). It uses 11x11 sub-apertures Shack-Hartmann wavefront sensors with cross correlation (see Figure 2B), applied on 5 to 9 selected solar guiding regions 18”x18” each (sunspot & granulation) within the 1.4' full FOV (see Figure 2C). We performed simulations for different telescope elevations. The simulations featured 100 realizations of a turbulent atmosphere of r0=7,10,15 cm at 550 nm. The atmosphere includes 19 frozen layers. In each simulation cases, the r0 is maintained for all the Third AO4ELT Conference Adaptive Optics for Extremely Large Telescopes Adaptive Optics for Extremely Large Telescopes III elevations, and the reason is that solar telescopes observe at low elevation during the first hours of the day, and at that moment the atmosphere is less turbulent. Therefore, we use the same value for all elevations. The atmospheric profile is given by the combined Cn profile from Rimmele [9]. The impact of anisoplanatism from 5’’x5’’ extended-source SH is not included yet in our modeling but is shown in table 1 for comparison. It has been shown in Béchet et al. [10], that the wider the WFS fieldof-view or the lower the elevation, the worse the Strehl ratio of the conventional solar AO. In the case of MCAO the impact should be smaller due to the MCAO correction of the WFS sub-fields themselves. DM Heights (km) 0.0, 2.0, 6.0 DMs Spatial Sampling (cm) 15.0, 15.0, 15.0 Sensing Fields 5 to 9