9.2 Multi-Conjugate Adaptive Optics (MCAO)

A severe limitation of conventional AO is its ability to fully correct only the seeing aberrations within the isoplanatic patch. Anisoplanatism causes the quality of the AO correction to gradually decline away from the diffraction limit with distance from the chosen lock center and as a consequence, the Strehl ratio decreases. In the visible, conventional AO limits the corrected FOV to a few arcseconds diameter or less (Equation 20View Equation). At near infrared wavelengths, where r0 is larger, a corrected FOV of a few tens of arcseconds might be achieved (Rimmele et al., 2006c). As was shown in Sections 6.1.3 and 9.1.3 the actual size of the isoplanatic patch depends strongly on telescope aperture size, the Fried parameter, wavelength, and, of course, the particular turbulence profile.

A sunspot or active region extends typically over 1 – 2’. Flares can occur “unannounced” anywhere in the extended FOV. Flare trigger mechanisms operate rapidly and on the smallest spatial scales and their location within the FOV is difficult to predict. Diffraction limited resolution over a FOV of 1 – 2’ is required in these cases.

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Figure 47: Principle of MCAO (from Rigaut et al., 2000Jump To The Next Citation Point).

MCAO is a technique that can provide real-time diffraction limited imaging over an extended FOV of 1 – 2’ (Beckers, 1988; Rigaut et al., 2000). Figure 47View Image illustrates the basic principle is tomography. Guide stars in different sky directions are used to probe the turbulent volume above the telescope. Two or more DMs are placed at conjugates of the main turbulence layers and provide correction over an extended FOV as is illustrated with Figure 48View Image. The operation of multiple DMs poses a challenging controls problem.

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Figure 48: Strehl vs. FOV for conventional AO and a 2DM MCAO system. This figure is intended to demonstrate the principle of MCAO and does not represent a realistic performance prediction for a solar MCAO system. Realistic performance estimates of MCAO at a 4 m solar telescope will be shown later in this section (courtesy of T. Berkefeld).

Night-time MCAO development has progressed to a point where scientific results with MCAO have been obtained. The ESO MAD system (Marchetti et al., 2003, 2008), which uses natural guide stars, demonstrated the power of MCAO for scientific discovery even with a demonstrator system. Wavefront sensing in night-time MCAO is difficult because for general use, multiple laser guide stars (LGSs) are needed for tomographic wavefront reconstruction. Thus, to accurately reconstruct 3-D turbulence, generally a number of laser guide stars are needed. The Sun, on the other hand, is an ideal target for MCAO. Any number and any configuration of “guide stars” can in principle be created from the omni-present granulation the same but multiple correlating SHWFS used for conventional AO. Implementing operational solar MCAO is an essential, but challenging task that faces the NST, GREGOR, ATST, and EST.

A number of successful on-the-sky MCAO experiments were performed at the DST and at the German VTT (Vacuum Tower Telescope) on the Canary Islands (Berkefeld et al., 2003; Langlois et al., 2004; Berkefeld et al., 2005; von der Lühe et al., 2005Jump To The Next Citation Point; Rimmele et al., 2009). As an example the KIS MCAO approach is shown in Figure 49View Image. A common theme of the solar MCAO experiments is the two stage approach, which uses the high order conventional AO to provide a good correction of the ground layer, and a second, low-order MCAO stage with multiple off-axis, extended “guide fields”, which are equivalent to the night-time MCAO guide stars. Figure 50View Image shows results from solar MCAO experiments performed at the VTT, Tenerife (von der Lühe et al., 2005Jump To The Next Citation Point) and at the DST (Rimmele et al., 2010a,cJump To The Next Citation Point). Both examples clearly demonstrate the MCAO’s ability to extend the corrected FOV significantly beyond the conventional AO FOV.

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Figure 49: WFS Lenslet and subaperture image arrangement of the conventional AO stage (high order, narrow field, right) and the additional MCAO stage (low order, wide field, left) (from Berkefeld et al., 2006Jump To The Next Citation Point). The upper panel shows a top level schematic of the conventional and MCAO stages (from von der Lühe et al., 2005Jump To The Next Citation Point).
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Figure 50: Top: Results from KIS MCAO experiment at the VTT, Tenerife. Shown is the field dependence of the generalized Fried parameter ρ0 derived from speckle interferometry. Without adaptive optics (left) ρ0 equals r0 and amounts to about 7 cm at 430 nm and is uniformly distributed across the field. Conventional adaptive optics (middle) corrects a FOV of a few arcseconds where ρ0 is about 10.5 cm. The correction extends over a much larger area with MCAO (right). The data for MCAO and without AO were taken about a minute apart (from von der Lühe et al., 2005). Bottom: Results from the DST MCAO experiment. Shown are maps of residual image motion measured with conventional AO (left) and the MCAO (right). Dark blue areas in these images indicate good correction. This example was obtained with the five guide region “asterism”. The square FOV is about 45” × 45”. The MCAO corrects to a best level of 0.01” rms and over a FOV of about 40 – 45” compared to typically less than 10” of the conventional AO (from Rimmele et al., 2010cJump To The Next Citation Point).

As an example of a typical MCAO optical implementation Figure 51View Image shows the MCAO optical path of GREGOR (Berkefeld et al., 2006Jump To The Next Citation Point). The collimator M12 images the entrance pupil onto the tip/tilt mirror M13 which is followed by the pupil plane DM1. The combination of M15MCAO and M16 produce 25 km and 8 km conjugate images at a convenient beam size in order to accommodate the MCAO DMs. The DMs can easily be moved to adjust the conjugate heights. M19 produces the final image at f/37.

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Figure 51: Optical implementation of MCAO for GREGOR (from Berkefeld, 2007)

The 1.5 m GREGOR telescope project is planning to implement a MCAO system shortly after first light (Berkefeld et al., 2006). The system is currently being tested in the lab. Further development is progressing at the DST with the goal to provide the ground work for operational solar MCAO at the BBSO NST. The optical path of the 4 m ATST, currently under construction, is designed to allow the high order conventional AO system to be easily upgraded to high order MCAO once the solar MCAO technology is mature (Rimmele et al., 2006b). The EST project is considering MCAO as a first light capability (Berkefeld et al., 2010Jump To The Next Citation Point; Soltau et al., 2010).

The EST MCAO goals are quite ambitious (Berkefeld et al., 2010Jump To The Next Citation Point). The requirement to achieve a Strehl ratio of S = 0.3 for r0 = 7 cm and S = 0.6 at r0 = 20 cm across a corrected FOV of 1’ at visible wavelengths is extremely challenging. The design uses a large number of DMs and off-axis WFS. In addition to the tip/tilt device there are five (5) DMs at conjugate heights of 0, 5, 9, 15, and 30 km as shown in Figure 52View Image. The science FOV envisioned is 60” square. Performance modeling of such a system was presented by Berkefeld et al. (2010Jump To The Next Citation Point) and illustrates the difficult issues facing solar MCAO. The best seeing conditions at solar telescopes are generally in the morning hours when ground layer turbulence due to ground heating is minimal. On the other hand the zenith angle is large and, consequently, the air mass through which the Sun is observed is multiple times that of observations performed at near zenith pointing.

The impact of zenith angle on MCAO field performance is seen in Figure 53View Image. The achievable maximum Strehl drops drastically with increasing zenith angle. The simulation indicates that even with a 5 DM MCAO system the Strehl does not exceed S = 0.2 once the zenith angle is 60 degrees or larger. In comparison, for zenith pointing high Strehl can be achieved with only four DMs. If the MCAO performance is optimized for a reduced FOV of 30” diameter good Strehl performance can be achieved at large zenith angles (Figure 53View Image, right).

These results point to fundamental issues with the implementation of solar MCAO. Trade-offs will have to be performed in order to optimize the MCAO performance toward specific science experiments. The FOV over which diffraction limited observations have to be performed to achieve scientific goals as well as the desired Strehl will drive the complexity of the MCAO and the zenith distance for which the observations are possible. Achieving a high visible Strehl over a large (≥ 2’) FOV does not appear to be practical during the typical prime solar observing hours. The probability of obtaining large corrected FOVs with MCAO in the infrared is much higher because of the 6 λ 5 dependence of key atmospheric parameters, such as the Fried parameter and the isoplanatic angle. MCAO observations are best performed for near zenith pointing. However, the build up of ground turbulence will increasingly stress the ground layer DM and drive it toward the highest possible order of correction. As was noted above the minimum subaperture size of the correlating SHWFS is around 7 – 8 cm. The order of the ground-layer DM can not be increased much beyond this limit unless ways to significantly improve the SNR performance of this WFS approach can be found (see Section 6.1.5) or different WFS approaches that do not have this limitation can be developed for solar AO application. The phase diversity approach (Paxman et al., 2007) to the solar WFS appears to be the most promising in this regard since it deploys full aperture WFS and, thus, is not limited by subaperture diffraction. The information content of the PD sensor is higher compared to the correlating SHWFS.

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Figure 52: EST MCAO linear optical arrangement. M1 and M2 constitute the main telescope. M5 and M8 are the collimator and camera optics of the conventional AO. The tip/tilt mirror is M6; the ground layer (AO) DM is M7. The four MCAO DMs M9-M12 are at conjugate heights of 30, 15, 9, and 5 km (from Berkefeld et al., 2010Jump To The Next Citation Point).
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Figure 53: Left: Strehl as a function of the field angle and zenith angles = 0° (upper curve), 30° (middle curve), 60° (lower curve). Even with 5 DMs the performance is unsatisfactory for large zenith angles. Right: Strehl as a function of the field angle and zenith angle 45° for a corrected FOV of 60” and 30”, respectively, and modified MCAO DM conjugated heights (from Berkefeld et al., 2010).

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