ADAPTIVE OPTICS FOR THE 8-METER CHINESE GIANT SOLAR TELESCOPE

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1 ADAPTIVE OPTICS FOR THE 8-METER CHINESE GIANT SOLAR TELESCOPE Jacques M. Beckers 1, Zhong Liu 2, Yuanyong Deng 3, and, Haisheng Ji 4 1 University of Arizona, College of Optical Sciences, Tucson, AZ USA 2 Yunnan Astronomical Observatory, CAS, Kunming, PR China 3 National Astronomical Observatories CAS, Beijing, PR China 4 Purple Mountain Observatory, Nanjing CAS, PR China Abstract. Solar Extremely Large Telescopes (ELTs) enable diffraction limited imaging of the basic physical structure of the solar atmosphere. Magneto-hydrodynamic considerations limit their size to about 0.03 arcsec. To observe them in the near-infrared 8-meter class telescopes are needed. The Chinese Giant Solar Telescope, or CGST, is such a near-infrared solar ELT. It is a Ring Telescope with an 8-meter outer diameter and a central clear aperture of about 6-meter diameter. At present various options for such a Gregorian type telescope are under study, like a continuous ring made out of segments or a multiple aperture ring made of 6 or 7 off-axis telescopes. The advantages of such a ring telescope is that its MTF covers all spatial frequencies out to those corresponding to its outer diameter, that its circular symmetry makes it polarization neutral, and that its large central hole helps thermal control while it provides ample space for MCAO and Gregorian focus instrumentation. We present the current status of the design of the CGST. Our thinking is guided by the outstanding performance of the 1-meter vacuum solar telescope of the Yunnan Solar Observatory which like the CGST uses both AO and image reconstruction. Using it with a ring-shape aperture mask the imaging techniques for the CGST are being explored. The CGST will have Multi-Conjugate Adaptive Optics (MCAO). The peculiarities of Atmospheric Wavefront Tomography for Ring Telescopes are aided by the ample availability of guide stars on the Sun. IR MCAO-aided diffraction limited imaging offers the advantage of a large FOV, and high solar magnetic field sensitivity. Site testing is proceeding in western China (e.g. northern Yunnan Province and Tibet). The CGST is a Chinese solar community project originated by the Yunnan Observatory, the National Astronomical Observatories, the Purple Mountain Observatory, the Nanjing University, the Nanjing Institute of Astronomical Optics and Technology and the Beijing Normal University. 1. Introduction Following the successful construction of the 1-meter New Vacuum Solar Telescope (NVST) at the Fuxian Solar Observatory (FSO) in Yunnan ( one of us (Liu) developed the concept for a Ring Solar Telescope or RST. Part of the implementation of the NVST was the site testing using the first solar version of the differential image motion monitor (S-DIMM). My (JMB) collaboration with the Yunnan Observatory started with the incorporation of this jbeckers@cox.net

2 S-DIMM in the seeing monitor that was developed for the site testing of the 4-meter US solar telescope (ATST) which also included also the SHABAR for the measurement of the height variation of C n 2 and a sky brightness monitor. This ATST site monitor was first tested at the FSO [1]. That collaboration led to further joint efforts on the definition of the Giant Solar Telescope (CGST) which uses the RST concept. This paper results from those. 2. Science Objectives of CGST The science objectives for the CGST are similar to those of the ATST and the 4-meter European Solar Telescope: the study of the smallest structures in the solar atmosphere that are significant in terms of the physical processes acting on the Sun and by implication stars. Because of its larger resolution diameter as well as its collecting diameter, the CGST will be able to do those studies at near-infrared wavelengths (0.8 2 μm) with higher magnetic field accuracy as those telescopes can do at visible wavelengths. To achieve its objectives the CGST will initially be equipped with infrared multi-conjugate adaptive optics (MCAO) and instrumentation. For study of active regions on the Sun the high Strehl Ratio field-of-view (FOV) should be at least 2 x 2 arcmin. In case it turns out that both 4-meter telescopes and CGST indicate smaller significant structures than can be resolved, the CGST and its MCAO will be designed so as to be adaptable to observe at visible wavelengths at full diffraction limited resolution. At longer IR wavelengths (> 2 μm) the CGST will of course outperform the 4-meter telescopes as well. Still under consideration is the desirability to make the CGST coronagraphic. Fig. 1, Two versions of the Ring Telescope. Left: Segmented Ring Solar Telescope (SRST). Right; Aperture Ring Solar Telescope (ARST) with N = 6.

3 3. Options for CGST Configuration Figure 1 shows the two RST options for the CGST currently being considered. The original one is the so-called Segmented Ring Solar Telescope. It consists of 24 segments each about 1 x 1 meter in size. The other is the Aperture Ring Solar Telescope. It consists of a ring of N (off-axis Gregorian?) telescopes whose images are combined. N = 6 in the version shown. For the CGST- SRST the outer diameter of the ring is taken as 8 meter and the width as 1 meter. The advantages of a SRST configuration is (a) that the image MTF includes all spatial frequencies out to that of an filled 8-meter telescope (b) that the heat load management is easier than for a filled aperture telescope, (c) that it does not introduce polarization although it will slightly modify the Stokes Parameters because of the non-normal reflection. We refer to that as being polarization neutral which in terms of its Müller matrix means only diametric elements which deviate slightly from unity, (d) that the diffraction limited point-spread-function (PSF) is symmetric, and (e) that there is ample space at the Gregorian focus for instrumentation, atmospheric dispersion compensation and multi-conjugate adaptive optics (MCAO). For the CGST-ARST things are not quite so optimal. Specifically the non-circularly symmetric configuration leads to a non-isotropic MTF and PSF and possibly worse polarization effects. By going to odd N values (like N=5 or 7) such effects are minimized because of the non-interferometric redundancy of the aperture array. Fig. 2, Aperture Ring Telescope configuration for N = 6 and 7. Different MTF curves correspond to different radial image cuts. The red curve is for the SRST telescope.

4 Figure 2 compares the N = 6 and 7 configurations. The edge-to-edge spacing between adjacent mirrors is taken to be 40 cm to allow for the co-alignment/co-phasing optics. For each SRT the resolution diameter is 8 meter. The collecting diameter equals 5.9 meter and 5.7 meter respectively. Recent descriptions of the SRST can be found in [2-5]. They include dynamical analyses of the concept. One of the major concerns is the requirement to co-align and co-phase the 24 segments. We consider the alternate ARST concept in order to provide other ways to do that. One way is that proposed for the unfunded US National New Technology Telescope (NNTT) which is also an Aperture Ring Telescope with N = 4 [6]. It included a Co-Alignment and Co-Phasing system (C 2 S) which underwent a successful lab-bench test. The Giant Magellan Telescope encounters similar requirements. 4. Adaptive Optics Considerations The requirement for a 2 x 2 arcmin diffraction limited FOV dictates the use of MCAO since the Sun as a single extended object makes multi-object AO (MOAO) not suitable. For our discussions here we will assume a system optimized at the g=3 FeI line at 1.56 μm and a site with an isoplanatic patch diameter 2Θ 0 at that wavelength of 20 arcsec. That will give a diffraction limited FOV diameter of 20 arcsec for Pupil Conjugate Adaptive Optics (PCAO = AO with M=1; M = number of conjugates). For M=3 MCAO, or Triple Conjugate Adaptive Optics (TCAO) the FOV diameter would then be about 100 arcsec. For these conditions figure 3 shows the area on a solar image at which diffraction limited imaging is achieved for the CaII lines at 0.86 μm (white), the FeI (g=3) line at 1.56 μm (yellow), the CO molecular lines at 4.80 μm (red) and the MgI emission line at 12.3 μm (purple). Fig. 3, Diffraction limited FOV for PCAO Fig. 4, Red: beam cross-sections at 12 km (M=1, small circles) and TCAO (M=3; distance for 3 (out of 6) solar guide stars. large circles) for 0.8, 1.56, 4.8 and Green dashed for PCAO (20 arcsec FOV); 12.3 μm wavelength) green full for TCAO 100 arcsec FOV).

5 Although the primary wavelength region for the CGST is the near-infrared, it should be noted that its capabilities at longer wavelengths are truly stunning as well. At the 12.3 μm lines the MCAO covers effectively the entire solar disk with an angular resolution of ~ 0.3 arcsec. That wavelength region contains a number of g 1 lines which could be used for magnetic field observations (including the MgI lines themselves [7-8]). At those wavelengths the Zeeman splitting for these lines is 8 times larger than near-infrared g=1 lines and almost 3 times larger than that that of the g= μm FeI line (expressed in line width). Fig. 5, Example of Shack-Hartman image for solar active region taken at the 76 cm Dunn Solar Telescope. Each image shows a 127 x 127 arcsec area on the Sun (from [9]). Because of the ring-shape of the CGST the light beams converging in each image point in the solar image crosses an orthogonal plane at each distance (height when viewing at zenith) in an

6 identical ring-shape (see figure 4). This affects the atmospheric tomography (AT) part of the MCAO system since tomography requires complete filling in that plane of wavefront information. To accomplish that more subareas on in the solar image (or solar guide stars ) are needed for the S-H correlation wavefront sensing than are necessary for a filled aperture 8-meter telescope. Figure 4 refers to the SRST option; the same is the case for an ARST. We estimate that 3 times the number of these guide stars is needed to do good AT in the ring telescopes. That is of course possible when solar disk imaging is done, but adds complexity to the wavefront sensing and atmospheric tomography of the MCAO system. Figure 5 shows an example of such a solar S-H wavefront sensor image [9]. For TCAO one might select about eighteen equally spaced 4 x 4 arcsec sized solar guide stars within a circular FOV in the 127 x 127 arcsec S-H images for wavefront sensing. 5. Siting Aspects The CGST will probably be located in lake site in Yunnan Province or Tibet selected because of its good seeing quality and large amount of sunshine. A high altitude site is to be preferred if coronal observations are included in its science objectives. Such a site is also preferred because the distance to the high altitude seeing layers, located at the tropopause, is less. This optimizes the diameter of the isoplanatic patch and the performance of the MCAO. One promising site under consideration is on the high altitude plane of Tibet in the Trari Namtso Lake (30 o 55 N & 85 o 36 E; ~25 x 35 km in size) at an altitude of 4600 meters (see figure 6). Fig. 6, Image of Trari Namtso Lake in Tibet (credit Yu Liu). There are of course higher mountain top sites especially in the Tibet area (see e.g. figure 6). The site selection of other large (4-meter) aperture telescopes (ATST, EST) resulted in the choice for mountain tops (Haleakala, La Palma or Izaña) rather than lake sites [10] because of their superior seeing characteristics especially in the early morning. Our present understanding is that in early morning those sites have good seeing because the higher quality boundary layer, late-night seeing is still preserved. A few hours after sunrise solar ground layer heating destroys this quality seeing causing poor image quality in midday until late afternoon when the seeing improves somewhat because solar heating decreases. For lake sites the situation is very different since the solar heating of the lake water is slow. The boundary layer seeing also increases during the day but less and more slowly. As a result the solar image quality is best in the middle of the day when the Sun is closer to zenith, but not as good as in the early morning for mountain sites. Another factor could be the surrounding land boundary layer deterioration penetrating the air mass above the

7 lake. In the early morning and late afternoon the longer pass through the atmosphere resulting from large solar zenith distances causes the seeing to be poorer for the lake sites. Why is a lake site more likely to be selected for the CGST? The reasoning is as follows: (i) the good experience with the 1-meter NVST of the FSO and the 1.6-meter NST at Big Bear Lake, (ii) the availability of high altitude, large lakes in the Tibet area, higher than any mountain site under consideration, should make lake sites look better too in the early morning. In fact a high altitude lake site probably combines the good qualities of mountain and lake sites for solar observations, (iii) the smaller zenith distances at good mid-day seeing conditions minimize the atmospheric airmass and hence the atmospheric absorption, (iv) the high altitude and absence of pollution may make any Tibetan site good for high spatial resolution corona observation, (v) the atmospheric dispersion is less at midday making its correction easier, and (vi) the smaller zenith distance ζ at midday maximizes the size of the isoplanatic patch Θ 0 which varies as cosζ 1.6 (see [11], table 2.1). This strong dependence of Θ 0 on the zenith distance is especially a serious issue. For example, for the mountain Haleakala and La Palma sites that were tested in the ATST site survey [11] the seeing is best in the early morning about one hour after sunrise At that time (at the equinox) ζ 76 0 so that cos 1.6 ζ equals approximately 0.1 or almost 8 times smaller than that for the Trari Namtso Lake site at midday (and equinox) for which it is To reach an equal MCAO diffraction limited area many more conjugate deformable mirrors would therefore be needed for those types of mountain sites thus enhancing the complexity and cost of the MCAO system substantially. At the lower latitude of Haleakala things are somewhat improved because of the lower Jetstream activity [12] 6. Conclusions We described the desired properties and location of the CGST. Its scientific goal is to do imagery and spectroscopy of substantial sized 2 x 2 arcmin regions on the solar disk at near-infrared wavelengths and to do so with an 8-meter diameter telescope located at a good seeing lake site at high altitudes using the inherent properties of the site combined with multi-conjugate adaptive optics. Emphasis will be on accurate magnetic field and velocity observations of the quiet and the active Sun. The facility would be capable to observe large spectral regions both at shorter and longer wavelengths and might be made to do coronal observations. The CGST is a joint effort from the Yunnan Astronomical Observatory, CAS; the National Astronomical Observatories, CAS; the Purple Mountain Observatory, CAS; the Nanjing University; the Nanjing Institute of Astronomical Optics and the Beijing Normal University. It is presently in a definition and engineering study. Site survey efforts are proceeding. 7 References 1. J.M. Beckers, Liu Zhong and Jin Zhenyu SPIE Proceedings SPIE 4853, 273, (2003) 2. Zhong, Liu, Zhenyu Jin, Proceedings SPIE 8336, , (2011) 3. Yichun Dai, Jing Line, Proceedings SPIE 8336, , (2011)

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