Astronomy Next generation adaptive optics for solar observation: a new view of the Sun The roiling surface of the Sun is periodically affected by dramatic explosive recording in optically downstream focal plane events releasing enormous amounts of energy in space, which when instrumentation. The resolution of the image directed earthward can disrupt satellite operations, as well as terrestrial is determined by the quality and size of the primary mirror, and there is a fundamental telecommunications and power grids. Prof Phil Goode, long time, and maximum of resolution achievable with a given now former Director of Big Bear Solar Observatory, New Jersey Institute telescope, called the diffraction limit, which of Technology and Dr Dirk Schmidt at the US National Solar Observatory is proportional to the wavelength of the light are working on the development of a new generation of adaptive optics collected and inversely proportional to the for solar observations, which will help to provide a deeper understanding diameter of the mirror. In actual observations, however, atmospheric turbulence, created of the dynamics of the solar atmosphere and of the origin of solar storms for instance by temperature gradients, can in what is broadly called “space weather”. distort and jitter images, reducing their quality well below the diffraction limit. The standard adaptive optics (AO) system tries to correct ll observations of stars, planets advances in ground astronomy, which came these distortions by rapidly changing the and the Sun using ground- near the end of the last century, has been the shape of a single small (hand-sized) mirror based telescopes suffer from development of methods to correct for the surface to counteract the blurring effects of the the distorting effects of the presence of the atmosphere – adaptive optics atmosphere. All major solar observatories in the Earth’s atmosphere, which (AO) – used in the effort to push the image world are equipped with a single, deformable dramaticallyA affects the level of resolution resolution toward its theoretical limit. mirror (DM) AO system. Lying just before the achievable in remote object imaging and limits focal plane instrumentation, adaptive optics the amount of information obtainable using ASTRONOMY WITH ADAPTIVE OPTICS uses a wavefront sensor to characterise the standard optical techniques. Air turbulence, In standard optical telescopes, the light from optical aberrations caused by the atmosphere, especially close to the ground, causes stars distant objects travelling through space and and a computer calculates the optimal to twinkle at night and planet and Sun images the Earth’s atmosphere is typically collected deformation that should be applied to the to blur when observed at high magnification and focused by a large mirror, and a magnified mirror to cancel the wavefront error. The whole through a telescope. One of the most important image is then created for observation and process happens very quickly, with the image correction applied on the order of a 1,000 times per second. The use of single DM adaptive The dynamics of the Sun’s surface and optics restores image details approaching the diffraction limit, but only in a narrow angular the origin of events like solar storms regime (~10” or about 7000 km on the surface are still largely unexplained of the Sun) near the centre of field of view. The correcting of wavefront distortion allows scientists to build and exploit larger and larger telescopes to observe ever finer details of Air is turbulent beneath blades but image blurring is only beneath hot exhaust air. remote objects. This technology has been steadily improving over the last twenty years, and, today, adaptive optics is considered an indispensable tool in astronomical observations with large telescopes.

SPACE WEATHER The main limitation of existing adaptive optics systems is in the use of a single deformable mirror, which is unable to restore fine image details over a wide field of view. This is caused by the fact that turbulence affects light in different ways depending on the distance from the ground while its path through the atmosphere expands through the troposphere.

86 www.researchfeatures.com 87 Astronomy

Below: Schematic of Shack-Hartmann Adaptive Optics. Green arrows indicate light path. 3 DMs are in upper middle of image facing the reader. Detail

RESEARCH OBJECTIVES Prof Goode has many years of research Despite the impressive successes technologies that are combined and need experience focusing on the Sun’s surface of space-based telescopes, like the to work (together) flawlessly in a complex and atmosphere. Other research interests Hubble Space Telescope, ground- system. Challenges involve both fast include helioseismology and climate based observatories continue to play digital imaging hardware and fast, parallel studies. Combining ground-based a crucial role in observational astronomy. processing computers as well as the quality observatory data with satellite data, Prof What are the advantages of ground- of the optical components, in particular of Goode explores the dynamic properties of based astronomy compared to space the DMs and wavefront sensors. Designed solar magnetic fields that are the origin telescopes? and built as an experimental pathfinder, explosive events, namely space weather. The advantages of ground-based Clear has allowed us to test and optimise telescopes are as follows: critical parameters and algorithms over FUNDING photosphere roughly trebling the corrected 1) Resolution: Largest aperture telescopes the years. National Science Foundation field over that from a single DM. The trebling are ground-based because mass and size is often the difference between fully resolving are not such critical considerations. Is Clear restricted to the observation COLLABORATORS the whole, rather than part of large-scale 2) Instrumentation: Much more complex of the Sun, or can it be extended to study • Dirk Schmidt (NSO) solar phenomena. These observations and sophisticated instrumentation are stars and other astronomical targets • Thomas Rimmele (NSO) When using a single mirror, all solar telescopes Bear Solar Observatory (BBSO) of the New represent the first-ever MCAO observations ground-based because mass and size as well? • Thomas Berkefeld (KIS) with AO have very good image correction in Jersey Institute of Technology, the US National of the Sun showing such an apparent, wide- are not such critical considerations. Clear is solely for the Sun. There are two a small field of view. In Big Bear, users of single Solar Observatory and the Kiepenheuer-Institut field elimination of the deleterious effects 3) Repair: If something goes wrong with MCAO systems in the world operating on BIO DM AO have two choices – the standard very für Sonnenphysik (KIS) in Germany, devoted of atmospheric turbulence. a ground-based telescope, engineers can telescopes. GeMS in near IR on the 8m Prof Phil Goode received an AB from good image over a small field or a medium- to developing and exploiting adaptive optics tackle the problem(s) without a space suit. Gemini South night-time telescope the University of California quality correction over a wider field (more techniques specifically for solar observation. CHALLENGES AND OUTLOOK 4) Cost: Space missions are vastly costlier and Clear. Clear uses what is called at Berkeley in 1964 and a PhD like 20000 km on the solar surface). This issue They have created an adaptive optics system, The development of Clear paves the way because the telescope has to be built with a Shack-Hartman WFS system, which from Rutgers University in 1969. is especially problematic in the case of solar which is capable of providing very good for next generation studies of the dynamics the realisation that post facto correction is is different than GeMS. Furthermore, He is a distinguished research observations, particularly in the study of the image corrections over wide observation and evolution of the solar storms so that extremely expensive (if it is even possible). GeMS uses laser and natural guide professor of physics at New powerful explosive events, like flaring and fields (~20,000 km). The technique they have one can address many basic questions that Then there is the cost of placing the stars, while Clear uses features on the Jersey Institute of Technology coronal mass ejections caused by the Sun’s created, generically referred to as multi- have so far eluded observational answers. telescope in orbit Sun’s surface as ‘guide stars’. (NJIT) and was founding Director of magnetic field dynamics, which appear to start conjugate adaptive optics, exploits three Despite the impressive results obtained the Center for Solar-Terrestrial Research. over wide regions of the solar atmosphere deformable mirrors operating simultaneously to date, however, multi-conjugate adaptive What are currently the most pressing Clear has been the product of a Prof Goode led the project to build the all at once, which is the attraction of the with each correcting turbulence within different optics remains at an experimental stage, questions in solar physics, and how will successful and long-standing GST – the world’s highest resolution solar medium quality, wider field single DM AO. altitude layers of the atmosphere, utilising a at least for the time being, owing to its the development of larger telescopes, collaboration involving the US and telescope at Big Bear Solar Observatory These phenomena are important drivers to single wavefront sensor to probe the optical complexity and hardware requirements. like DKIST, help to answer them? Germany. What is the importance of this (BBSO), California. The telescope has been what is broadly called “space weather”, i.e., aberrations over the targeted field of view and A number of issues need to be addressed We at the dawn of having the spatial and international aspect of the project, and in operation since early 2009. He was the variable conditions on the Sun and in the guide the deformable mirrors. Their system, before this technique reaches the levels of temporal resolution to be able to observe how do you see the role of international director of BBSO since NJIT took over Solar System, including the solar wind, that called Clear (https://cuna.nso.edu/clear), has robustness and reliability required for routine the Sun’s dynamics on their fundamental collaborations develop in current and BBSO from Caltech in 1997 until 2013. can influence the performance of technology been installed and widely tested on the Goode observations. For instance, further refinement scales from the surface to corona. The GST future observational astronomy? He now concentrates his research efforts on Earth and in its vicinity, potentially causing Solar Telescope (GST) at BBSO, which, with a and optimisation are needed to handle can probe at the edge in the Sun’s surface As scientific technologies become more on adaptive optics. damage to critical infrastructures, like power 1.6 m clear aperture, is currently the highest continuously developing images of the Sun’s and lower atmosphere, while DKIST will capable, they also become more complex grids and satellite communication. In this case, resolution solar telescope ever built and has surface for times longer than a few minutes, as have more than twice the resolution of the and expensive (both hardware and CONTACT being able to work at the diffraction limit and to been in regular operation for nine years. most dynamic solar processes are characterised GST and the DKIST will also reach out to the implementation). It is most cost-effective Prof Phil Goode detect fine details over wide areas of the Sun’s Extensive work carried out in 2016 and 2017 by longer lifetimes. The successes at Big corona. Currently, we do not understand and efficient to work in teams across Big Bear Solar Observatory surface, to understand the cause and evolution has shown that the Clear system provides Bear are built on earlier pioneering work at the fundamental origin of space weather. institutions and national borders. This serves 40386 North Shore Lane of these solar events, is a crucial and long imaging capabilities that are far superior to NSO and KIS. The successes at Big Bear will More broadly, the Sun has the largest scale the interests of the taxpayers who ultimately Big Bear City sought-after goal. those of existing single-mirror adaptive optics lead to a wider adoption of multi-conjugate magnetic field that we can resolve and support the scientific enterprise that California setups for solar observation in a variety of adaptive optics in new telescopes, including understanding how it is driven is a basic underlies progress, like MCAO, and derive 92314-9672 LOOKING AT THE SUN: MULTI- the Sun’s and Earth’s atmospheric conditions. the 4 m clear aperture Daniel K Inouye Solar problem of wide interest. its economic and intellectual benefits. USA CONJUGATE ADAPTIVE OPTICS For instance, comparable image quality can Telescope (DKIST), currently under construction Prof Phil Goode and Dr Dirk Schmidt lead be obtained using the Clear system for both atop Haleakala, Maui, Hawai’i, will bring this What were the main technical E: [email protected] an international collaboration involving the Big active (sunspots) and quiet regions of the solar technique to maturity in the coming years, challenges you encountered during T: +1 908 249 1943 and potentially transform solar observations the development of multi-conjugate W: https://cuna.nso.edu/clear/ and, thus, our understanding of the powerful adaptive optics and the Clear system? Adaptive optics is used to remove transient phenomena occurring on the surface A solar MCAO system cannot be bought of the Sun. The DKIST project director is off the shelf, but it is a custom development atmospheric distortions in astronomical Thomas Rimmele, who has been the key that involves various state of the art collaborator in all three generations of AO images, to exploit the resolution in Big Bear. capabilities of large telescopes

88 www.researchfeatures.com www.researchfeatures.com 89