What is adaptive optics?

The adaptive optics is a modern technique that allow us to surpass the atmosphere so we can get better images and better resolution of the things that we want to look in the space.

As we know, the atmosphere have certain conditions that can affect the vision of a sky, even if we have a clear one. Among those, the humidity, for example, is the main factor that doesn’t allow the astronomers, engineers or researchers, to obtain images that are effectively showing what we are actually seeing in the space, so we can guarantee a highly precision on the studies that we want to make.

The Center for Adaptive Optics of Valparaíso (CAOVA) is searching for the solutions to this issue, using world class research in instrumentation to do so.  How can we achieve this in the era of multidisciplinary research, artificial intelligence and Internet of Things (IoT), among other challenges for the engineering related fields? That’s the question we’re trying to solve.The ground based telescopes have matched the characteristics of the space telescopes.

Nowadays Adaptive Optics (AO) has become a de facto technique for the current generation of telescopes—all of the eight-meter class telescopes and many retrofitted smaller ones—which allow reaching near diffraction limit resolution even when the seeing conditions are unfavorable. This widely used control-system technique characterizes the wavefront fluctuations coming from known reference sources (artificial or natural guide stars nearby the target of interest) with wavefront sensors (WFS), and then feedback adaptive correction signals to deformable mirrors within the optical path of the telescope to compensate the perturbations introduced by the atmosphere. Aided by the technological advancements in the fabrication of more reliable laser sources, the continuous improvements of deformable mirrors and detectors for wavefront sensing, AO has been smoothly evolving from single conjugate AO (correcting for a single turbulent layer) towards more complex schemes such as multi-conjugate AO (MCAO) or laser tomography AO (LTAO) that relies on a more comprehensive characterization of the whole turbulent atmospheric volume above the telescope.

A brief review of current AO technologies shows that turbulence is modeled after Obukhov- Kolmogorov (OK) classical theories laid out by Tatarskĭ more than sixty years ago. Thus, phase corrections are framed around them ignoring most of our current knowledge about Atmospheric Optics. The arguments run as follows: observations are performed mostly upwards, and thus the complexities of the whole turbulent volume are lost into a few layers; the dimensions of telescopes make the inner and outer scales rarely needed; and finally, the frozen turbulence hypothesis is a de facto law translating spatial properties into temporal ones. Of course, our level of success in
atmospheric corrections for our current generation of telescopes have somewhat consolidated these assumptions.
Nevertheless, as we embrace the arrival of the ELTs some of these arguments have come to bite us back. For instance, Taylor’s frozen turbulence hypotheses has been observed to fail —the surface layer may have a greater influence than previously anticipated. Sodium LGS are known to resemble cigars, rather than ideal punctual sources, and wander in anisotropic paths (tilt components residuals are anisotropic)—see and references therein. The mesospheric region, where they are located, is known to be affected by gravity waves and shear constituted between many layers changing within minutes or hours: a recipe for anisotropic turbulence.

This behavior has led to suggest the use of real-time profiling [6]. In the case of the ELTs, since the ground layer may account for up to a 65% of strength for some scientific objectives, real-time profiling is fundamental. Tomographic AO systems can act as internal profilers, combining information from multiple off-axis wavefront sensors to estimate the turbulence in a given target direction, which can also provide information to build an atmospheric profile using the internal wavefront sensor data using the SLODAR method [7]. On the other hand, external profilers such as the stereo SCIDAR can provide extremely useful information for telescope operations with a high-altitude resolution irrespective and independent of the AO system; however, the profile resolution required for the Concurso Fondo QUIMAL 2019 – Programa de Astronomía ELTs exceeds that of existing profiling instrumentation. In particular, and whether using the current ideas and instrumentation for external or internal turbulence profiling, we are uncertain on the effects of the optical turbulence produced by the gigantic domes housing them—effect known as dome-seeing.

Wavefront sensing is the core technology to provide AO feedback and tomographic profiling, which is fundamental for ELT operations. However, wavefront dynamics due to the optical turbulence may be neither timely nor spatially characterized by traditional WFS technology. Current state-of-the-art zonal WFS techniques such as Shack-Hartmann and Pyramidal WFSs have fundamental
tradeoffs between speed and resolution, even cost, given current technological limitations of sensor-array technologies (EMCCD or sCMOS) and the available signal to noise ratio given the choice of natural or artificial LGS. In addition, zonal WFSs require a heavy computational step to provide with a reconstruction of the aberrated wavefront layer or volume. Moreover, none of the current WFSs technologies are capable of properly handling elongated spots from the LGSs, thus the need to look for possible alternatives based on other approaches such as holographic, plenoptic, binary amplitude modulation and machine learning.