We explore the feasibility of adaptive optics for observations of prominences off the solar limb. We installed a wavefront sensor prototype on the GST at the Big Bear Solar Observatory. This sensor is used to conduct open and closed loop experiments to characterize the limitations of this application following up on first demonstrations at the DST. The sensor will enable us to optimize parameters and algorithms for a potential future implementation on NSF’s Daniel K. Inouye Solar Telescope. Unlike the granular structure of the solar photosphere that has served wavefront sensors in solar telescopes for decades with useful reference structures, prominences are much more challenging to use: they are faint and their fine structure is barely visible at the short exposures needed for an AO wavefront sensor.
The MCAO pathfinder Clear on the 1.6-meter Goode Solar Telescope has been enabling us to advance solar MCAO from early conceptual demonstrations to science grade wide-field image correction. We report on recent improvements to the control loop and we comment on issues such as the co-aligning of wavefront sensors and deformable mirrors and the sensitivity of wavefront sensor gains. Further, we comment on the challenges to wavefront sensing and the control system architecture faced when scaling up to a 4-meter aperture. Finally, we present an early concept of the future MCAO upgrade for the Daniel K. Inouye Solar Telescope.
We present improvements to the telescope and the Adaptive Optics (AO) upgrades that will be installed in 2018 to the 1.5m GREGOR solar telescope. An exchange of the telescope’s secondary mirror has improved the contrast in the AO wavefront sensor (WFS). The repair of the telescope building’s faulty facade plates will result in better dome/building seeing. The daytime (solar) wavefront sensor camera will be replaced by a high full well capacity model, enabling the AO to lock on granulation and faculae close to the solar limb during bad seeing conditions. A 2x2 guide region ground layer AO (GLAO) mode will provide a 10” FoV with a much flatter PSF as compared to conventional AO. A change of the night-time wavefront sensor camera will result in a larger WFS FoV, improving the AO tracking of the large solar system planets like Jupiter and Saturn. A new AO mode of operation will allow the tracking of of solar prominences using a narrow-band H-alpha filter.
Adaptive Optics (AO) that compensates for atmospheric turbulence is a standard tool for high angular resolution observations of the Sun at most ground-based observatories today. AO systems as deployed at major solar telescopes allow for diffraction limited resolution in the visible light regime. Anisoplanatism of the turbulent air volume limits the effectivity of classical AO to a small region, typically of order 10 seconds of arc. Scientifically interesting features on the solar surface are often larger thus multi-conjugate adaptive optics (MCAO) is being developed to enlarge the corrected field of view. Dedicated wavefront sensors for observations of solar prominences off the solar limb with AO have been deployed. This paper summarizes wavefront sensing concepts specific to solar adaptive optics applications, like the correlating Shack-Hartmann wavefront sensor (SH-WFS), multi-directional sensing with wide-field SH-WFSs, and gives a brief overview of recent developments.
The multi-conjugate adaptive optics (MCAO) system for solar observations at the 1.6-meter clear aperture New Solar Telescope (NST) of the Big Bear Solar Observatory (BBSO) in Big Bear Lake, California, enables us to study fundamental design questions in solar MCAO experimentally. It is the pathfinder for MCAO of the upcoming Daniel K. Inoyue Solar Telescope (DKIST). This system is very flexible and offers various optical configurations such as different sequencings of deformable mirrors (DMs) and wavefront sensors (WFS), which are hard to simulate conclusively. We show preliminary results and summarize the design, and 2016 updates to the MCAO system. The system utilizes three DMs. One of which is conjugate to the telescope pupil, and the other two to distinct higher altitudes. The pupil DM can be either placed into a pupil image up- or downstream of the high-altitude DMs. The high-altitude DMs can be separately and quickly conjugated to various altitudes between 2 and 8 km. Three Shack-Hartmann WFS units are available, one for low-order, multi-directional sensing and two high-order on-axis sensing.
Solar adaptive optics (AO) simulations are a valuable tool to guide the design and optimization process of current and future solar AO and multi-conjugate AO (MCAO) systems. Solar AO and MCAO systems rely on extended object cross-correlating Shack-Hartmann wavefront sensors to measure the wavefront. Accurate solar AO simulations require computationally intensive operations, which have until recently presented a prohibitive computational cost. We present an update on the status of a solar AO and MCAO simulation tool being developed at the National Solar Observatory. The simulation tool is a multi-threaded application written in the C++ language that takes advantage of current large multi-core CPU computer systems and fast ethernet connections to provide accurate full simulation of solar AO and MCAO systems. It interfaces with KAOS, a state of the art solar AO control software developed by the Kiepenheuer-Institut fuer Sonnenphysik, that provides reliable AO control. We report on the latest results produced by the solar AO simulation tool.
We present the properties of the adaptive optics (AO) system of the German 1.5m solar telescope GREGOR, located on the island of Tenerife, Spain. The conventional AO system uses a correlating Shack-Hartmann-Sensor with a 92mm subaperture size and a 256-actuator stacked-piezo deformable mirror (DM). AO performance results and practical experience based on the last four years of operation are presented. A recently installed second wavefront sensor with exchangeable lenslets / subaperture sizes in combination with an EM-CCD camera is used for low light observations such as polarimetric measurements of the solar system planets. Further developments include algorithmic improvements, the use of the night-time sensor for solar (off-limb) observations and solar MCAO.
We are currently implementing a solar adaptive optics (AO) and multi-conjugate adaptive optics (MCAO) simulation package that provides a full simulation, including wavefront sensor cross-correlations, and is able to operate at quasi-realtime performance. This is made possible by modern personal computers with many cores, which allow the operation of a solar AO system with relatively inexpensive off-the-shelf computers. The simulation package uses KAOS, a mature AO controller software used at the GREGOR solar telescope, to operate the simulated AO system. It provides a simulated environment that is presented to KAOS to achieve a highly realistic and fast simulation of solar AO.
A multi-conjugate adaptive optics (MCAO) system is being built for the world's largest aperture 1.6m solar telescope, New Solar Telescope, at the Big Bear Solar Observatory (BBSO). The BBSO MCAO system employs three deformable mirrors to enlarge the corrected field of view. In order to characterize the MCAO performance with different optical configurations and DM conjugated altitudes, the BBSO MCAO setup also needs to be flexible. In this paper, we present the optical design of the BBSO MCAO system.
A multi-conjugate adaptive optics systems has been deployed at the 1.5-meter solar telescope GREGOR for on-sun experiments of MCAO in November 2013. GREGOR MCAO incorporates three deformable mirrors (DMs) conjugate to 0, 8, and 25 km line of sight distance. Two correlating Shack-Hartmann wavefront sensor units are deployed: a high-order on-axis wavefront sensor (OA-WFS) with 10-cm subapertures and 10 arcsec field of view, and a low-order multi-direction wavefront sensor (MD-WFS) with 50-cm subapertures that sample the wavefront in 19 guide regions distributed over one arcminute. The MCAO loop was closed repeatedly in November ’13, as well as in January and May ’14. However, in particular strong static aberrations that were not removed well by the system, derogated the image in the MCAO compensated focal plane. GREGOR MCAO is now permanently installed and available for experiments that shall advance the development of solar MCAO.
We report on the multi-conjugate adaptive optics (MCAO) system of the New Solar Telescope (NST) at Big Bear Solar Observatory which has been integrated in October 2013 and is now available for MCAO experiments. The NST MCAO system features three deformable mirrors (DM), and it is purposely flexible in order to offer a valuable facility for development of solar MCAO. Two of the deformable mirrors are dedicated to compensation of field dependent aberrations due to high-altitude turbulence, whereas the other deformable mirror compensates field independent aberrations in a pupil image. The opto-mechanical design allows for changing the conjugate plane of the two high-altitude DMs independently between two and nine kilometers. The pupil plane DM can be placed either in a pupil image upstream of the high-altitude DMs or downstream. This capability allows for performing experimental studies on the impact of the geometrical order of the deformable mirrors and the conjugate position. The control system is flexible, too, which allows for real-world analysis of various control approaches. This paper gives an overview of the NST MCAO system and reveals the first MCAO corrected image taken at Big Bear Solar Observatory.
Observing the Sun with high angular resolution is difficult because the turbulence in the atmosphere is strongest during day time. In this paper we describe the principles of solar adaptive optics exemplified by the two German solar telescopes VTT and GREGOR at the Observatorio del Teide. With theses systems we obtain near diffraction limited images of the Sun. Ways to overcome the limits of conventional AO by applying multiconjugate adaptive optics (MCAO) are shown.
We look back on two years of experience with the laboratory MCAO testbed for the GREGOR solar telescope.
GREGOR’s MCAO features four adaptive mirrors, i. e. one tip-tilt mirror, and three DMs to compensate for
turbulence around 0 km, 5 km, and 15.5 km above ground. Two different Hartmann-Shack wavefront sensor units
are used for wavefront tomography. A sensor with a narrow field of view and smaller subapertures is dedicated to
high-order aberrations on the optical axis. This sensor directly follows the pupil plane DM and does not see the
high-altitude DMs. The second sensor features larger subapertures and 19 guide regions spread over a wide field
of view for off-axis wavefront sensing. We show that high-altitude DMs cause rapidly changing pupil distortions
and thus misregistration, which renders the interaction of a pupil-plane DM and a subsequent wavefront sensor
We rewrote the control software for cleaner and more flexible code, and we switched to modal wavefront
reconstruction from direct reconstruction. The original digital interfacing of the DMs high-voltage electronics
didn’t prove to be reliable. Thus, we developed a new interface board that is based on CameraLink/ChannelLink
technology to transmit the DM commands from the control computer.
In this paper we present the innovations and some of the first experimental performance measurements with
two DMs. One DM failed before scientific grade data was recorded with three DMs. This DM will be replaced
soon. We conclude that GREGOR’s MCAO system is now ready for first on-sky tests at the telescope.
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 next generation large aperture solar telescopes is thus a top priority. The Sun is an ideal
object for the development of MCAO since solar structure provides multiple "guide stars" in any desired configuration.
At the Dunn Solar Telescope (DST) we implemented a dedicated MCAO bench with the goal of developing wellcharacterized,
operational MCAO. The MCAO system uses two deformable mirrors conjugated to the telescope
entrance pupil and a layer in the upper atmosphere, respectively. The high altitude deformable mirror can be placed at
conjugates ranging from 2km to 10km altitude. We have successfully and stably locked the MCAO system on solar
granulation and demonstrated the MCAO system's ability to significantly extend the corrected field of view. We present
results derived from analysis of imagery taken simultaneously with conventional AO and MCAO. We also present first
results from solar Ground Layer AO (GLAO) experiments.
The testbed of the MCAO for the new 1.5 meter solar telescope GREGOR is now operational. Most of the
components will be moved to the telescope after commissioning. The testbed features 4 adaptive mirrors (1 tiptilt,
and 3 DMs), and two Hartmann-Shack sensor units for wavefront tomography in a guide-region oriented
approach. First system characteristics gained from setting up operation of the testbed are presented. We
also comment on the effect of high-altitude deformable mirrors on subaperture alignment, and misregistration.
We conclude that on-axis wavefront sensors should not be located behind high-altitude deformable mirrors.
Furthermore, we present a general opto-geometric characteristic of micro-lens arrays needed for a
Hartmann-Shack sensor which shall be used for extended fields of view - be it solar surface or laser guide stars, for example.
This characteristic can be useful to have custom-made arrays manufactured for reasonable prices.
We present the latest concept of the multi-conjugate adaptive optics system for the 1.5-meter solar telescope Gregor. This
system will employ three deformable mirrors in order to compensate for seeing introduced by the ground layer, and by
shear winds in 5 and 15 km above the telescope ground. Thus, the compensated field of view will grow compared to ground
layer compensation only. We describe the design and the used components and present a testbed which is used to improve
control algorithms and to test all the components before installing them at the Gregor telescope.
We present the results of our first experimental tests of the concept of an alternative wavefront sensor for extended, incoherent
light sources such as the sun. This concept is not associated with subapertures and therefore does not suffer from
involved restrictions. In theory, this wavefront sensor also needs very little light from the telescope. The sensor employs a
liquid crystal display as used in digital video projectors for masking an image plane in an aberrated telescope. We describe
a laboratory setup and an advanced prototype used at the German Vaccum Tower Telescope (VTT), Tenerife.
Pulsed Er:YAG and Er:YSGG lasers are well known to be effective instruments for the ablation of dental hard tissues. Developments in the last years made it possible to transmit the laser radiation at these wavelengths with flexible fibers. Therefore the application in the periodontal pocket may be possible. The aim of this study was to evaluate the in-vitro conditions to generate a bioacceptable root surface. Twenty extracted human teeth, stored in an antibiotic solution, were conventionally scaled, root planed and axially separated into two halves. Two main groups were determined. With the first group laser radiation was carried out without and in the second group with spray cooling. The laser beam was scanned about root surface areas. Laser parameters were varied in a selected range. The biocompatibility was measured with the attachment of human gingival fibroblasts and directly compared to conventionally treated areas of the root surfaces. The fibroblasts were qualified and counted in SEM investigations. On conventionally treated areas gingival fibroblasts show the typical uniform cover. In dependance on the root roughness after laser treatment the fibroblasts loose the typical parallel alignment to the root surface. With spray cooling a better in-vitro attachment could be obtained. Without spray cooling the higher increase in temperature conducted to less bioacceptance by the human gingival fibroblasts to the root surface. These results show the possibility of producing bioacceptable root surfaces with pulsed laser radiation in the range of very high water absorption near 3 micrometer.