We present recent experimental results obtained with CILAS deformable mirrors (DMs) or demonstration prototypes in solar and night-time astronomy (with ground-based telescopes) as well as observation of the Earth (with space telescopes). These important results have been reached thanks to CILAS technology range composed of monomorph and piezostack deformable mirrors, drivers and optical coatings. For instance, the monomorph technology, due to a simple architecture can offer a very good reliability for space applications. It can be used for closed or open loop correction of the primary mirror deformation (thermal and polishing aberrations, absence of gravity). It can also allow a real-time correction of wavefront aberrations introduced by the atmosphere up to relatively high spatial and temporal frequencies for ground-based telescopes. The piezostack technology is useful for very high order correction at high frequency and under relatively low operational temperature (down to -30°C), which is required for future Extremely Large Telescopes (ELTs). This wide range of applications is exposed through recent examples of DMs performances in operation and results obtained with breadboards, allowing promising DMs for future needs.
Seeing measurements are crucial for the optimum design of (multi-conjugate) adaptive optics systems operating at solar
telescopes. For the design study of the 4-meter European Solar Telescope, to be located in the Canary Islands, several
instruments have been constructed and operated, at the Observatorio del Roque de los Muchachos (La Palma) and at the
Observatorio del Teide (Tenerife), to measure the properties of the ground layer and medium-high altitude turbulence.
Several units of short (42.34 cm) and two long (323.06 cm) scintillometer bars are, or are to be, installed at both
observatories. In addition to them, two wide-field wavefront sensors will be attached to the optical beams of the Swedish
tower, on La Palma, and of the German VTT, on Tenerife, simultaneously used with the normal operation of the
telescopes. These wavefront sensors are of Shack-Hartmann type with ~1 arcminute field of view. In this contribution,
the instruments setup and their performance are described.
A consortium of more than 20 European solar physics institution from 15 different countries is conducting a design
study for a 4 m class solar telescope which shall be situated at the Canary Islands. In this paper we introduce the AO and
MCAO design concept for EST. A ground layer deformable mirror is combined with an arrangement of four deformable
layer mirrors. A combination of Shack-Hartmann wave front sensors with wide and narrow fields of view is used to
control the system and to achieve a corrected field of view of one arcmin.
The 1-meter Swedish Solar Telescope (SST) obtains images of the solar surface with an unprecedented resolution of
0.1 arcsec. It consists of a relatively slender tower with on top only the vacuum turret for reflecting downward the solar
beam and no protective dome. This is a favourable situation to get good local seeing. Just in the case of some wind,
seeing is best for daytime observations, therefore the precision bearings and drives of the elevation- and azimuth axis
of the turret have to be stiff against wind. This requires line contact between the meshing teeth of the large gear wheel
and the pinion. High preload forces to achieve line contact are not allowed because of appearing stick-slip effects. To
reduce the risk on stick-slip a special design of the teeth for high stiffness combined with low friction and smooth
transition from one tooth to the next was made. Furthermore, extreme precision in the fabrication was pursued such that
relatively small contact forces give already line contact. This required a special order of the successive fabrication steps
of the combination of bearing and gear teeth. An additional problem was the relatively thin section of the bearings
required for a compact turret construction, needed for best local seeing and minimum wind load. Solutions for all these
problems will be discussed. For the large gears the exceptional good DIN quality class 4 for the pitch precision and
straightness plus direction of the teeth faces was achieved.
The 1-meter Swedish solar telescope is a new solar telescope that was put in operation on the island of La Palma in the Canary Islands at the end of May 2002. The goal of this telescope is to reach its diffraction limited resolution of 0.1 arcsec in blue light. This has already been achieved by use of a low-order adaptive optics (AO)system. This paper describes the AO system initially developed for the former 50-cm Swedish Vacuum Solar Telescope (SVST) and further improved for the new telescope. Both systems use a combination of bimorph modal mirrors and Shack-Hartmann wavefront sensors. Unique to these systems are that they rely on a single workstation or a PC to do all the computations required to extract and pre-process the images, measure their positions using cross correlation techniques and for controlling the deformable mirror. This is in the present system possible by using the PERR instruction available on Compaq's Alpha architecture and in the new system using the PSADDBW instruction, available on Pentium 4 and Athlon processors. We describe both these systems with an emphasis on the performance, the ease of support and upgrades of performance.
We also describe the optimization of the electrode geometry for the new 37-electrode bimorph mirror, supplied by AOPTIX Technologies, Inc., for controlling Karhunen--Loeve modes. Expected performance, based on closed-loop simulations, is discussed.
We report on the use of a new joint phase diverse speckle code, an implementation of a method where a single object and individual phases are estimated from several pairs of phase diverse data. The code was used on 430.5 nm G-band data collected with the newly installed Swedish 1-meter solar telescope in La Palma, equipped with a low-order adaptive optics system. We describe the algorithm briefly, show wavefront statistics and object estimates from the processing and discuss the results. We demonstrate a resolution of 0.12 arc seconds for a time sequence and a large field of view, which is a break-through for ground based solar telescopes.
We describe the 1-meter Swedish solar telescope which replaces the former 50-cm solar telescope (SVST) in La Palma. The un-obscured optics consists of a singlet lens used as vacuum window and two secondary optical systems. The first of these enables narrow-band imaging and polarimetry with a minimum of optical surfaces. The second optical system uses a field mirror to re-image the pupil on a 25 cm corrector which provides a perfectly achromatic image, corrected also for atmospheric dispersion. The adaptive optics system is integrated with the design of the telescope but is sufficiently flexible to allow future upgrades. It consists of a low-order bimorph modal mirror with 37 electrodes, allowing near-diffraction-limited imaging a reasonable fraction of the observing time on La Palma.
The new telescope became operational at the end of May 2002 and has already proven to be the most highly resolving solar telescope ever built. In this paper, we describe its mechanical and optical design, the polishing and testing of the optics and the instrumentation in use or planned for this telescope.
We present a novel and fast method for utilizing wavefront information in closed-loop phase-diverse image data. We form a 2D object-independent error function using the images at different focus positions together with OTFs of the diffraction limited system. Each coefficient in an expansion of the wavefront is estimated quickly and independently by calculating the inner produce of a corresponding predictor function and the error function. This operation is easy to parallelize. The main computational burden is in pre- processing, when the predictors are formed. This makes this method fast and therefore attractive for closed loop operation. Calculating the predictors involves error function derivatives with respect to the wavefront parameters, statistics of the parameters, noise levels and other known characteristics of the optical system. The predictors are optimized so that the RMS error in the wavefront parameters is minimized rather than consistency between estimated quantities with image data. We present simulation results that are relevant to the phasing of segmented mirrors in a space telescope, such as the NGST.
The microprocessors used in off-the-shelf workstations double in performance every eighteen months. The Swedish Vacuum Solar Tower (SVST) uses off-the-shelf workstations for all aspects of its on-line telescope control and data acquisition. Since 1995 workstation performance has been adequate for a correlation tracker of solar granulation controlling a tip- tilt corrector. In 2000 workstation performance permits the construction of a 20 - 50 subimage Shack-Hartmann based low- latency adaptive optics system. It is argued that workstations provide a cost-effective, upgradable, low-risk and flexible means of construction of stellar and solar adaptive optics systems. We give an overview of the adaptive optics system installed at the SVST in May 1999. The system uses a bimorph modal mirror with 19 electrodes from Laplacian Optics. For use with extended targets, such as solar fine structure, cross- correlations with 16 X 16-pixel sub-images are used. For use with point sources, a centroiding algorithm is implemented. The work station used is capable of completing all processing required by the adaptive optics system in 0.5 ms (cross-correlations) or 0.3 ms (centroiding), with potential for significant performance improvements.
We show with simulation experiments that closed-loop phase- diversity can be used without numerical guard-bands for wavefront sensing of low-order wavefronts from extended objects using broad-band filters. This may allow real-time correction at high bandwidth for certain applications. We also present a proper maximum likelihood treatment of Shack- Hartmann data, which includes an imaging model to extract curvature information from the lenslet images. We demonstrate by simple simulations that this approach should allow higher-order wavefront information to be extracted than with traditional Shack-Hartmann wavefront sensing for a given number of lenslets.
We describe the use of a reconfigurable interface board based on FPGAs and a UNIX workstation to implement a correlation tracker with 3.8ms latency. The correlation tracker is part of an active mirror system in use at the Swedish Vacuum Solar Telescope, La Palma, Canary Islands. The reconfigurable interface is used to leverage the workstation CPU, relieving it of tasks that it performs poorly such as rapid context switching and low-level bit manipulation. The reconfigurable interface handles control of external devices, high- performance input (16 MB/s) and data preformatting. The workstation CPU, a 64-bit microprocessor, performs the bulk of the computation. For the key computations of the correlation tracker we are able to treat 8 pixels in parallel in the CPU's 64-bit integer datapath. We present the structure of the CCD interface configuration and the implementations of the key algorithms on the workstation CPU. We describe the design trade-offs that arose during the development of the system, and demonstrate the symbiosis between components implemented in software and configurable hardware.
We have implemented a least-squares technique for recovering phase information and alignment parameters from simultaneously obtained focused and defocused solar images. Small subfields are used, in order to deal with anisoplanatism. The method is applied to sequences of 100 8-bit solar granulation images. These data enable a number of consistency tests, all of which demonstrate that the technique works. Alignment parameters derived from averaged images in a sequence are highly consistent and wavefronts derived from different subfields and different sequences recorded close in time are virtually identical. The wavefronts derived from averaged images are also virtually identical to the average of wavefronts derived from individual images. These aberrations vary with time in a way which is consistent with a major contribution from the moving elements of the alt-az tower telescope. Independently derived wavefronts from single images show high correlation between neighboring subfields and smooth variations across large fields-of-view, consistent with the impression that the image quality is more or less uniform across the image. Restored images in a sequence show a high degree of consistency and much more fine structure than the corresponding observed images.