The next generation of extremely large telescopes requires the use of segmented mirrors. This technology needs specific wave front sensors to measure the alignment and phasing state. This paper compares two specific technologies for the measurement of wavefront steps between segments: a simple pin hole and a phase contrast sensor. The efficiency of each sensor will be quantified by calculating the Fisher information, first, under ideal conditions, then including the effects of sampling and atmospheric turbulence.

We study a novel focal plane wavefront sensing and active optics control scheme at the VST on Cerro Paranal, an f/5.5 survey telescope with a 1x1 degree field of view and a 2.6m primary mirror. This scheme analyzes the elongation pattern of stellar PSFs across the full science image (256 Mpixels) and compares their second moments with an analytical model based on 5th-order geometrical optics. We consider 11 scalar degrees of freedom in mirror misalignments and deformations (M2 piston, tip/tilt and lateral displacement, detector tip/tilt, plus M1 figure astigmatism and trefoil). Using a numerical optimization method, we extract up to 4000 stars and complete the fitting process in under one minute. We demonstrate successful closed-loop active optics control based on maximum likelihood filtering.

This paper describes the active optics system of the VLT Survey Telescope, the 2.6-m survey telescope designed for visible wavelengths of the European Southern Observatory at Cerro Paranal, in the Atacama desert. The telescope is characterized by a wide field of view (1.42 deg diameter), leading to tighter active optics than in conventional telescopes, in particular for the alignment requirements. We discuss the effects of typical error sources on the image quality and present the specific solutions adopted for wavefront sensing and correction of the aberrations, which are based on the shaping of a monolithic primary mirror and the positioning of the secondary in five degrees of freedom.

A one year database has been gathered from the VLT active optics Shack-Hartmann (S-H) wavefront sensor images taken at each operating focus about every 30 seconds. The VLT telescope control software includes a dedicated code to extract the median full width at half maximum of the unvignetted S-H spots which is used for this study. This code applies a 1-D fit, assuming circular Hartmann spots, which allows to work only on foci equipped with atmospheric dispersion correction, or when the telescopes are observing close to zenith. The S-H image size measured inside the 30m enclosures is compared the outside seeing measured at 6m above ground by the VLT Astronomical Site Monitor (DIMM). A method for correcting DIMM measurements from surface layer turbulence contamination is proposed.

In wide-field survey telescopes, the patterns of spot sizes and ellipticities can be used to determine wavefront aberrations generated by the telescope. The calculation of spot sizes and ellipticities generated by telescope aberrations is most conveniently done if the aberrations are expressed in terms of Zernike-type polynomials whose derivatives are orthonormal. The field dependence of the spot sizes and ellipticities generated by the telescope can conveniently be expressed by low-order Zernike polynomials. Because the exposure times in astronomical survey work are typically rather short, this information may be used for a quasi-closed loop control of the telescope optics. The ability to accurately subtract ellipticities generated by telescope errors could also be useful for observations such as gravitational lensing surveys.

We study a novel active optics control scheme at the VST on Cerro Paranal, an f/5:5 survey telescope with a 1x1 degree field of view and a 2.6m primary mirror. This scheme analyzes the elongation pattern of the star PSFs across the full science image (267 Mpixels) and compares their second moments with an analytical model based on 5th-order geometrical optics, comprising 9 degrees of freedom in mirror misalignments and deformations. Using a numerical optimization method, we can complete the star extraction and fitting process in under one minute, fast enough for effective closed-loop active optics control in survey observing cadences.

The wavefront sensor in active and adaptive telescopes is usually not in the optical path toward the scientific detector. It may generate additional wavefront aberrations, which have to be separated from the errors due to the telescope optics. The aberrations that are not rotationally symmetric can be disentangled from the telescope aberrations by a series of measurements taken in the center of the field, with the wavefront sensor at different orientation angles with respect to the focal plane. This method has been applied at the VLT Survey Telescope on the ESO Paranal observatory.

The paper describes the envisaged positioning and drive system for telescopes of the 2m class, gives some basic analysis for power consumption and positioning performance and proposes next steps for verification of the concept. A possible application could be in optical stellar interferometry where an additional delay could be spared if the positioning performance is better than 10nm. Fast re-positioning of the telescopes should allow for multiple baselines during one observing night giving the opportunity to obtain high spatial stellar images with high time resolution.

The active optics system of the VLT Survey Telescope (VST) adopts a positioning system for the secondary mirror, a system to support and modify the shape of the 2.6-m primary mirror, and a Shack-Hartmann wavefront sensor. This paper describes the concepts of the VST active optics and the commissioning of the whole system on the ESO's Paranal Observatory.

In a wide-field telescope like the VST, the requirements for alignment are tighter than for traditional instruments. The same amount of misalignment can be negligible in traditional telescopes with fields of some arc minutes, but unacceptable when the field is one order of magnitude larger. We describe the alignment procedure implemented during the telescope commissioning on the Paranal ESO's observatory, as well as the final results.

The Zernike phase contrast sensor has been studied in the framework of the Active Phasing Experiment in the laboratory and on sky at the Very Large Telescope. Atmospheric turbulence strongly affects the shape of the signal of the Zernike phase contrast sensor. The first part of these proceedings is dedicated to a study of the influence of atmospheric turbulence on the signal of the Zernike phase contrast sensor. The second part is dedicated to the phasing of segmented deformable mirrors. A new technology of segmented deformable mirrors for adaptive optics made from silicon wafers with bimorph piezoelectric actuation has been proven to work. A demonstrator with three hexagonal segments of 90 mm corner to corner has been built. The morphing capability of the segmented mirror has been studied and validated by simulations and on a test bench. In this paper, we demonstrate with simulations the phasing of the segmented bimorph mirror with the Zernike phase contrast method. Aspects such as phasing in the presence of segment aberrations have been investigated.

New optical phasing sensor technologies have been studied with a test bench experiment, called Active Phasing Experiment, on-sky at the European Southern Observatory Very Large Telescope. One of the sensors was of the Shack-Hartmann type using cylindrical lenslets across the segment borders for the measurement of the phasing errors. With bright stars, the precision of the measurement of piston steps at a single border was better than 9 nm wavefront RMS, and the precision of the closed-loop correction of the piston errors of the segments across the whole mirror was better than 10 nm wavefront RMS. With dimmer stars of magnitude up to 14.5, precisions of the order of 22 nm wavefront RMS were obtained.

Since the invention of the telescope the mirrors in reflecting telescopes have evolved from small pieces of polished speculum to sophisticated, computer-controlled systems. This review describes the major problems with the casting, figuring and support of these mirrors, and how fundamental inventions like depositing a silver layer on glass or electronic devices like computers opened the path to new solutions for fabricating larger mirrors with improved performance.

The Shack-Hartmann Phasing Sensor (SHAPS) has been integrated in the Active Phasing Experiment (APE) at ESO. It is currently under test in the laboratory. The tests on sky are foreseen for the end of 2008, when APE will be mounted at the Nasmyth focus of one of the VLT unit telescopes. SHAPS is based on the Shack-Hartmann principle: the lenslet array is located in a plane which is optically conjugated to the Active Segmented Mirror (ASM) of APE and is composed of two types of microlenses, circular and cylindrical, which give information about the wavefront slope and the piston steps, respectively. This proceeding contains a description of SHAPS and of the algorithms implemented for the wavefront reconstruction and for the phasing. The preliminary results obtained during the laboratory tests are discussed and compared with the theoretical predictions. The performances of SHAPS at the VLT and at the European Extremely Large Telescope (E-ELT) are estimated.

Large telescopes pose a continuous challenge to systems engineering due to their complexity in terms of requirements, operational modes, long duty lifetime, interfaces and number of components. A multitude of decisions must be taken throughout the life cycle of a new system, and a prime means of coping with complexity and uncertainty is using models as one decision aid. The potential of descriptive models based on the OMG Systems Modeling Language (OMG SysML(TM)) is examined in different areas: building a comprehensive model serves as the basis for subsequent activities of soliciting and review for requirements, analysis and design alike. Furthermore a model is an effective communication instrument against misinterpretation pitfalls which are typical of cross disciplinary activities when using natural language only or free-format diagrams. Modeling the essential characteristics of the system, like interfaces, system structure and its behavior, are important system level issues which are addressed. Also shown is how to use a model as an analysis tool to describe the relationships among disturbances, opto-mechanical effects and control decisions and to refine the control use cases. Considerations on the scalability of the model structure and organization, its impact on the development process, the relation to document-centric structures, style and usage guidelines and the required tool chain are presented.

The shape correction of the mirrors is a crucial operation to obtain diffraction limited images in actively controlled telescopes. If the mirror is not monolithic, the segmentation errors introduced by piston, tip and tilt of the segments are superimposed on the continuous aberrations. In the case of a sensor based on the measurement of the wave front slopes, like the Shack-Hartmann wave front sensor, an algorithm which allows separating the different contributions is necessary for a proper correction. In the framework of the Active Phasing Experiment (APE) carried out at ESO, we have developed a simple algorithm which can be applied to compute the aberrations and the tip-tilt coefficients using the information obtained with a Shack-Hartmann sensor. It is based on the construction of an orthogonal base in the space of the wave front slope functions. The description of the algorithm and its performance in the cases of low-order aberrations superimposed on tip-tilt misalignment of the segments are reported. A particular application of this technique in the case of the European Extremely Large Telescope (E-ELT) is discussed and the expected upper limits for the residual errors after correction are estimated.

Integrated models including optics, structures, control systems, and disturbances are important design tools for Extremely Large Telescopes (ELTs). An integrated model has been formulated for the European ELT and it includes telescope structure, main servos, primary mirror segment control system, wind, optics, wavefront sensors, deformable mirror, and an AO reconstructor and controller. There are three model phases: Initialization, execution of a solver to determine time responses, and post-processing. In near future, the model will be applied for performance studies and design trade-offs for the European ELT.

The purpose of the Active Phasing Experiment, designed under the lead of ESO, is to study new phasing technologies and to validate wavefront control concepts for Extremely Large Telescopes. The Active Phasing Experiment is currently tested in the laboratory at the ESO headquarters and will be tested on sky at a Nasmyth focus of a VLT unit telescope at the end of 2008. The test bench contains four different phasing sensors which are tested in parallel to compare them under the same conditions. They have been developed by Istituto Nazionale di Astrofisica in Florenze, Instituto Astrofisica Canarias in Tenerife, Laboratoire d'Astrophysique de Marseille and ESO. It includes also an Active Segmented Mirror which simulates the segmentation of a primary mirror. A non-contact optical metrology has been developed by Fogale Nanotech to control it. The VLT focus and the VLT atmospheric conditions are simulated in the laboratory with a turbulence generator producing a seeing between 0.45 and 0.85 arcsec. Once installed on a VLT unit telescope, the control system of the Active Phasing Experiment will be able to control the phasing of the ASM, but also the guiding and the active optics of the VLT. This proceeding gives a brief summary of the opto-mechanical aspects of the Active Phasing experiment, describes its control system and gives an analysis of the preliminary results obtained in the laboratory.