Isabelle Surdej

OWL phase A status report

Progress in the conceptual design phase of ESOs OWL 100-m optical and near-infrared telescope is reported, with emphasis on the development of the science case. The Phase A opto-mechanical design is now basically completed, and provides a clean, symmetrical geometry of the pupil, with a near-circular outer edge. We also report about the latest outcome of industrial studies, introduce the essential definition of the wavefront control systems, and outline operational concepts and instruments priorities. Finally, we elaborate on the favorable cost factors associated to the telescope design, its compatibility with low industrial risks, and argue that progressive implementation allows for competitive timescales. In particular, we show that suitable fabrication and integration schemes should accommodate for a start of science operation at unequalled potential and within a time frame comparable to that of smaller designs, while at the same time maximizing R&D time for critical subsystems.

On-sky performance of the Zernike phase contrast sensor for the phasing of segmented telescopes

The Zernike phase contrast method is a novel technique to phase the primary mirrors of segmented telescopes. It has been tested on-sky on a unit telescope of the Very Large Telescope with a segmented mirror conjugated to the primary mirror to emulate a segmented telescope. The theoretical background of this sensor and the algorithm used to retrieve the piston, tip, and tilt information are described. The performance of the sensor as a function of parameters such as star magnitude, seeing, and integration time is discussed. The phasing accuracy has always been below 15 nm root mean square wavefront error under normal conditions of operation and the limiting star magnitude achieved on-sky with this sensor is 15.7 in the red, which would be sufficient to phase segmented telescopes in closed-loop during observations.

THE ACTIVE PHASING EXPERIMENT PART I: CONCEPT AND OBJECTIVES

In a framework of ELT design study our group is building an Active Phasing Experiment (APE), the main goals of which is to demonstrate the non-adaptive wavefront control scheme and technology for Extremely Large Telescope (ELT). The experiment includes verification and test of different phasing sensors and integration of a phasing wavefront sensor into a global scheme of segmented telescope active control. After a sufficient number of tests in the laboratory APE will be mounted and tested on sky at a Nasmyth focus of a VLT unit telescope. The paper presents APE as a demonstrator of particular aspects of ELT and provides a general understanding concerning the strategy of segmented mirrors active control.

Design and performances of the Shack-Hartmann sensor within the Active Phasing Experiment

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.

On-sky performances of an optical phasing sensor based on a cylindrical lenslet array for segmented telescopes.

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.

Pattern recognition and signal analysis in a Mach-Zehnder type phasing sensor

The primary mirror of future Extremely Large Telescopes will be composed of hundreds of individual segments. Misalignments in piston and tip-tilt of such segments must be reduced to a small fraction of the observing wavelength in order not to affect the image quality of these telescopes. In the framework of the Active Phasing Experiment carried out at ESO, new phasing techniques based on the concept of pupil plane detection will be tested. The misalignments of the segments produce amplitude variations at locations on a CCD detector corresponding to the locations of the segment edges. The position of the segment edges on a CCD image must first be determined with pixel accuracy in order to localize the signals which can be analyzed in a second phase with a robust signal analysis algorithm. A method to retrieve the locations of the edges and a phasing algorithm to measure the misalignments between the segments with an accuracy of a few nanometers have been developed. This entire phasing procedure will be presented. The performance of the pattern recognition algorithm will be studied as a function of the number of photons, the amplitude of the segment misalignments and their distribution. Finally, the accuracy achieved under conditions similar to the ones met during observation will be discussed.

Fresnel phasing of segmented mirror telescopes.

Shack-Hartmann (S-H) phasing of segmented telescopes is based upon a physical optics generalization of the geometrical optics Shack-Hartmann test, in which each S-H lenslet straddles an intersegment edge. For the extremely large segmented telescopes currently in the design stages, one is led naturally to very large pupil demagnifications for the S-H phasing cameras. This in turn implies rather small Fresnel numbers F for the lenslets; the nominal design for the Thirty Meter Telescope calls for F=0.6. For such small Fresnel numbers, it may be possible to eliminate the lenslets entirely, replacing them with a simple mask containing a sparse array of clear subapertures and thereby also eliminating a number of manufacturing problems and experimental complications associated with lenslets. We present laboratory results that demonstrate the validity of this approach.

Preliminary results obtained with the ZEUS phasing sensor within the APE experiment

In the framework of the Active Phasing Experiment (APE), four different phasing techniques are tested. The ZErnike Unit for Segment phasing sensor (ZEUS) is integrated on the APE bench. APE has been tested in the laboratory before it will be installed on one of the Nasmyth platform of a Very Large Telescope (VLT) Unit Telescope to perform on sky tests. The ZEUS phasing sensor concept has its origins in the Mach-Zehnder interferometer equipped with a spatial filter in its focal plane. In this paper, the ZEUS phasing sensor is described together with its theoretical background and deployment within the APE experiment. The algorithms and its elements used to reconstruct the wavefront are described. Finally, the preliminary results obtained in the laboratory are presented.

Challenges in precision and vibration control for physics experiments

The present paper examines some of the ongoing development projects for new facilities for astronomy, high-energy particle physics and gravitational wave detection, from the viewpoint of precision and vibration control requirements.

Influence of atmospheric turbulence on the Zernike phase contrast method and the first steps towards the phasing ofsegmented deformable mirrors

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.