Philippe Dierickx

Future of filled aperture telescopes: is a 100-m feasible?

We explore the scientific case and the conceptual feasibility of giant filled aperture telescopes, in the light of science goals needing an order of magnitude increase in aperture size, and investigate the requirements (and challenges) these imply for possible technical options in the case of a 100 m telescope. The 100-m f/6.4 telescope optical concept is of a four mirror design with segmented, spherical primary and secondary mirrors, and 8-m class aspheric tertiary and quaternary mirrors, providing a 3 arc minutes field of view. Building on the experience of the VLT and other large telescope projects, we investigate mirror fabrication issues, a possible mechanical solution, the requirements for the absolutely essential adaptive optics system and for the instrumentation package, and the implications for budget and schedule.

Analytical study of diffraction effects in extremely large segmented telescopes

We present an analysis of the diffraction effects from a segmented aperture with a very large number of segments-prototype of the next generation of extremely large telescopes. This analysis is based on the point-spread-function analytical calculation for Keck-type hexagonal segmentation geometry. We concentrate on the effects that lead to the appearance of speckles and/or a regular pattern of diffraction peaks. These effects are related to random piston and tip-tilt errors on each segment, gaps between segments, and segment edge distortion. We deliver formulas and the typical numerical values for the Strehl ratio, the relative intensity of higher-order diffraction peaks, and the averaged intensity of speckles associated with each particular case of segmentation error.

Mach–Zehnder interferometer for piston and tip–tilt sensing in segmented telescopes: theory and analytical treatment

A study is presented of a Mach-Zehnder interferometer for the measurement of phasing errors of the type found in segmented telescopes. We show that with a pinhole much larger than the Airy disk and an optical path difference between the arms equal to a quarter of the wavelength, the interferometric signal is related to the second derivative of the wave front. In this condition the signal is produced mostly by the segmentation errors and is marginally sensitive to other aberrations including atmospheric turbulence. The signal has distinguishable symmetric and antisymmetric properties that are related to segment aberrations. We suggest using the antisymmetric component of the signal to retrieve piston, tip, and tilt. The symmetric component of the signal serves as an estimate of the measurement error. In this way we proceed with a study of the errors associated with the misalignment of the interferometer, the segment edge imperfections, and the nonaveraged atmospheric perturbations. The entire study is performed on a theoretical basis, and numerical simulations are used to cross check the analytical results.

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.

Progress of the OWL 100-m telescope conceptual design

The European Southern Observatory is developing a concept of ground-based, 100-m class optical telescope, with segmented primary and secondary mirrors, integrated active optics and multi-conjugate adaptive optics capabilities. Preliminary analysis have confirmed feasibility of the major telescope components within a cost on the order of 1,000 million Euros and within a competitive time frame. The modular design allows progressive transition between integration and science operation, and the telescope would be able to deliver full resolution and unequalled collecting power 11 to 12 years after project funding. The concept owes much of its design characteristics to features of existing telescopes, namely the Hobby-Eberly for optical fabrication, the Keck for optical segmentation, and the VLT for active optics control. The only critical area in terms of needed development seems to be multi-conjugate adaptive optics, but its principles have recently been confirmed experimentally and rapid progress in the underlying technologies is taking place and benefits from consumer applications. Further studies are progressing, confirming initial estimates, and a baseline design is taking shape. The primary objective of those studies is to demonstrate feasibility within proven technologies, but provisions are made for likely technological progress allowing either cost reduction or performance improvement, or both.

The eye of the beholder: designing the OWL

Preliminary requirements and possible technological solutions for the next generation of ground-based optical telescopes were laid down at ESO in 1998. Since then, a phase A study has been commissioned, the objective of which is to produce a conceptual design compatible, to the maximum possible extent, with proven technology, and establish realistic plans for detailed design, site selection, construction and operation for a 100-m class optical, diffraction-limited telescope. There was no doubt about how daunting such a challenge would be, but, somewhat surprisingly, it turns out to be firmly confined to adaptive optics concepts and technologies. The telescope itself appears to be feasible within the allocated budget and without reliance on exotic assumptions. Fabrication of key subsystems is fully within the reach of a properly engineered, industrialized process. A consolidated baseline is taking shape, and alternative system and subsystem solutions are being explored, strengthening the confidence that requirements could be met. Extensive development of wavefront measurement techniques enlarges the palette of solutions available for active wavefront control of a segmented, active telescope. At system level, ESO is developing enabling experiments to validate multi-conjugate adaptive optics (MAD for Multi-conjugate Adaptive optics Demonstrator) and telescope wavefront control (APE, for Active Phasing Experiment).

APE: a breadboard to evaluate new phasing technologies for a future European Giant Optical Telescope

The point spread function of a segmented aperture is seriously affected by the misalignment of the segments. Stringent requirements apply to position sensors and their calibration. The Active Phasing Experiment (APE) will be a technical instrument aimed at testing possible phasing techniques for a European Giant Optical Telescope (EGOT) in a representative environment. It will also integrate simultaneous control of segmented and monolithic, active surfaces. A mirror composed of 61 hexagonal segments is conjugated to the primary mirror of the VLT. Each segment can be moved in piston, tip and tilt and can be controlled in open or closed loop. Three new types of Phasing Wave Front Sensors dedicated to the measurement of segmentation errors will be tested, evaluated and compared: a modified Mach-Zehnder sensor developed by the LAM and ESO, a Pyramid Sensor developed by Arcetri, and a Curvature Sensor developed by IAC. A reference metrology developed by FOGALE will be added to measure directly the deformation of the segmented mirror and check the efficiency of the tested wavefront sensors. This metrology will be based on a synthetic wavelength instantaneous phase stepping method. This experiment will first run in the laboratory with point-like polychromatic sources and a turbulence generator. In a second step, it will be mounted at a Nasmyth focus of a VLT unit telescope. These activities are included in a proposal to the European Commission for funding within Framework Program 6.

Progress of ESO's 100-m OWL optical telescope design

Even as a number of 8- to 10-m class telescopes come into operation worldwide, the scientific challenges these instruments and their space-based counterparts already address imply that future increases in light-gathering power and resolution will have to exceed conventional scaling factors. Indeed, it can be expected that the same progress in telescope diameter and resolution achieved throughout the century must now be realized within, at most, one or two decades. The technologies required to assert the validity of such an extrapolation appear to be within reach. Large telescopes successfully comissioned within the last decade have demonstrated key technologies such as active optics and segmentation. Furthermore, current design methods and fabrication processes imply that the technological challenge of constructing telescopes up to the 100-m range could, in some critical areas, be lower than those underlying, two decades ago, the design and construction of 8 to 10-m class telescopes. At system level, however, such giants are no size-extrapolated fusion of VLT and Keck, but fully integrated adaptive systems. In this paper we elaborate on some of the science drivers behind the OWL concept of a 100-m telescope with integrated adaptive optics capability. We identify major conceptual differences with classical, non-adaptive telescopes, and derive design drivers accordingly. We also discuss critical system and fabrication aspects, and the possible timeline for the concept to be realized.

Mach Zender wavefront sensor for phasing of segmented telescopes

Segmented mirror technology has been successfully applied to 10m class telescopes (Keck, HET, GTC) and is widely recognized as mandatory for Extremely Large Telescopes. For optimal performance the wavefront error associated with segmentation should remain within conservative limits, typically 1/20th of a wave. Several phasing techniques and associated metrologies are under development, with a view to extrapolate such methods to the 100-m OWL telescope. We investigate a novel technique based on Mach-Zehnder interferometry, whereby the wavefront in one of the interferometer arms is spatially filtered so as to provide a reference wave, prior to having the two arms recombined to produce suitable interferograms. We introduce a theoretical description of the interferometer, as well as results of simulations, showing that with proper settings of the interferometers parameters, the technique can be made insensitive to atmospheric turbulence and, more generally, to almost any error source not associated with the segmentation. It also appears that, in a telescope that would include more than one segmented mirror, simple processing allows to disentangle the signal associated to each of them. Finally, we outline the development still required to complete a full qualification of this approach.

E-ELT update of project and effect of change to 39m design

During the last year a modified baseline design for the E-ELT has been developed. The aim of this revision was both to achieve a significant cost saving and to reduce risk on major items. The primary mirror diameter was slightly reduced to 39 m and the total height of the telescope also decreased accordingly. This paper describes the work performed by ESO and a variety of contractors to review the EELT design to match the modified baseline. Detailed design and construction planning, as well as detailed cost estimates were updated for the 39-metre baseline design. In June 2011, ESO Council formally endorsed this modified design as the E-ELT revised baseline. The design drivers and balancing cost factors will be described along with the risk reduction measures taken during this phase. This will culminate in the design which has been agreed as being ready to move forward to construction once approval from ESO Council has been achieved.