For many applications, including future giant telescopes, there is increased interest in structural cables. Usually integrated models, i.e. combined models of structures, optics and control systems are used for modeling of telescopes. Performance of structural cables is non-linear, so often a linearization is needed to include cable models in integrated models of telescopes. We present two different linear models describing static and dynamic performance of a cable near an equilibrium operating point. The models are plug-in" models that in a black box form give end point forces as outputs, with end point movements as inputs. Using an example, we compare the two models and we outline the benefits of each of the models. Although we here focus on modeling of telescopes, the techniques are useful also for other applications.
While it is attractive to integrate a deformable mirror (DM) for adaptive optics (AO) into the telescope itself
rather than using relay optics within an instrument, the resulting large DM can be expensive, particularly for
extremely large telescopes. A low-cost approach for building a large DM is to use voice-coil actuators, and rely
on feedback from mechanical sensors to improve the dynamic response of the mirror sufficiently so that it can
be used in a standard AO control system. The use of inexpensive voice-coil actuators results in many lightly-
damped structural resonances within the desired control bandwidth. We present a robust control approach for
this problem, and demonstrate performance in a closed-loop AO simulation, incorporating realistic models of
low-cost actuators and sensors. The first contribution is to demonstrate that high-bandwidth active damping
can be robustly implemented even with non-collocated sensors, by relying on the "acoustic limit" of the structure
where the modal bandwidth exceeds the modal spacing. Next we introduce a novel local control approach, which
significantly improves the high spatial frequency performance relative to collocated position control, but without
the robustness challenges associated with a global control approach. The combination of these "inner" control
loops results in DM command response that is demonstrated to be sufficient for integration within an AO system.
To improve the mechanical characteristics of actively controlled continuous faceplate deformable mirrors in adaptive optics, a strategy for reducing crosstalk between adjacent actuators and for suppressing low-order eigenmodes is proposed. The strategy can be seen as extending Saint-Venant's principle beyond the static case, for small local families of actuators. An analytic model is presented, from which we show the feasibility of the local control. Also, we demonstrate how eigenmodes and eigenfrequencies are affected by mirror parameters, such as thickness, diameter, Young's modulus, Poisson's ratio, and density. This analysis is used to evaluate the design strategy for a large deformable mirror, and how many actuators are needed within a family.
We study a concept for a low-cost, large deformable mirror for an Extremely Large Telescope. The use of inexpensive voice-coil actuators leads to a poorly damped faceplate, with many modes within the desired control bandwidth. A control architecture, including rate and position feedback to add damping and stiness, for the faceplate has been presented in our previous papers. An innovative local control scheme which decouples adjacent actuators and suppresses low-order eigenmodes is a key feature in our controller. Here, we present an integrated
model of a partially illuminated large deformable mirror in an experimental laboratory setup with a limited amount of actuators. From the model, conclusions are drawn regarding the number of actuators needed to identify the key features, such as local control performance, dynamic range, and controllability and robustness of the deformable mirror.
The Carlina hypertelescope is a planned sparse aperture 100 m telescope with pupil densification. The telescope
has a spherical primary with segments located in a valley between mountains, and additional optical elements in a
gondola suspended in eight cables some 100 m above the primary mirror. The resolution is about 1.2×10<sup>-3</sup> arcsec.
It is imperative that the position and attitude of the gondola be maintained within tight tolerances during
observation and star tracking. The present design has servo-controlled winches on the ground for control of
the gondola via the cables. An integrated model of the system, including optics, cables, gondola, position and
attitude control system, and wind disturbances has been set up. The structural and control models are linear.
Calculations in the frequency domain and simulations in the time domain show that the performance of the
telescope with the present design seems adequate for short exposures. However, for long-exposure operation, the
gondola stability should be improved by about two orders of magnitude. Recommendations are given on possible
approaches for performance improvement.
Lund Observatory is presently designing and constructing a robotic telescope dedicated to studies of the Earth's albedo
by measuring the ratio between the intensity of the dark and bright sides of the Moon. The telescope will operate both in
broadband and narrow-band modes over the entire visible wavelength range and will transmit observational results back
to the operation team over the Internet. Design challenges, in particular related to choice of CCD and stray light
suppression, are described, together with the design of the optics, control system, and enclosure. Finally we present
results from laboratory tests. The telescope will go into operation in the first half of 2011.
In this paper we identify and attempt to quantify all effects of Atmospheric Dispersion (AD) which might affect the
performance of a European ELT. For each effect, we examine the implications and possible methods of correction. We
produce "demonstration-of-possibility" ADC designs for the proposed 42-m ("E-ELT"). The rotational (RADC) and
linear ADC (LADC) optical designs are compared at the Nasmyth focus and at the intermediate focal plane.
The Earthshine telescope project is a collaborative effort between Lund Observatory (LO) in Sweden and The Institute of
Meteorology in Demark (DMI) with the purpose of constructing one or more robotic telescopes to record the albedo of
the Earth over a long time. The objective is to measure long-term development of the global cloud coverage and
reflectivity for climate modeling. A 1% change in the Earth's albedo will result in an average temperature change of 0.5
K of the Earth, calling for high precision of the albedo measurements. This poses strict demands on the telescope design,
in particular with respect to suppression of straylight. The paper describes our proposed optical and mechanical design of
the Earthshine telescope, and presents a preliminary straylight analysis of the design.
Planned Extremely Large Telescopes will rely on availability of large Deformable Mirror in the 2-3m class. Design
and construction of such mirrors are challenging and call for powerful simulation tools. We present an evaluation
model which is used to study performance of a large deformable mirror for three actuator topologies.
Back sensors topologies are discussed from the point of view of sensor noise propagation. Two methods for
estimating the deflection at the actuator locations on the basis of sensor signal are presented and compared
regarding the computational power needed.
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.
When performing adaptive optics (AO) correction for the effect of atmospheric fluctuations, different effects, related to
measuring the wavefront at one wavelength and observing at another one, may show up. Some of these effects are
related to the dispersive properties of the atmosphere. Others are related to diffraction, both regarding light propagation
through the turbulent atmosphere to the entrance pupil of the telescope and regarding the inevitable Fraunhofer
diffraction taking place for light propagating from the exit pupil to the image plane of the telescope. In this paper some
of these effects are revised and discussed, in particular the way in which uncorrected wavefront errors in the point spread
function (PSF) will scale with color, and the way in which dispersion affects the Strehl ratio and the background level of
the AO corrected PSF. Also discussed is the trade-off between AO spectral bandwidth and Poisson noise.
Large, high-bandwidth deformable mirrors (DMs) with thousands of actuators for adaptive optics are of high interest for existing large telescopes and indispensable for construction of efficient future extremely large telescopes. Different actuation and sensing principles are possible. We propose a novel concept using commercially available voice coil actuators attached to the back of the mirror with suction cups and using LVDT sensors on the actuators for local stabilization. Also, a new low-cost sensor for easy measurement of DM displacement or velocity has been developed. It has a sensitivity better than 20 nm and a bandwidth wider than 20 to 1000 Hz. Finally, studies are in progress of global, hierarchical mirror form controllers based on many parallel multiple-input, multiple-output regulators of low order.
Atmospheric dispersion represents a relatively overlooked problem in connection with the ultimate quality of ELT images corrected by adaptive optics (AO). The aim of this paper is to evaluate the contribution from atmospheric dispersion to the background level of the point-spread function (PSF). Since proper suppression of this level is important for the prospects for direct exo-planet observation, it is necessary to quantify the contributions from all possible sources to it. Atmospheric dispersion will in principle result in three different kinds of contributions. The first one is related to the fact that two rays of different color following the same path through the atmosphere to the telescope do not have the same optical path-length difference (OPD). The second one is related to the fact that two coinciding rays of different color entering the atmosphere at a non zero zenith angle will be separated due to refraction before they reach the telescope. The third one is related to the fact that rays are diffracted by inhomogeneities in the atmosphere and that the diffraction angle is dependent on color. This last effect is small and will not be treated here. As a consequence of dispersion phase fluctuations can, in principle, only be compensated at a single wavelength by AO systems with deformable mirrors (DMs). Hence looking for an exo-planet in a certain spectral bandwidth there will be a contribution from the parent star uncorrected background level. Hence it will be crucial to perform observations in a narrow spectral bandwidth and to ensure that the wavefront measurements used for AO correction are performed within the same narrow bandwidth. The last point affects the needed magnitude of the parent star, which is used for wavefront measurements.
The current status of a real-time real-sky dual-conjugate adaptive optics experiment is presented. This experiment is a follow-up on a lab experiment at Lund Observatory that demonstrated dual-conjugate adaptive optics on a static atmosphere.
The setup is to be placed at Lund Observatory. This means that the setup will be available 24h a day and does not have to share time with other instruments. The optical design of the experiment is finalized. A siderostat will be used to track the guide object and all other optical components are placed on an optical table. A small telescope, 35 cm aperture, is used and following this a tip-tilt mirror and two deformable mirrors are placed. The wave-front sensor is a Shack-Hartmann sensor using a SciMeasure Li'l Joe CCD39 camera system.
The maximum update rate of the setup will be 0.5 kHz and the control system will be running under Linux. The effective wavelength will be 750 nm. All components in the setup have been acquired and the completion of the setup is underway. Collaborating partners in this project are the Applied Optics Group at National University of Ireland, Galway and the Swedish Defense Research Agency.
For extremely large telescopes, there is strong need for thin deformable mirrors in the 3-4 m class. So far, feasibility of such mirrors has not been demonstrated. Extrapolation from existing techniques suggests that the mirrors could be highly expensive. We give a progress report on a study of an approach for construction of large deformable mirrors with a moderate cost. We have developed low-cost actuators and deflection sensors that can absorb mounting tolerances in the millimeter range, and we have tested prototypes in the laboratory. Studies of control laws for mirrors with thousands of sensors and actuators are in good progress and simulations have been carried out. Manufacturing of thin, glass mirror blanks is being studied and first prototypes have been produced by a slumping technique. Development of polishing procedures for thin mirrors is in progress.
The Euro50 is a proposed 50m extremely large telescope for optical and infrared wavelengths. To study and predict the performance of the complete telescope system, an integrated model combining the structural model of the telescope, optics models, the control systems and the adaptive optics has been established. Wind and atmospheric disturbances are also included in the model. The model is written in MATLAB and C. It is general and modular and built around dedicated ordinary differential equation solvers. The difference in time constants between subsystems is exploited to speed up calculations. The solvers can handle discontinuities and subsystem mode changes. The high degree of modularity allows different telescope designs to be modelled by rearranging subsystem blocks. Certain subsystems, for instance adaptive optics, can also run in a standalone fashion. Parts of the model are parallelized for execution on a large shared memory machine. The resulting architecture of the integrated model and sample results using the code for different telescope models are presented.
With Euro50 as a convenient telescope laboratory, the Euro50 team has continued development aiming at a European
extremely large telescope (ELT). Here, we give a progress report. The needs of science and instrumentation are briefly
discussed as is the importance of photometric stability and precision. Results are reported from work on integrated
modelling. Details are given concerning point-spread functions (PSFs) obtained with and without adaptive optics (AO).
Our results are rather encouraging concerning AO photometry and compensation of edge sensor noise as well as
regarding seeing-limited ELT operation. The current status of our development of large deformable mirrors is shown.
Low-cost actuators and deflection sensors have been developed as have hierarchic control algorithms. Fabrication of
large thin mirror blanks as well as polishing and handling of thin mirrors has been studied experimentally. Regarding
adaptive optics, we discuss differential refraction and the limitations imposed by dispersive optical path differences
(OPDs) and dispersive anisoplanatism. We report on progress in laser guide star (LGS) performance and a real-time online experiment in multi-conjugate AO (MCAO). We discuss ELTs, high-resolution spectroscopy and pupil slicing with
and without use of AO. Finally, we present some recent studies of ELT enclosure options.
The Euro50 is a proposed 50m extremely large telescope for optical and infrared wavelengths. To study and predict the performance of the complete telescope system, an integrated model combining the structural model of the telescope, optics models, the control systems and the adaptive optics has been established. Wind and atmospheric disturbances are also included in the model. The integrated model is written in MATLAB and C. To satisfy memory demands and to achieve acceptable execution times, 64-bit MATLAB is used and part of the model is run on a shared memory machine using OpenMP. We present results from simulations with a complete integrated single conjugate adaptive optics model. Various sensor and actuator geometries are evaluated. A comparison of wind loading and atmospheric turbulence effects is also presented. The model shows that the telescope will be essentially seeing limited under wind load and no AO correction.
The Euro50 is an extremely large telescope for optical and infrared wavelength with a 50 m primary mirror. It has a segmented, aspherical primary mirror and an aspherical, deformable secondary in a Gregorian layout. A tentative conceptual design exists and has been documented in a study report. Recent activities have concentrated on the science case for extremely large telescopes in the 50 m class and on identification of potential technical "show stoppers". The science case investigation has identified four fields of particular interest. The studies of critical technical issues have concentrated on atmospheric dispersion effects for high-resolution adaptive optics for extremely large telescopes, and on the influence of wind and other disturbances on wavefront control. Wind load on the telescope, the primary mirror and the enclosure has been studied using wind tunnel measurements and computational fluid dynamics. The impact of wind on the total system has been investigated using an integrated model that includes the telescope structure, the primary mirror segment alignment system, the secondary mirror alignment system, and single conjugate adaptive optics using the deformable secondary mirror. The first, tentative results show that wind disturbances may be significant and that the task of correcting for wind residuals may be at least as large for the adaptive optics system as that of correcting for atmospheric aberrations. The results suggest that use of extremely large telescopes for observations of earth-like planets around nearby stars may imply a considerable challenge.
Previously the effect of atmospheric dispersion on telescope performance has attracted only relatively little attention. This may be due to the fact that the dispersion effects have been evaluated in relation to the size of the diffraction limited resolution angle of current telescopes, or to seeing limited telescopes. However, since the resolution angle is inversely proportional to telescope diameter, dispersion and dispersion compensation becomes increasingly important for extremely large telescopes (ELTs). In this paper we present a simple model for the dispersion effects in telescopes with adaptive optics (AO). The model addresses the expected loss in Strehl ratio when the atmospheric wavefront error is measured at a wavelength different from the wavelength of observation. Also, the bandwidth over which the correction will be of a given quality is evaluated. Related to AO performance, the consequence of using laser guide stars (LGSs) for probing the atmosphere may be that the measured wavefront error must be rescaled to the wavelength of observation. This places special demands on the AO control loop. Since linear atmospheric dispersion compensation need not cover a larger bandwidth than the AO compensation, an atmospheric dispersion compensator (ADC) can be designed for narrow band operation. As an example of the benefits to be obtained from this, we briefly present the proposed ADC for the Euro50.
The Euro50 is an astronomical extremely large telescope for optical and infrared wavelength with a 50 m primary mirror. The telescope will have an elaborate control system ("live optics") to correct for atmospheric and telescope aberrations. To study and predict performance of the complete telescope system, an integrated model combining the structural model of the telescope, optics models, the control systems, and the adaptive optics has been established. Wind is taken into account on the basis of wind tunnel measurements and computer fluid dynamics calculations. Atmospheric aberrations are included using a seven-layer atmosphere model. The integrated model is written in Matlab and is run on a cluster computer to achieve acceptable execution times. Dedicated ordinary differential equation solvers have been written and a special toolkit for communication between Matlab processes on different nodes of the cluster computer has been set up. Preliminary results from the complete integrated model, including adaptive optics, are shown.
To obtain full sky coverage, astronomical adaptive optics systems require Na Sodium Beacons (SBs) (also referred to as Laser Guide Stars or LGSs) located at heights extending from 85 to 100 km. When viewed at the edge of large telescopes these SBs appear elongated. For the Euro50 50 meter aperture telecopes this elongation amounts to 6 to 9 arcseconds when the laser is launched from a point on the telescope axis. This is substantially larger than the -0.6 arcsec FWHM SB when viewed near the telescope center. This so-called "perspective elongation" substantially decreases the sensitivity of SB aided adaptive optics. We describe a way of removing this elongation when using pulsed lasers. It uses rapid (microsecond) refocusing of the telescope with the aid of birefringent lenses and polarization modulators. We present an outline of the SB wavefront sensor for the Euro50.
The Euro50 is a telescope for optical and infrared wavelengths. It has an aspherical primary mirror with a size of 50 meters and 618 segments. The optical configuration is of Gregorian type and the secondary mirror is deformable for adaptive optics. Observations can take place in prime focus, Gregorian foci, and Nasmyth foci using additional relay mirrors. The telescope provides seeing limited observations, partial adaptive optics with ground layer correction, single conjugate adaptive optics and dual-conjugate adaptive optics. For prime focus observations, a clam-shell corrector with a doublet lens is used. The primary mirror segments can be polished using the precessions polishing technique. "Live Optics" denotes the joint segment alignment system, secondary mirror control system, adaptive optics and main axes servos. An overview is given of the live optics architecture, including feedback from wavefront sensors for natural and laser guide stars, and from primary mirror segment edge sensors. A straw man concept of the laser guide star system using sum-frequency YAG lasers is presented together with a solution to the laser guide star perspective elongation problem. The structural design involves a large steel structure and a tripod of carbon fiber reinforced polymer to support the secondary mirror. Integrated models have been set up to simulate telescope performance. Results show that an enclosure is needed to protect the telescope against wind during observations. The enclosure is very large box-shaped steel structure.
This laboratory demonstrator setup is a downscaled version of a 7.5-m aperture telescope with dual-conjugate adaptive optics, a representative static atmosphere in the K-band and five natural guide stars in a cross. The demonstrator has been used to evaluate different modes of adaptive optics; conventional single-conjugate adaptive optics, field-averaged single-conjugate adaptive optics and dual-conjugate adaptive optics.
This paper presents an analysis of the point-spread functions (PSFs), which may be expected in the so-called improved seeing limited mode for the Euro50 (Extremely large telescope with a 50 m aperture). This mode comprises adaptive control of the deformable secondary mirror of the aplanatic Gregorian telescope configuration using guide stars in a wide field. I may result in efficient compensation of turbulence-induced wavefront distortion near the ground layer. The prime goal of the investigations has been to evaluate the performance, in particular the full width at half maximum (FWHM) of the K-band PSFs, as a function of both the guide star field size and the outer scale of the atmospheric fluctuations. For this reason we have adopted a very simple guide star configuration consisting of a circular ring of homogeneously distributed stars. The PSFs are analyzed in the center of the ring. The atmosphere is implemented as the nine layer extended ORM atmosphere described in the paper. One result of the investigations is that the FWHM of the PSFs is very sesitive to the size of the outer scale. For a layer-common outer scale of 20 m, that is slightly less than half of the telescope diameter, the FWHM will stay close to the diffraction limit even for a guide star ring diameter of 10'. This makes the improved seeing limited mode very well suited for a fiber-fed spectrograph located at the F/5 Nasmyth focus of the Euro50.
The Euro50 is a proposed 50 m optical and infrared telescope. It will have thousands of control loops to keep the optics aligned under influence of wind, gravity and thermal loads. Cross-disciplinary integrated modeling is used to study the overall performance of the Euro50. A sub-model of the mechanical structure originates from finite element modeling. The optical performance is determined using ray tracing, both non-linear and linearized. The primary mirror segment alignment control system is modeled with the 618 segments taken as rigid bodies. Adaptive optics is included using a layered model of the atmosphere and sub-models of the wavefront sensor, reconstructor and controller. The deformable mirror is, so far, described by a simple influence function and a second order dynamical transfer function but more detailed work is in progress. The model has been implemented using Matlab/Simulink on individual computers but it will shortly be implemented on a Beowulf cluster within a trusted network. Communication routines between Matlab on the cluster processors have been written and are being benchmarked. Representative results from the simulations are shown.
To obtain full sky coverage, astronomical adaptive optics systems require Na Sodium Beacons (SBs) (also referred to as Laser Guide Stars or LGSs) located at heights extending from 85 to 100 km. When viewed at the edge of large telescopes these SBs appear elongated. For the Euro50 50 meter aperture telescopes this elongation amounts to 6 to 9 arcseconds when the laser is launched from a point on the telescope axis. This is substantially larger than the ~0.6 arcsec FWHM SB when viewed near the telescope center. This so-called "perspective elongation" substantially decreases the sensitivity of SB aided adaptive optics. We describe a way of removing this elongation when using pulsed lasers. It uses rapid (microsecond) refocusing of the telescope with the aid of birefringent lenses and polarization modulators. We present an outline of the SB wavefront sensor for the Euro50.
Modeling the performance of future ground-based extremely large telescopes with the AO system requires an atmospheric model and a corrective algorithm. The atmospheric model employed here is the ORM 7 layer model with an outer scale distribution. Using an analytical algorithm for atmospheric correction the performance of the Euro50 telescope is analyzed. The influence of a distributed outer scale on the strokes of the adaptive mirrors is investigated. It is concluded that for modeling the AO system performance the account for variations of outer scale with altitude is of not great importance, and a simplified atmospheric model with the effective outer scale common for all layers may be used. The knowledge of effective outer scale is sufficient for setting requirements on the mirror strokes.
The scheme presently envisaged for the EURO50 adaptive optics is presented. The Euro50 adaptive optics will primarily work with laser guide stars (LGSs) and control of either one or two deformable mirrors (SCAO and DCAO respectively), but operation using a natural guide star (NGS) is also foreseen. The point spread function (PSF) for SCAO operation using a single NGS is evaluated. An algorithm for optimal control of the deformable mirrors (DMs) using LGSs and Shack-Hartman wavefront sensors is presented and commented upon. It is an extension of a recently developed algorithm for optimal control using NGSs and working in the spatial Fourier domain. In addition the concept of a virtual wavefront sensor is introduced to overcome the difficulty in transmitting a large number (37) of LGSs to the final DCAO focus with both adequate field and adequate aberrations. The expected performance is estimated in form of maps of the Strehl ratio versus field angle using a standard seven layer atmospheric model for the Observatorio del Roque de los Muchachos (ORM) site on la Palma for the case of the outer scale being either 20 m (nominal for ORM) or infinity (Kolmogorov - most pessimistic case).
The optical design for the proposed Euro50 extremely large telescope with integrated adaptive optics (AO) is presented. For atmospheric turbulence correction, we propose using single and dual-conjugate AO systems working with natural and laser guide stars. The corrective shape of the deformable mirrors (DMs) is derived from an analytical algorithm based on minimization of the sum of the residual power spectra of the phase fluctuations seen by guide stars after correction. Predictions for performance of the Euro50 ELT with Dual-conjugate AO are given for the K band using a seven layer atmospheric model for the atmosphere at the Observatorio del Roque de los Muchachos (ORM) on La Palma. The average Strehl ratio is used to quantify the system performance for different values of actuator pitch and DM conjugation altitudes. The influence of the outer scale and telescope pointing on the RMS stroke of the DMs is presented. It is concluded that construction of such a system is feasible and that there is a need for development of a simulation tool to verify the analytical calculations. Precise knowledge of the outer scale of the atmosphere at the ORM is needed to establish the dynamical range of the mirrors.
Euro50 is a proposed optical telescope with an equivalent primary mirror diameter of 50 m. Partners of the collaboration are institutes in Sweden, Spain, Ireland, Finland, and the UK. The telescope will have a segmented primary mirror and an aplanatic Gregorian configuration with two elliptical mirrors. For a 50 m telescope there would be no economical advantage in going to a spherical primary. The size of the primary mirror segments (2 m) has been selected on the basis of a minimization of cost. An adaptive optics system will be integrated into the telescope. The telescope will have three operational modes: Seeing limited observations, single conjugate adaptive observations in the K-band, and dual conjugate observations also in the K-band. An upgrade to adaptive optics also in the visible down to 500 nm is foreseen. There will be an enclosure to protect the telescope against adverse weather and wind disturbances. Integrated simulation models are under development. The project time will be 10 years and the cost some 591 MEuros.
The Euro50 is a proposed optical telescope with an equivalent aperture of 50 m. It will have a segmented primary mirror and full adaptive optics. To study the interaction of the telescope structure, the control system and the optics, an integrated simulation model has been formulated. The mechanical model is a modal version of an Ansys finite element model. The optics model is based on ray tracing and physical optics. The segments model takes the alignment servos and the segment dynamics into account. Wind variation over the primary mirror is included. Segment control system modeling is in progress. First results clearly demonstrate that a good enclosure is needed to protect the telescope well against wind. The results also suggest that the segment alignment system must have a bandwidth well above the lowest eigenfrequencies of the telescope.
The Swedish ELT is intended to be a 50 m telescope with multiconjugate adaptive optics integrated directly as a crucial part of the optical design. In this paper we discuss the effects of the distributed atmospheric turbulence with regard to the choice of optimal geometry of the telescope. Originally the basic system was foreseen to be a Gregorian with an adaptive secondary correcting adequately for nearby turbulences in both the infrared and visual regions, but if the performance degradation expected from changing the basic system to a Cassegrain keeping the adaptive secondary could be accepted, the constructional costs would be significantly reduced. In order to clarify this question, a simple analytical model describing the performance employing a single deformable mirror for adaptive correction has been developed and used for analysis. The quantitative results shown here relates to a wavelength of 2.2 micrometers and are based on the seven layer atmospheric model for the Cerro Pachon site, which is believed to be a good representative of most good astronomical sites. As a consequence of the analysis no performance degradation is expected from changing the core telescope to a Cassegrain (Ritchey- Chretien). The paper presents the layout and optical performance of the new design.
For a very large telescope for optical and adjacent wavelengths, we have studied a number of parameters that influence choice of aperture size. These are defined by scientific drivers, aspects of segmentation, f-ratio considerations, provisions for interferometry, and structural limitations as well as cost. Alignment systems and adaptive optics operation have been studied. Primary mirror layout and provisions for interferometric operation are commented. Cost estimates have been made based on a number of selected parameters. We conclude that a range of 25 - 50 m seems most interesting for the diameter of the primary mirror. Tentatively, we opt for a 40 m telescope with fully adaptive operation and with optional non-adaptive wide-field operation. The primary mirror is composed of 2 m super-segments, divided into 15 cm adaptive segments.
A 25 m four mirror live optics telescope is studied. M1 is spherical with 141 segments and f/0.96. M1 is re-imaged onto M4, also with 141 segments. Image FWHM is less than 0.10 arc sec over greater than 20 arc min. A horseshoe solution with a simple azimuth platform is applied. M1 segments are supported by a fine meniscus form truss structure, tied to the horseshoes by a coarser mesh. A FEM with 10<SUP>4</SUP> dof was developed and applied. Live optics control M1 and M4 segments (the latter with potentially high bandwidth), M1/M4 segment balancing and servos. Correction signals in tilt, coma and defocus are traced. A correlation tracker and a laser guide star system are included. Low and high wind speed regimes are studied. An end-to-end simulation model is developed, based on modal representation of our FEM. Image quality dependence on wind load is studied from segment piston and tilt deflections. Eigenmodes are recorded. Using wind time series, we study dynamic effects and image quality resulting from the 141 segment spots. Automatic segment control at a bandwidth of only 1 Hz gives excellent image quality. We foresee to reach a bandwidth greater than 50 Hz, securing a system partly adaptive, with effects of atmospheric wave front tilt removed through M4 segment tilting at high frequency. Further progress include optimization of mechanical design and end-to-end simulation model, wind tunnel testing and studies of wave front sensor, correlation tracker and instruments. A fully adaptive system is tentatively studied as is coherent operation at IR wavelengths.
The basic design and an analysis of the performance possibilities of a 25 m class optical telescope are presented here. The configuration consists of a 28 m segmented spherical primary M1 followed by three highly aspherical corrective mirrors M2, M3 and M4 which also deviates from Cartesian shape. The construction is axially folded. The combination M1- M2 forms a focus close to a coupling aperture in M4, and the combination M3-M4 relays this focus to the final focus behind M1 and M3. The combination M2-M3 images M1 on the segmented M4 to be used for correction of wavefront errors induced by M1 form errors arising from gravitational sag and windbuffeting. Several types of aspherical figuring of M2, M3 and M4 all resulting in a field performance better than characterized by a rms spot radius smaller than 0.1 arcseconds within a full FOV of 21 arcminutes are presented.
Results from the design of a 28 m optical telescope with a spherical segmented primary are presented. The telescope is a four mirror configuration reimaging M1 on M4. The wish for a small and compact structure resulting in a need for controlling high order aspherical mirror coefficients has initiated development of a design procedure satisfying Fermat principle and Abbes Sine condition. Thus the only remaining point aberration will be astigmatism. For a given shape of M4, the design procedure delivers the Taylor expansion coefficients for the shapes of M3 and M4 to be directly used for optical analysis by software capable of handling the needed number of coefficients.
A study of a 25 m class telescope is presented. The scientific case is described together with imaging and spectroscopy aspects. Spectroscopy is found possible also at high resolution with the telescope proposed. Light efficiency and mirror coating are discussed. The optical design and corresponding performance requirements are presented. With a spherical segmented f/0.9 primary mirror and an exit focal ratio of f/2.86, an on-axis four mirror system with segmented primary and quaternary mirrors is found optimal. It gives an image quality of 0.27 arcsec FWHM over a field of 90 arcsec. Mechanical design is based on a tripod configuration similar to those of radio telescopes. The alignment system proposed is discussed. Total alignment is divided into three main sub-tasks. First, low frequency alignment is established using a slow wavefront sensor. Second, a high frequency alignment is maintained with an internal laser measurement system. Third, a high frequency correction for wavefront tilt errors is made with a correlation tracker. The enclosure is co-rotating with two sections sliding apart for observations. It has adjustable wind screens and double skin panels with internal air circulation. Control facilities are installed in a thermal jacket and the observing floor is cooled.
A feasibility study of a 25 m class telescope for optical wavelengths is presented. A short summary of the scientific background is given. A possible optical design is presented and discussed. Technical and engineering aspects are detailed. Tentative solutions are proposed for manufacture of mirrors and mirror segments. The suitability of metal mirrors is discussed. Comments are given on procedures for optical testing. Details on the mechanical design of the telescope are given. A first tentative proposal for the enclosure is presented. The control system is briefly discussed. Brief comments are given on the maintenance of the telescope. Finally, a tentative implementation plan is presented, including budget and time schedule.
A possible scheme for performing vibrational phase and amplitude mapping using pulsed interferometry is presented. Both the case of electronic speckle pattern interferometry (ESPI) and of conventional holographic recording (HI) are treated. The proposed methods utilize the possibility of performing optical phase shifting, and the number of required interferograms are kept at a minimum.
This paper deals with the problems encountered when unwrapping a noisy phase map obtained in connection with phase shifting speckle interferometry. Due to decorrelation of the speckle patterns obtained before and after loading the object, these phase maps are very noisy and unwrapping requires smoothing. Traditional procedures based upon averaging or median filtering in a window have some disadvantages. Averaging tends to smooth the jumps to be identified in the phase map and median filtering is very slow. A new fast-edge preserving approach is presented and compared to some traditional methods by means of evaluating their performance when filtering and unwrapping a computer-generated noisy sawtooth pattern.