A study is presented of the impact on science data from extremely large telescopes of a transformation of wavelength
base for optimization of actuator architecture from 2 200 to 1 250 nm. From the optical path difference (OPD) data for
Euro50, we transform to E-ELT OPDs. We compute the corresponding power spectrum, in which we simulate a higher
actuator density via high-pass filtering to convert from K to J band actuator-pitch optimization. From the modified
power spectrum we derive the correspondingly modified OPDs, PSFs and Strehl ratios. A massive improvement is
demonstrated resulting from converting from AO@K to AO@J. This result is followed up by model-based E-ELT
imaging in a field in a galactic disc at a distance of 4 Mpc. The improvements in image quality, background and limiting
magnitude are very large as are the increases in photometric precision derived from the field imaging. Further, the great
science benefit and large opportunities provided by partial AO is demonstrated. In conclusion, while admittedly
challenging, pushing AO optimization to wavelengths as short as possible is of prime concern for the science output of
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.
A number of Extremely Large Telescopes for visual-infrared and adjacent wavelengths are in various degrees
of progress. All have primary mirrors with equivalent diameters larger than 20 m and are intended for operation with
adaptive optics systems. We discuss several ELT observing parameters as functions of wavelength. Stellar energy
distributions and atomic line spectra are inspected as are the transmission of the Earth's atmosphere, the emissivity of the
sky and telescope and instruments as well as detector sensitivity, resolution and signal-to-noise ratio. The spatial
resolution depending on the size of the diffraction limited adaptive optics point spread function is discussed. We have
evaluated the ELT efficiency in terms of Johnson V to N band photometry, simulating diffraction-limited ELT images of
a stellar field at 4 Mpc and 4 kpc, respectively. We conclude that the information content at shorter wavelengths is of
dominant nature and that there is every reason to do the utmost to include shorter wavelengths in the AO regime. We
propose to adopt a short-wavelength goal of 1 000 nm for first light AO with later updates reaching down to visual
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.
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.
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.
Designs for Extremely Large Telescopes (ELTs) are quite well advanced, but the requirements of instruments have had limited impact. Since provision of a suitable environment for instruments is a critical aspect of all telescopes, we outline some well-known and some less-appreciated challenges of designing instruments for ELTs. A wide-field spectrometer (WFSPEC) with ~10 arcmin field-of-view, probably with AO correction of ground-layer seeing, illustrates the well-known difficulty of matching modern detector pixels to large (~0."3) images. The challenges of exploiting wide-field (1'-2' FOV) high-performance AO systems on ELTs are illustrated by a Multi-Object Multi-field Spectrometer and Imager (MOMSI), which provides imaging and integral-field spectroscopy, at near-diffraction-limited pixel scales, of targets in approximately 300 subfields each. This instrument, roughly equivalent to all the astronomical spectrometers yet built, extracts ~200 times less of the available information from the ELT's FOV than near-future instruments on 8-m class telescopes will do for their hosts. We emphasise the great size of such instruments (40-100 tonnes, 100-200 m3) and the need to accommodate this size in telescope plans. A third area of challenge is the exploitation of the potential capabilities of ELTs in the mid-IR, where they would offer powerful complements to JWST and ALMA; low-emissivity telescope designs and, possibly, cryogenic AO, may be needed. Finally, we outline the potential challenges of correcting atmospheric dispersion effects.
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.
The formation, shaping and evolution of galaxies are processes of high interest but poorly known. This is true also for our closest galactic neighbours. Of key importance is a representative sample of all major types of galaxies with solid evolutionary parameters. Stellar clusters are excellent probes of galactic evolution, albeit so far useful at smaller distances only, mainly due to the limiting effects of image crowding. With Extremely Large Telescopes (ELTs) with full adaptive optics (AO) and near diffraction limited performance, the effects of image crowding will be drastically reduced. Thus, the excellence of clusters of stars as evolutionary probes can be extended out to cosmological distances. We have studied this extension. With data on the Strömgren uvby system, based on direct measurements taken from the literature, we synthesised an open test cluster as well as a galactic background field. The cluster was embedded in the background and located at distances between one and 500 Mpc. y and b data were measured with a 50 m ELT, reduced and analysed, out to 20 Mpc for individual cluster members and between 10 and 500 Mpc for the properties of the integrated cluster. Based on individual stellar data, colour-magnitude diagrams (CMDs) and luminosity functions (LFs) were constructed and studied for age
parameters. For the integral cluster data, we studied the cluster colour index (b-y). We conclude that with a 50 m AO ELT, turn-off point (TOP) determination in CMDs provide ages of excellent quality out to 8 Mpc, of high quality out to 16 Mpc and of reasonable quality to 20 Mpc. At this distance level, the luminosity function provides good support. The integral colour, (b-y), as an age parameter for the clusters, can be determined with high precision out to at least 300 Mpc and with somewhat reduced accuracy to 500 Mpc. Thus, using a 50 m ELT and observing stellar clusters, we can study evolutionary parameters of galaxies out to 20 Mpc and obtain age parameters out to around 500 Mpc from the Sun.
Programmes driving ELT design are discussed. The nature and evolution of circum-stellar shells and planetary discs are attractive, while the small angular sizes place basic properties, asymmetry and warps, beyond VLT capability. An ELT is neeed to reveal the conversion processes shell - disc - planets, planetary systems, Earth-like planets, habitability and life signatures. Formation and evolution of massive stars are dicussed as are stellar rotation and shape, surface spatial and temporal resolution and evolution. Stellar clusters are discussed as probes of galactic evolution, resolving CMDs for Virgo and Fornax cluster galaxies. High-resolution studies of AGNs are discussed as are supernovae with bearing on early star formation, galactic activity and cosmology. Finally, the requirements defined from the prime science drivers are confronted with technical realities.
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.
Among the science challenges of the Extremely Large Telescopes (ELTs), four object types are studied for performance with a 50 m ELT with adaptive optics (AO), Euro50. Emphasis is on planetary systems and very distant objects. For planetary systems and their evolution, we examine high resolution imaging of the nuclei of comets and high-resolution imaging, photometry and low and intermediate resolution spectroscopy of Kuiper-Belt objects. Imaging of Earth-like planets is discussed. The very high contrast imaging necessary for these purposes is discussed together with the relevant error sources. Finally, photometry and classification of supernovae is discussed and examined. The performance of a 50 m AO ELT is compared to corresponding data obtainable with current VLTs equipped with AO.
Some leading science programs undertaken with Very Large Telescopes and challenges driving the progress of the Extremely Large Telescopes are discussed together with the corresponding requirements. They concern expolanets, Earth-like planets, habitable zones, formation of stars and galaxies, first stars and cosmology. A description is attempted.
Stellar clusters are highly useful as tools for determination of distances, ages and abundances of heavy elements of galaxies, also at larger distances. Their utility for these purposes has, so far, been severely limited, mainly due to image crowding. The introduction of Extremely Large Telescopes (ELTs) with full adaptive optics (AO) and near diffraction limited performance should imply a drastic improvement concerning the usefulness of clusters and the limiting distances of high quality data. We have made a study of stellar clusters as probes of distance, evolution and chemistry of galaxies at distances from one to twenty Mpc. From data on the Stromgren uvby system, partly from direct measurements taken from the literature, we have synthesized test clusters, one open and one globular, as well as galactic backgrounds. The clusters have been embedded in the backgrounds and located at distances between one and twenty Mpc. Here, vby data have been measured, reduced and analyzed. Color-magnitude diagrams (CMDs), metallicity diagrams (MDs) and luminosity functions (LFs) have been constructed. They have been evaluated absolutely and compared to the corresponding template data. We conclude that with a 50 m AO ELT, for open as well as globular clusters, MDs are of high quality for clusters out to and beyond 5 Mpc and useful out to 10 Mpc. CMDs are of very high quality well beyond 5 Mpc. They are of high scientific value out to and beyond 10 Mpc and valuable for clusters even out to 20 Mpc. LFs are highly informative well beyond 10 Mpc and still rather valuable at 20 Mpc. With sufficient measurement data available, LFs are useful for clusters in galaxies even beyond 20 Mpc.
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.
ELT science drivers stress aperture, Strehl ratio, PSF definition and stability, field of view, wavelength range, flexibility, low polarisation and thermal emittance, auxiliary instruments, site and infrastructure. Applicable science categories are planets and planetary systems, stars and stellar systems, galaxies and galaxy clusters, and cosmology. ELT observations are needed for our own and other planetary systems. The study of planetary disks and formation requires ELT data. ELT results, with emphasis on PSF quality and stability, are crucial to the search for earth-like planets, especially those favourable for life. Investigation of star formation and stellar evolution requires ELT performance, as does the study of final stellar stages. ELTs are necessary for extremely high time resolution, details of stellar surfaces and astroseismology. Galaxy formation studies will benefit dramatically from ELT data, as will studies of large-scale development of galaxies and galaxy clusters over cosmological time scales. Detecting active galactic nuclei requires ELTs. ELT data are crucial for examining the structure and evolution of the universe. Observations of supernovae and other standard sources over very large distances are necessary for mapping the expansion of the universe and determining its acceleration or deceleration. Comparisons of Euro50 with VLTs and HST show a dramatic gain. The complementarity of Euro50, NGST and ALMA is noted.
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.
The optical prescription of the primary mirror of an astronomical telescope is an essential design driver, with major implications on cost and complexity. The sheer size of an alt-az 25-m class telescope emphasizes the need for a short focal ratio of the primary mirror. The optical solution for the 25-m telescope is that of a 4-mirror axial design. Cost and fabrication constraints require the segmented primary mirror to be spherical; on similar grounds, we conclude that the same statement most likely applies to the secondary mirror as well. This makes the correction of spherical aberration and field aberrations comparatively more complex than with a 2-mirror Ritchey- Chretien design, and the very high asphericity of the fourth mirror will constrain the minimal achievable focal ratio of the primary mirror. A parametric study will be presented, showing the available field of view and the difficulty of fabrication of the optics of the telescope as a function of the f/ratio of the primary mirror. The peculiar properties of the design will be reviewed, and it will be shown that the f/ratio of the primary mirror shall most likely fall in the f/1.20 to 1.40 range, as a result of fabrication, testing and alignment constraints. Novel optical testing methods will be proposed as well.
We discuss a device for real time compensation of image quality deterioration induced by atmospheric turbulence. The device will permit ground based observations with very high image resolution. We propose an instrument with two channels. One is an ordinary image detection channel, while the other uses a Hartmann-Shack wavefront detector to measure image degradation. This information is obtained in the form of a set of lenslet focus shifts, each corresponding to the local tilt of the wavefront. Through modeling, the entire wavefront is reconstructed. Consequently, we can estimate the optical transfer function and its corresponding point spread function. Through convolution techniques, the distorted image can subsequently be restored. Thus, image correction is performed in software, eliminating the need for expensive live optics designs. Due to the nature of atmospheric turbulence, detection and correction have to be made with 50 - 100 frames per second. This implies a need for very high computing capacity. A study of the mathematical operations involved has been made with special emphasis on implementation in the hardware architecture known as radar video image processor (RVIP). This hardware utilizes a high degree of parallelism. Results available show that RVIP together with complementary units provide the necessary high-speed computing capacity. The detection system in both channels must meet very high demands. We mention high quantum efficiency, fast readout at low noise levels and a wide spectral range. A preliminary investigation evaluates suitable detectors. ICCDs are so far the most promising candidates.
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 104 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.
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.
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 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.
Results are presented on the evaluation of the site for the Nordic Optical Telescope (NOT) at its site at the Roque de los Muchachos Observatory. Basic design features of the NOT are described together with the parameters that define image quality, with special consideration given to the role of atmospheric turbulence and the factors taken into account in the selection of the telescope site. Attention is given to the optical elements of the NOT and to its mechanical structure as well as to the thermal control of the telescope, the enclosure, and the ancillary instrumentation. Results from first observations at the NOT point to excellent observing conditions in terms of transparency and extinction stability as well as image quality.