Large Optical Telescopes Research Articles

Hartmann method for monitoring the shape of convex aspheric mirrors of large optical telescopes

This paper discusses the possibility of using the Hartmann method to monitor the shape of convex aspheric mirrors of telescopes under manufacturing conditions. A Hartmann stop in the form of regular small-diameter apertures in an opaque screen is placed outside the mirror to be monitored and is illuminated by a pencil of parallel rays directed perpendicular to the mirror's symmetry axis. The role of recording device can be played by a linear array of CCDs or a photographic plate on whose sensitive surface a so-called Hartmann pattern is recorded. Subsequent mathematical processing makes it possible to determine the shape errors of the meridional cross section. To provide monitoring of the entire surface, the latter is rotated around the symmetry axis by any given angle any number of times, and this makes it possible to completely automate the monitoring process of both ground and polished surfaces under manufacturing conditions.

The Night Sky at the Calar Alto Observatory

ABSTRACTWe present characterization of the main properties of the night sky at the Calar Alto observatory for the time period between 2004 and 2007. We use optical spectrophotometric data, photometric‐calibrated images taken in moonless observing periods, together with the observing conditions regularly monitored at the observatory, such as atmospheric extinction and seeing. We derive, for the first time, the typical moonless night‐sky optical spectrum for the observatory. The spectrum shows a strong contamination by different pollution lines, in particular from mercury lines, whose contribution to the sky brightness in the different bands is of the order of ∼0.09, ∼0.16, and ∼0.10 mag in B, V, and R, respectively. Regarding the strength of the sodium pollution line in comparison with the airglow emission, the observatory does not fulfill the IAU recommendations for a dark site. The zenith‐corrected values of the moonless night‐sky surface brightness are 22.39, 22.86, 22.01, 21.36, and 19.25 mag arcsec - 2 in U, B, V, R and I, which indicates that Calar Alto is a particularly dark site for optical observations up to the I band. The fraction of astronomical useful nights at the observatory is ∼70%, with ∼30% of photometric nights. The typical extinction at the observatory is κV∼0.15 mag in the winter season, with little dispersion. In summer the extinction has a wider range of values, although it does not reach the extreme peaks observed at other sites. The analysis of the winter and summer extinction curves indicates that the Rayleigh scattering is almost constant throughout the year. The rise of the extinction in the summer season is due to an enhancement of the aerosol extinction, most probably associated with an increase of dust in the atmosphere. The median seeing for the last two years (2005–2006) was ∼0.90 ″, being smaller in the summer (∼0.87 ″) than in the winter (∼0.96 ″). We conclude in general that after 26 years of operations, Calar Alto is still a good astronomical site. Its main properties are similar in many aspects to those of other major observatories where 10 m–like telescopes are under operation or construction, thus being a natural candidate for future large aperture optical telescopes.

Correlation of CFD predictions and wind tunnel measurements of mean and unsteady wind loads on a large optical telescope

Complementary studies, involving computational fluid dynamics (CFD) analyses and wind tunnel (WT) testing, have been carried out at the Institute for Aerospace Research (IAR) to investigate the wind-loading effects on the prototype, spherical Canadian/US Very Large Optical Telescope (VLOT) structure. The VLOT is currently in the first phase of design and these preliminary studies were part of an IAR/HIA (Herzberg Institute of Astrophysics) collaboration supporting the development of an integrated design model (IM) of the VLOT. The 51-m-diameter enclosure is spherical in shape, with the lower one-quarter removed to provide a flat base. The opening through which the telescope sees is 24 m in diameter and can be positioned at a range of zenith (elevation) angles from 0° to 60° for the full 360° azimuth range. The primary purpose of the CFD/WT studies was to assess the capability of a fully unsteady Lattice-Boltzmann CFD code to predict wind loads on the enclosure and telescope structures for use in telescope design by performing comparisons with the wind tunnel measurements. Both cavity resonance due to flow over the opening and vortex shedding from the spherical structure were observed in the wind tunnel experiments and the CFD computations. The CFD code predicted three simultaneously excited cavity modes that were identical to those measured.

Computational fluid dynamics simulations and wind tunnel measurements of unsteady wind loads on a scaled model of a very large optical telescope: A comparative study

Thorough theoretical and experimental investigations were performed to analyze the main characteristics of unsteady flows past a 1:100-scale wind tunnel (WT) model of a very large optical telescope housed within a spherical enclosure. The investigations were focused on the prediction and measurements of unsteady pressures on the inner and outer surfaces of the enclosure and on the telescope primary mirror. The WT measurements were performed essentially to provide aerodynamic data on the telescope structure and also to build a database for correlation with numerical simulation of the flow using computational fluid dynamics (CFD). Unsteady viscous flow solutions were computed for different telescope orientations using the lattice Boltzmann method coupled with the RNG k-ε turbulence model. For WT testing, unsteady pressure measurements were performed in an open jet WT for different telescope orientations and wind speeds, using a number of pressure taps distributed around the inner and the outer surfaces of the enclosure and on the primary mirror surface. A smoke stream visualization technique was also used to study the flow behavior around and inside of the telescope enclosure. The flow solutions were computed using the WT flow conditions. Correlations were obtained between CFD and WT data in terms of the mean pressure coefficients on the enclosure and the primary mirror surfaces, and for their standard deviations. Power spectral density analyses were also carried out for a number of pressure signals collected on the primary mirror surface. Both CFD solutions and WT measurements demonstrated that the flow inside and outside the enclosure was unsteady and massively separated on the back of the enclosure. The mean values and standard deviations of the pressure coefficients on the enclosure and the primary mirror surfaces correlated well with the experimental data. Using the WT Mach number in the simulation, the shear layer over the enclosure opening and the resulting acoustic wave effects were well captured, and there was excellent agreement between the CFD results and the WT measurements.

World's largest optical telescope sees first light

World's largest optical telescope sees first light

Statistical estimates of fundamental constraints on the use of commercial CCD cameras in TV guide systems of large optical telescopes

The parameters of TV guide cameras of the BTA and Zeiss-1000 telescopes are analyzed. The formation of optical images by the “atmosphere + telescope” system is analyzed with allowance for the laws of photoelectron statistics in order to justify the applicability of commercial CCD cameras in the guiding systems of large optical telescopes. The analysis focuses on the estimates of fundamental constraints imposed on the method of TV observations of the sky through a turbulent atmosphere. The possible ways of reducing the main constraining factors in the case of the use of highly sensitive commercially produced CCDs in TV guide cameras are outlined.

The optical system of an adaptive 25-m telescope with a composite primary mirror composed of spherical segments

This paper presents the results of a calculation of the four-mirror two-stage optical system of a large optical telescope with a composite primary mirror (CPM), whose aspheric reflecting surface is approximated by hexagonal segments of spherical shape. Our analysis of the image quality showed that, when a definite size is chosen for the elements of the CPM, the proposed approximation makes it possible in principle to achieve an equivalent angular resolution of the optical system of the telescope better than 0.1″. Elements of the CPM about 1 m across can be used for the 25-m telescope. It is expedient to put such a technical solution into practice both in the processing aspect and from the viewpoint of applying adaptive control of the shape of the reflective surface of the CPM.

PILOT – the Pathfinder for an International Large Optical Telescope

PILOT is proposed as a partnership between Australia and Europe to develop a 2.4 m optical/infrared telescope for Dome C, Antarctica. Funding for a detailed designed study is being sought from Australian sources, with a view to commencing construction in early 2008. The current “strawman” design is for an f/10 dual Nasmyth configuration with provision for both a silicon carbide fast tip-tilt secondary mirror for the thermal infrared, and an adaptive secondary mirror to achieve diffraction-limited imaging at wavelengths as short as V -band.

Wind loads on ground-based telescopes

One of the factors that can influence the performance of large optical telescopes is the vibration of the telescope structure due to unsteady wind inside the telescope enclosure. Estimating the resulting degradation in image quality has been difficult because of the relatively poor understanding of the flow characteristics. Significant progress has recently been made, informed by measurements in existing observatories, wind-tunnel tests, and computational fluid dynamic analyses. We combine the information from these sources to summarize the relevant wind characteristics and enable a model of the dynamic wind loads on a telescope structure within an enclosure. The amplitude, temporal spectrum, and spatial distribution of wind disturbances are defined as a function of relevant design parameters, providing a significant improvement in our understanding of an important design issue.

Ecological Consequences of Artificial Night Lighting

Review: Ecological Consequences of Artificial Night Lighting Catherine Rich and Travis Longcore (Eds.) Reviewed by Kathy Piselli Atlanta Fulton Public Library, USA Catherine Rich and Travis Loncore (Eds.). Ecological Consequences of Artificial Night. Washington, DC: Island Press, 2006. ISBN: 1-55963-129-5. $29.95 (Trade paper). ISBN: 1-55963-128-7 $65.00 (Trade cloth). The Urban Wildlands Group, a Los Angeles–based conservation organization, convened a gathering of ecologists in 2002 to review the state of knowledge about the ecological consequences of artificial night lighting. This book provides edited proceedings from that conference. When New York City’s Pearl Street Station opened in 1882, the age of modern urban lighting was ushered in. Coincidentally, this was also a time when large optical refracting telescopes were built. By the 1950s, astronomers were complaining that “sky glow” from city lights were keeping them from doing their work. Sky glow happens when lights from urban areas reflect off clouds, causing light from even a small urban area to obliterate much of what there is to see in the night sky. The effect was most problematic in the western United States, location of several large telescope facilities and the traditional home of big, starry skies. By 2002, nine U.S. states and one entire country - the Czech Republic - had adopted so-called dark sky provisions, and 11 other states were considering similar laws. These legal efforts were based on aesthetic concerns. Some statistics frequently quoted came out of the First World Atlas of the Artificial Night-Sky Brightness (2001): half of the world’s population has never seen the Milky Way, 40 percent of Americans no longer have to adjust their eyes to night vision, two thirds of the world's population lives under light-polluted night skies. In March 2006, the National Park Service’s Night Sky Team released comprehensive data on the amount of light in the night sky in national parks, finding pollution in places that once seemed distant from urban centers. But to paraphrase one of this book’s editors, the idea that light pollution might pose a broad problem for whole ecosystems remained mostly in the gray literature. The topic is relatively new and unstudied and does not even have proper terminology yet. Lighting professionals and environmental regulators speak of “lux” to communicate a quantity of visible light. But for ecological purposes, “lux” fails to express the fact that different light sources

S18 すばる望遠鏡に用いた材料力学 : 渥美先生が教えてくれなかったもの?(SS(5)「渥美光」記念セッション)

Subaru telescope has been leading the world as the best instrument for astronomical observation in the visible and infrared region. Before Subaru, Japan had almost no experience to engineer and design such a large and accurate optical telescope. At the development stage, various state-of-the-art technologies were studied. It is, however, the basis of "The mechanics of materials" that played one of the most important roles in the study. The important roles include 1) how to make the huge and complicated telescope structure behave as an elastic body so as to show almost perfect repeatability of the deformation, and 2) how to obtain force vectors to correct the deformation of primary mirror with 8.3m diameter. This is to simplify the drive control system of the telescope structure and the figure control system of the primary mirror. The author learnt "The mechanics of materials" and "The theory of elasticity" from Prof. Atsumi in Tohoku University late in the 1970s, and led the Subaru telescope project as a system engineer and project manager in Mitsubishi Electric from 1987 to 2001. The basis of the learning was fully utilized in the design of Subaru with some additional application which Prof. Atsumi would have not taught. Some differences of the practical engineering from the academic learning and some key technologies which led the success of Subaru are introduced.

Astronomy from Antarctica

Astronomers have always sought the very best locations for their telescopes. From observatories in city centres, astronomers moved first to nearby mountain tops, then to remote sites in distant countries, to aircraft, and into space. In the past decade we have come to realize that the best astronomical observing conditions on the surface of the earth are to be found on the Antarctic plateau. The combination of high altitude, low temperature, low absolute humidity, low wind and extremely stable atmosphere offers astronomers gains in sensitivity and measurement precision that can exceed two orders of magnitude over even the best temperate sites. In addition, spectral windows are opened up – particularly in the far-infrared and terahertz regions – that are otherwise only accessible from high-flying aircraft or from space. Established and highly successful telescopes at the South Pole are soon to be joined by a new generation of facilities at Concordia Station, including large telescopes and interferometers. It has even been suggested that the largest optical telescopes currently proposed, with diameters of up to 100 m, might achieve their science goals at a lower overall cost if they are built on the Antarctic plateau rather than at a temperate site. Such telescopes offer the possibility of not only detecting earth-like planets in other star systems, but also of analysing their atmospheres spectroscopically.

Buffeting of large telescopes: Wind-tunnel measurements of the flow inside a generic enclosure

The design of future large optical telescopes must take into account the wind-induced vibration of the telescope structure that is caused by large-scale flow structures and turbulence inside the telescope enclosure when the aperture in the enclosure dome is open. However, estimating the resulting degradation in image quality is difficult due to our relatively poor understanding of the flow inside the enclosure. Data has been collected in a wind-tunnel test of an empty telescope enclosure to understand the flow-field around the region near the dome opening where the secondary mirror and supporting structure would be subjected to wind loads. Digital particle image velocimetry (DPIV) data was collected in a vertical plane near the dome opening to obtain mean velocity and fluctuation kinetic energy, and hot-wire data was collected along the telescope axis to obtain temporal spectra of the velocity. Data was collected both with and without additional venting, in order to understand its influence on the flow. The temporal spectrum of the broadband turbulence is of von Karman type. The flow field also includes tonal shear layer modes, with the mode selection influenced by the enclosure Helmholtz mode, and the overall amplitude strongly influenced by the vented area. In addition to its direct use in telescope modelling and design, the data presented herein is of particular value in validation of computational fluid dynamic (CFD) analyses, so that CFD can be used with confidence in future design work.

Rising star

As construction on the world's largest optical telescope nears completion in Spain, the country's astronomers are gearing up for an expanded role on the global stage. Mark Peplow follows the preparations for first light.

Analytical expressions for optimum alignment modes of highly segmented mirrors

The major sources causing deterioration of optical quality in extremely large optical telescopes are misadjustments of the mirrors, deformations of monolithic mirrors and misalignments of segments in segmented mirrors. For active optics corrections, all three errors, which can partially compensate each other, are measured simultaneously. It is therefore of interest to understand the similarities and differences between the three corresponding types of modes which describe these errors. The first two types are best represented by Zernike polynomials and elastic modes respectively, both of them being continuous and smooth functions. The segment misalignment modes, which are derived by singular value decomposition, are by their nature not smooth and in general discontinuous. However, for mirrors with a large number of segments, the lowest modes become effectively both smooth and continuous. This paper derives analytical expressions for these modes, using differential operators and their adjoints, for the limit case of infinitesimally small segments. For segmented mirrors with approximately 1000 segments, it is shown that these modes agree well with the corresponding lowest singular value decomposition modes. Furthermore, the analytical expressions reveal the nature of the segment misalignment modes and allow for a detailed comparison with the elastic modes of monolithic mirrors. Some mathematical features emerge as identical in the two cases.

Craig Nance: Engineer to the stars

Craig Nance keeps the world's largest optical telescopes running for the W.M. Keck Observatory in Waimea, Hawaii. He gets a sneak peak at news-making astronomical discoveries; lives, works, plays golf, and enjoys sunset drives with his wife, Laura, on the Big Island of Hawaii.

Science Programs for a 2-m Class Telescope at Dome C, Antarctica: PILOT, the Pathfinder for an International Large Optical Telescope

AbstractThe cold, dry, and stable air above the summits of the Antarctic plateau provides the best ground-based observing conditions from optical to sub-millimetre wavelengths to be found on the Earth. Pathfinder for an International Large Optical Telescope (PILOT) is a proposed 2 m telescope, to be built at Dome C in Antarctica, able to exploit these conditions for conducting astronomy at optical and infrared wavelengths. While PILOT is intended as a pathfinder towards the construction of future grand-design facilities, it will also be able to undertake a range of fundamental science investigations in its own right. This paper provides the performance specifications for PILOT, including its instrumentation. It then describes the kinds of projects that it could best conduct. These range from planetary science to the search for other solar systems, from star formation within the Galaxy to the star formation history of the Universe, and from gravitational lensing caused by exo-planets to that produced by the cosmic web of dark matter. PILOT would be particularly powerful for wide-field imaging at infrared wavelengths, achieving near diffraction-limited performance with simple tip–tilt wavefront correction. PILOT would also be capable of near diffraction-limited performance in the optical wavebands, as well be able to open new wavebands for regular ground-based observation, in the mid-IR from 17 to 40 μm and in the sub-millimetre at 200 μm.

Pathfinder for an International Large Optical Telescope

PILOT (Pathfinder for an International Large Optical Telescope) is a proposed 2 m class optical/infrared telescope to be sited at Dome C on the Antarctic plateau. Recent site testing results from Dome C have shown remarkable ground level seeing, a large isoplanatic angle, and a long atmospheric coherence time. These factors significantly improve the resolution and increase the capabilities of adaptive optics systems. Additionally, the low sky emission at Dome C gives a significant increase in infrared sensitivity compared to typical mid-latitude sites. PILOT is envisaged as a winterised version of a commercially available 2 m class telescope, with a simple low cost dome (sufficient for the low ground level wind speeds), and a tip-tilt and/or a deformable secondary mirror. A number of science cameras covering the visible to the sub-millimetre are being considered.

Book Review: The Design and Construction of Large Optical Telescopes. By Pierre‐Y. Bely

Book Review: The Design and Construction of Large Optical Telescopes. By Pierre‐Y. Bely

Shape Achievement of Optical Membrane MirrorsUsing Coating/Substrate Intrinsic Stresses

The U.S. Air Force, as well as many other organizations, has evinced considerable interest in developing large gossamer membrane radio antennas and optical quality telescopes. A concept is discussed of global shape maintenance of a reflective polymer membrane element after curing, coating, and release from a precision casting mold of the desired parabolic shape. The idea involves manipulation of intrinsic stresses in the coating and its membrane substrate. Results from an axisymmetric, geometrically linear, shallow-shell theory are presented as closed-form solutions for the displacement fields. A unique condition emerges from these solutions, suggesting that zero deformations can occur in a 0-g environment. Results of a finite element analysis are presented, showing that if a coated membrane satisfying this condition is subjected to a 1-g loading, rms errors of 9.3 µm and 2.8 mm can be expected fo r1 -and 20-m-diam mirrors, respectively. However, in the case of a 1-g load, another condition follows from the theory, which, if met, predicts zero deformation in the 1-g environment. Additional results of the finite element analysis are presented that lend support to these conclusions.