Ground-based Optical Astronomy Research Articles

Intercomparison of flux-, gradient-, and variance-based optical turbulence (C n2) parameterizations

For free-space optical communication or ground-based optical astronomy, ample data of optical turbulence strength (C n 2) are imperative but typically scarce. Turbulence conditions are strongly site dependent, so their accurate quantification requires in situ measurements or numerical weather simulations. If C n 2 is not measured directly (e.g., with a scintillometer), C n 2 parameterizations must be utilized to estimate it from meteorological observations or model output. Even though various parameterizations exist in the literature, their relative performance is unknown. We fill this knowledge gap by performing a systematic three-way comparison of a flux-, gradient-, and variance-based parameterization. Each parameterization is applied to both observed and simulated meteorological variables, and the resulting C n 2 estimates are compared against observed C n 2 from two scintillometers. The variance-based parameterization yields the overall best performance, and unlike other approaches, its application is not limited to the lowest part of the atmospheric boundary layer (i.e. the surface layer). We also show that C n 2 estimated from the output of the Weather Research and Forecasting model aligns well with observations, highlighting the value of mesoscale models for optical turbulence modeling.

Demonstrating 24-hour continuous vertical monitoring of atmospheric optical turbulence.

We report what is believed to be the first example of fully continuous, 24-hour vertical monitoring of atmospheric optical turbulence. This is achieved using a novel instrument, the 24-hour Shack-Hartmann Image Motion Monitor (24hSHIMM). Optical turbulence is a fundamental limitation for applications such as free-space optical communications, where it limits the achievable bandwidth, and ground-based optical astronomy, restricting the observational precision. Knowledge of the turbulence enables us to select the best sites, design optical instrumentation and optimise the operation of ground-based optical systems. The 24hSHIMM estimates the vertical optical turbulence coherence length, time, angle and Rytov variance from the measurement of a four-layer vertical turbulence profile and a wind speed profile retrieved from meteorological forecasts. To illustrate our advance we show the values of these parameters recorded during a 36-hour, continuous demonstration of the instrument. Due to its portability and ability to work in stronger turbulence, the 24hSHIMM can also operate in urban locations, providing the field with a truly continuous, versatile turbulence monitor for all but the most demanding of applications.

Night sky brightness measurement, quality assessment and monitoring

Ground-based optical astronomy necessarily involves sensing the light of astronomical objects along with the contributions of many natural sources ranging from the Earth's atmosphere to cosmological light. In addition, astronomers have long contended with artificial light pollution that further adds to the 'background' against which astronomical objects are seen. Understanding the brightness of the night sky is therefore fundamental to astronomy. The last comprehensive review of this subject was nearly a half-century ago, and we have learned much about both the natural and artificial night sky since. This Review considers which influences determine the total optical brightness of the night sky; the means by which that brightness is measured; and how night sky quality is assessed and monitored in the long term.

Around the World

In science news around the world this week, Thomas Insel discussed the aims, structure, and temporary leadership of the new National Center for Advancing Translational Sciences; a joint German-Israeli research center on archaeology and human evolution is planned; Prime Minister Manmohan Singh has vowed to more than double India's spending on R&D over the next 5 years and build two major research facilities; sea snakes could be going missing from a haven in the Pacific Ocean; a new study suggests the Canadian High Arctic is also a good spot for ground-based optical astronomy; and a report has challenged an ambitious U.S. climate research plan.

S-Cam 3: Optical astronomy with a STJ-based imaging spectrophotometer

S-Cam 3 is the third generation of a cryogenic camera, based on superconducting tunnel junctions (STJs), for ground-based optical astronomy, deployed at the 4.2 m William Herschel Telescope (WHT) at La Palma (Spain). It exploits a 10×12 pixel array of Ta/Al STJs, covering a field of view of ∼9″×11″ on the sky. The wavelength band extends from 330–750 nm, with a wavelength resolving power of ∼10 at 500 nm. The detectors are operated at ∼285 mK, achieved with a double stage 4He– 3He sorption cooler. Each pixel has its own electronic readout chain at room temperature, with a JFET-based charge sensitive preamplifier. The instrument has undergone extensive testing and calibration, followed by the first observation campaign at La Palma in July 2004. This campaign has focused on point sources with time variability, exploiting the instrument's unique combination of spectrophotometry with high time resolution.

A high resolution, high frame rate detector based on a microchannel plate readout with the Medipix2 counting CMOS pixel chip

The future of ground-based optical astronomy lies with advancements in adaptive optics (AO) to overcome the limitations that the atmosphere places on high resolution imaging. A key technology for AO systems on future very large telescopes are the wavefront sensors (WFS) which detect the optical phase error and send corrections to deformable mirrors. Telescopes with >30 m diameters will require WFS detectors that have large pixel formats (512 /spl times/ 512), low noise (<3 e/sup -//pixel) and very high frame rates (/spl sim/1 kHz). These requirements have led to the idea of a bare CMOS active pixel device (the Medipix2 chip) functioning in counting mode as an anode with noiseless readout for a microchannel plate (MCP) detector and at 1 kHz continuous frame rate. First measurement results obtained with this novel detector are presented both for UV photons and beta particles.

LYMANα EMITTERS BEYOND REDSHIFT 5: THE DAWN OF GALAXY FORMATION

The 8m class telescopes in the ground-based optical astronomy together with help from the ultra-sharp eye of the Hubble Space Telescope have enabled us to observe forming galaxies beyond red shift z = 5. In particular, more than twenty Ly<TEX>$\alpha$</TEX>-emitting galaxies have already been found at z > 5. These findings provide us with useful hints to investigate how galaxies formed and then evolved in the early universe. Further, detailed analysis of Ly<TEX>$\alpha$</TEX> emission line profiles are useful in exploring the nature of the intergalactic medium because the trailing edge of cosmic reionization could be close to z <TEX>$\~$</TEX> 6 -7, at which forming galaxies have been found recently. We also discuss the importance of superwinds from forming galaxies at high redshift, which has an intimate relationship between galaxies and the intergalactic medium. We then give a review of early cosmic star formation history based on recent progress in searching for Ly<TEX>$\alpha$</TEX>-emitting young galaxies beyond red shift 5.

120-pixel array of superconducting tunnel junctions as spectrophotometer for optical astronomy

Superconducting tunnel junctions (STJs), applied as photon counting detectors, combine intrinsic energy resolution with good detection efficiency and high count rate capability in a wide energy range, from the near infrared to the X-ray. The Science Payload and Advanced Concepts Office of the European Space Agency (ESA), is developing S-Cam: a cryogenic camera for ground-based optical astronomy exploiting an array of Ta STJs. The first generations of this instrument ( S-Cam1–2), using a 6×6 pixel array, have undergone four campaigns at the 4.2 m William Herschel Telescope at La Palma (Spain). The goals for S-Cam3, presently under development, are to increase the field of view of the instrument from 4×4″ to 11×9″, to optimize IR rejection, to extend the ‘red’ response and to increase the electronics readout speed. The camera will use a 10×12 array of Ta/Al STJs. Results are presented on the performance of these larger arrays.

Superconducting tunnel junctions as photon- counting imaging spectrometers from the optical to the x-ray band

Superconducting tunnel junctions (STJs) have been exten- sively investigated as photon detectors. They combine intrinsic energy resolution with good detection efficiency and high responsivity, thus pro- viding energy-resolved photon-counting performance in a wide energy range, from the near IR (NIR) to the x rays. On this basis, STJs offer advantages with respect to alternative photon detection systems, such as an intrinsic energy resolution, higher speed, and when compared to wavelength dispersive systems, increased detection efficiency. We over- view the STJ development in the Astrophysics Division of the European Space Agency (ESA), including STJ fabrication and operation, spectro- scopic performance of single STJs, small arrays, and STJ-absorber structures in the UV-NIR and x-ray bands. As a first application we de- scribe S-Cam, a cryogenic camera for ground-based optical astronomy exploiting a 636 array of Ta STJs. This camera has undergone four campaigns at the 4.2-m William Herschel Telescope at La Palma (Spain), and future generations of the camera are under active develop- ment. For the soft x-ray band (50 to 3000 eV) a STJ-based instrument with an active area of 73 7m m 2 is proposed as part of the payload of the X-ray Evolving Universe Spectroscopy (XEUS) mission, which is cur- rently under study at ESA. Future developments include devices based on lower Tc superconductors for improved energy resolution as well as larger format detector arrays combined with alternative readout schemes. © 2002 Society of Photo-Optical Instrumentation Engineers.

Soft X-ray performance of superconducting tunnel junction arrays

A number of 6/spl times/6 element arrays of Ta-based superconducting tunnel junctions have been manufactured for photon counting applications with moderate energy resolution in ground-based optical astronomy. The individual array elements show low leakage, uniform responsivity across the array, good simultaneous Josephson current suppression and minor crosstalk between adjacent pixels. The same arrays have been characterized in the soft X-ray range (E=270-1500 eV). The base electrode response shows good energy resolving power (E//spl Delta/E/spl ap/140). Unwanted spectral features originating from other parts of the detector can be largely eliminated by rise-time filtering. Modifications in the layering are necessary in order to improve the soft X-ray detection efficiency.

Software Tools for Engineering Dark Skies

ABSTRACTAtmospheric scattering of outdoor nighttime electric illumination produces the principal component of background sky luminance that seriously affects ground-based optical astronomy. The sources for this scattering are Tight emitted skyward directly from luminaires, and light reflected off the ground and other illuminated objects. Careful illumination engineering can thus significantly reduce background sky luminance in two ways: 1) by providing outdoor electric lighting equipment that controls the directions in which light is emitted, and; 2) by proper design of outdoor lighting systems which make efficient use of the east amount of light. Recent developments in applied mathematics and computer software have produced computational tools that are being used to design lighting equipment and lighting systems. The software system for luminaire design significantly reduces the cost of this process by eliminating the need for extensive prototyping and provides for inexpensive experimentation with new designs. The system for outdoor lighting calculations permits the design of highly controlled lighting systems that eliminate glare and upward directed light while providing light appropriate for the visual task. These two software systems are described, along with examples of their use in areas that directly affect astronomical observations.

Seeking Supernovas Systematically

Since its discovery last February, Supernova 1987A has been much in the news. The unanticipated find is giving astronomers computer memories full of information (SN: 8/22/87, p.117 & 122), but as more and more details of the development of this nearby supernova become known, astronomers need a context in which to put them. Many aspects of the supernova have not been seen before, and astronomers wonder whether these are unique to it or common to a class or classes of supernovas. To tell the common from the uncommon, the usual from the anomalous about supernovas, astronomers need good statistical information about them. This will not come from nearby supernovas like 1987A. The odds against another blowing off as near to us as the Large Magellanic Cloud are probably too astronomical to calculate. Where the odds are good is in the random, more or less distant galaxies scattered over the sky To find a lot of supernovas to make up the statistical base, automated observational patrols, which were under development long before SN 1987A appeared, are now beginning to operate. Supernova action is not infrequent in the distant galaxies, when those galaxies are taken as a whole. As of Aug. 22, according to International Astronomical Union Circular 4441, the count had gone as far as SN 1987L, the twelfth supernova to be discovered this year. However, in the absence of automated patrols, most supernovas are found by accident during other investigations, as Ian Shelton of the University of Toronto found SN 1987A. And most discoveries are not as lucky as his. Usually a supernova is not noticed until its astrophysically most interesting period -its earliest days-are already past. According to Carl Pennypacker of the University of California at Berkeley, only 10 type II supernovas, which are in many respects the most interesting kind, have been discovered before their light output reached its maximum level. Astronomers would like to find supernovas before maximum light so that they can follow their development from as early an epoch as possible. At the recent Workshop on Instrumentation for Ground-Based Optical Astronomy, held at the University of California at Santa Cruz, Pennypacker and Sterling Colgate of New Mexico Institute of Technology in Socorro and Los Alamos (N.M.) National Laboratory described the supernova patrol programs that each of them leads.

Prospects for the attainment of high angular resolution in ground-based optical astronomy

Prospects for the attainment of high angular resolution in ground-based optical astronomy

Perspektivy dostizheniya vysokogo uglovogo razresheniya v nazemnoi opticheskoi astronomii

Perspektivy dostizheniya vysokogo uglovogo razresheniya v nazemnoi opticheskoi astronomii

Prospects for development of ground-based optical astronomy

A number of refinements in ground-based optical telescopes and their detecting equipment, and also serious development of measures for neutralization of interference effects of the earth's atmosphere, permit the efficiency of observation of faint astronomical objects to be improved by at least an order of magnitude in the coming years. In the case of extragalactic astronomy and observational cosmology it is shown that progress in large ground-based optical telescopes has a decisive role in formation of contemporary concepts of the universe. Some of the problems of astronomy which will be solved by means of these instruments in the near future are discussed.

Perspektivy razvitiya nazemnoi opticheskoi astronomii

Perspektivy razvitiya nazemnoi opticheskoi astronomii