A means for improving telescope derives

In seeking to minimize local seeing, some large telescopes are being planned with less wind protection than has been traditional and some sites with very good seeing appear to suffer significantly from ground-transmitted vibration. A great care is needed in designing new telescopes to ensure that vibration does not become a dominant cause of image degradation. A supplementary drive system is proposed which could improve on past tracking performances even in such adverse conditions. The system may also reduce the costs of very large telescopes by easing the demands on the stiffness and accuracy of their mountings and drives.Paper presented at the Symposium on JNLT and Related Engineering Developments, Tokyo, November 29–December 2, 1988.     

Toward high spatial resolution in solar observations

To achieve high spatial resolution capability for IR solar observations, we suggest the use of a special mask placed before the entrance aperture of large conventional optical telescopes. Such a mask would match well the resolution of a non-redundant array and would provide safe operation of a large telescope used for solar observations. The proposed solution has the additional advantage of relatively low cost because already-existing optical telescopes are used.     

Optical intensity interferometry with the Cherenkov Telescope Array

With its unprecedented light-collecting area for night-sky observations, the Cherenkov Telescope Array (CTA) holds great potential for also optical stellar astronomy, in particular as a multi-element intensity interferometer for realizing imaging with sub-milliarcsecond angular resolution. Such an order-of-magnitude increase of the spatial resolution achieved in optical astronomy will reveal the surfaces of rotationally flattened stars with structures in their circumstellar disks and winds, or the gas flows between close binaries. Image reconstruction is feasible from the second-order coherence of light, measured as the temporal correlations of arrival times between photons recorded in different telescopes. This technique (once pioneered by Hanbury Brown and Twiss) connects telescopes only with electronic signals and is practically insensitive to atmospheric turbulence and to imperfections in telescope optics. Detector and telescope requirements are very similar to those for imaging air Cherenkov observatories, the main difference being the signal processing (calculating cross correlations between single camera pixels in pairs of telescopes). Observations of brighter stars are not limited by sky brightness, permitting efficient CTA use during also bright-Moon periods. While other concepts have been proposed to realize kilometer-scale optical interferometers of conventional amplitude (phase-) type, both in space and on the ground, their complexity places them much further into the future than CTA, which thus could become the first kilometer-scale optical imager in astronomy.     

History of solar telescopes

Solar observations have been done with telescopes since their invention—already Galileo looked at the Sun. Despite the Sun’s unusual brightness, telescopes which specialize in solar observations are fairly recent, dating from the late nineteenth century onwards. Today, many solar telescopes have rather little in common with nighttime telescopes. They are adapted to high light flux, a limited range of declination, and to the specifications of solar spectrographs and polarimeters. This paper presents the history of the modern optical solar telescope on the ground and in space, the accompanying evolution of scientific capabilities, and a brief outlook into the future.     

The Swift Ultra-Violet/Optical Telescope: a view of today and tomorrow

Currently there are four operating near-UV imaging space telescopes, one of which is the Swift Ultra-Violet/Optical Telescope (UVOT). Although the UVOT was primarily built for observations of γ-ray bursts, it has become a powerful instrument for studying other types of UV and optical astronomical phenomena. Here we discuss the properties of the UVOT, summarize some of the science undertaken with the UVOT, and present other possible science goals for the UVOT that have not yet been pursued. We also discuss some lessons learned that apply to future UV telescopes.     

Stellar intensity interferometry: Prospects for sub-milliarcsecond optical imaging

Using kilometric arrays of air Cherenkov telescopes at short wavelengths, intensity interferometry may increase the spatial resolution achieved in optical astronomy by an order of magnitude, enabling images of rapidly rotating hot stars with structures in their circumstellar disks and winds, or mapping out patterns of nonradial pulsations across stellar surfaces. Intensity interferometry (once pioneered by Hanbury Brown and Twiss) connects telescopes only electronically, and is practically insensitive to atmospheric turbulence and optical imperfections, permitting observations over long baselines and through large airmasses, also at short optical wavelengths. The required large telescopes (～10 m) with very fast detectors (～ns) are becoming available as the arrays primarily erected to measure Cherenkov light emitted in air by particle cascades initiated by energetic gamma rays. Planned facilities (e.g., CTA, Cherenkov Telescope Array) envision many tens of telescopes distributed over a few square km. Digital signal handling enables very many baselines (from tens of meters to over a kilometer) to be simultaneously synthesized between many pairs of telescopes, while stars may be tracked across the sky with electronic time delays, in effect synthesizing an optical interferometer in software. Simulated observations indicate limiting magnitudes around m_V = 8, reaching angular resolutions ～30 μarcsec in the violet. The signal-to-noise ratio favors high-temperature sources and emission-line structures, and is independent of the optical passband, be it a single spectral line or the broad spectral continuum. Intensity interferometry directly provides the modulus (but not phase) of any spatial frequency component of the source image; for this reason a full image reconstruction requires phase retrieval techniques. This is feasible if sufficient coverage of the interferometric (u, v)-plane is available, as was verified through numerical simulations. Laboratory and field experiments are in progress; test telescopes have been erected, intensity interferometry has been achieved in the laboratory, and first full-scale tests of connecting large Cherenkov telescopes have been carried out. This paper reviews this interferometric method in view of the new possibilities offered by arrays of air Cherenkov telescopes, and outlines observational programs that should become realistic already in the rather near future.     

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.     

I S L A An Astronomical World Space Observatory for The First Century of the 3 rd Millenium

ISLA will be an astronomical observatory, operating at the upper limit of our planet Earth atmosphere, offering space like observing conditions in most aspects. ISLA can be maintained easily, modified easily if necessary, always kept at the state of the art and operated for very extended periods without polluting the stratosphere. ISLA is ideally suited to become the first world space observatory as the observing conditions are very much space-like – diffraction limited angular resolution, very low ambient temperature, remote control – however ISLA is easily accessible, telescopes and instruments can be continuously improved and ISLA's costs corresponds only to those of ground-based modern astronomical installations like the ESO-VLT-, KECK- and GEMINI-observatories. The cost of observing and experimenting on ISLA will be orders of magnitudes lower than those of building and operating any space telescope, allowing the astronomers of developing nations to participate in the ISLA observatory within their limited financial possibilities as competent and full partners. ISLA's 4-m and 2-m telescopes will operate diffraction limited from 0.3 μm in the optical, over the infrared, far-infrared to the sub-mm spectral range. ISLA's individual telescopes will permit imaging with 20 milli-arcsec spatial resolution in the optical, 5 times better than the Hubble Space Telescope. ISLA's telescopes can be combined to form an interferometer with a maximum baseline of 250 m with nearly complete coverage of the u,v plane. Interferometric resolution will be of the order of 20 micro-arcsec for the optical. ISLA will thus offer spatial resolution comparable or better than the intercontinental VLBI radio interferometers. ISLA's telescope efficiency will be many orders of magnitude better than comparable ground-based telescopes. The light collecting power of ISLA's interferometric telescopes will be orders of magnitudes greater than the future space interferometers under discussion. ISLA, being an aviation project and not a space project, can be realised in the typical time scale for the development of aviation: about 5 years. ISLA's cost for the whole observatory, including its movable ground station etc. will be of the order of a typical medium size ESA space mission. ISLA's lifetime will be in excess of many decades, as it can easily be maintained, modernised, repaired and improved. ISLA will become the origin of a new astronomical international organisation with worldwide participation. ISLA's telescopes will be of the greatest importance to all astronomical fields, as it will permit to study much fainter, much more distant objects with microscopic spatial resolution in wavelength regions inaccessible from ground. ISLA's many telescopes permit easily simultaneous observations at many wavelengths for rapidly varying objects, from continuously monitoring the surfaces of the planets in our solar system, surfaces of close-by stars, nuclei of galaxies to QSO's. This revised version was published online in July 2006 with corrections to the Cover Date.     

Time variations of hard X-rays from Sco X-1

Simultaneous hard X-ray and optical observations of Sco X-1 were carried out on 1971 May 1 at Hyderabad, India, when Sco X-1 was optically bright. The X-ray intensity observed by balloon-borne counter telescopes increased in coincidence with optical enhancements, while the plasma temperature derived by fitting the X-ray spectrum in the energy range 20–40 keV to the thermal bremsstrahlung spectrum did not appreciably change over the whole period of observation.     

On the relation between supersoft X‐ray sources and VY Scl stars: The cases of V504 Cen and VY Scl

We summarise our optical monitoring program of VY Scl stars with the SMARTS telescopes, and triggered X‐ray as well as optical observations after/during state transitions of V504 Cen and VY Scl (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-resolution spectrographs on medium-diameter telescopes

The problem of high-resolution spectroscopic observations on telescopes with diameters of D = 0.5–1.5 m is considered. Examples of the optimum technical solutions and survivability of certain telescopes and methods of observation are given in retrospect. Improvements in the technique of spectroscopy allow one to also consider the potential of those instruments that were not originally intended for use in high-resolution spectroscopy. Arguments in favor of using medium-diameter telescopes in modern high-resolution spectroscopy are discussed. This review is connected to the 100th anniversary of the 122-cm telescope of the Crimean Astrophysical Observatory.     

Productivity and impact of astronomical facilities: A statistical study ofpublications and citations

In calendar years 2001 and 2002, 20 journals of astronomy and astrophysics published 7768 papers that reported or analyzed observations at wavelengths from meter radio to ultrahigh energy gamma rays. In the three calendar years after publication, these papers were cited more than 97 000 times, according to the Science Citation Index/Web of Science data base (the most complete, we believe, available), for an average rate of 4.19 citations per paper per year.We slice these data up several ways, by subject matter, wavelength band, and the telescopes (etc.) used. Most of the results will not surprise: There are hot topics (cosmology, exoplanets) and not so hot topics (binary stars, planetary nebulae). Papers reporting spacebased data are cited a bit more often and radio papers a bit less often than optical papers, but multi‐wavelength studies do the best. The total number of telescopes involved is surprisingly large, about 330 optical and infrared (mostly ground based but including HST), 109 radio (including COBE and VSOP satellites), and 90 space based (including satellites, interplanetary probes, things carried on rockets, balloons, the Shuttle, and so forth). The superstar telescopes are (mostly) the ones you would expect, though having the most papers does not always go with largest ratios of citations per paper. HST produces the largest number of optical papers, but SDSS the most highly‐cited ones, while the VLA is responsible for the largest number of radio papers and the most highly cited (apart from balloon‐borne CMB observatories), and among things that fly, the most recent tend to dominate both paper and citation numbers. If you have to choose, it is probably better to opt for a small telescope on a well‐supported site than a larger one with less support, and service to the community, in the form of catalogues and mission definitions, is rewarded, at least in citation counts, if not always in other ways. A few comparisons are made with other studies. The main difference is that we have included all the papers and all the telescopes for the years chosen, rather than focussing on one or a few observatories or skimming the cream of most‐cited papers or ones from the highest‐profile journals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stellar spectroscopy in ground-based ultraviolet. I: Observational technique

We present a historical note on the main spectroscopy instruments close to the short-wave boundary of the optical atmospheric window. Ground-based ultraviolet observations imposemore stringent requirements to the optics of telescopes and spectrographs, as well as to the calibration means. We have to bear in mind the seasonal variations of the optical properties of the Earth’s atmosphere. We consider the capabilities of ultraviolet observations with high spectral resolution on the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences.     

A Road Map for the Generation of a Near-Infrared Guide Star Catalog for Thirty Meter Telescope Observations

The near-infrared instruments in the upcoming Thirty Meter Telescope (TMT) will be assisted by a multi conjugate Adaptive Optics (AO) system. For the efficient operation of the AO system, during observations, a near-infrared guide star catalog which goes as faint as 22 mag in J_Vega band is essential and such a catalog does not exist. A methodology, based on stellar atmospheric models, to compute the expected near-infrared magnitudes of stellar sources from their optical magnitudes is developed. The method is applied and validated in JHKs bands for a magnitude range of J_Vega 16–22 mag. The methodology is also applied and validated using the reference catalog of PAN STARRS. We verified that the properties of the final PAN STARRS optical catalog will satisfy the requirements of TMT IRGSC and will be one of the potential sources for the generation of the final catalog. In a broader context, this methodology is applicable for the generation of a guide star catalog for any existing/upcoming near-infrared telescopes.     

Eigenvector Sky Subtraction

We develop a new method for estimating and removing the spectrum of the sky from deep spectroscopic observations. Our method does not rely on simultaneous measurement of the sky spectrum with the object spectrum. The technique is based on the iterative subtraction of continuum estimates and eigenvector sky models derived from singular value decompositions of sky spectra and sky spectra residuals. Using simulated data derived from small-telescope observations, we demonstrate that the method is effective for faint objects on large telescopes. We discuss simple methods to combine our new technique with the simultaneous measurement of sky to obtain sky subtraction very near the Poisson limit.     

Optical and X-Ray Observations of the Symbiotic System Ag Draconis During Quiescence and Outburst

We review the main characteristics of the symbiotic system AG Draconis, with special emphasis on its optical and X-ray variations. We also discuss the X-ray to visual energy distribution during quiescence and outburst and describe our spectroscopic and X-ray observations during the 2003 outburst. Based on X-ray observations collected with the XMM–Newton Observatory, on INES data from the IUE satellite, and on optical spectra collected with the Asiago–Cima Ekar, Bologna–Loiano and La Palma–Galileo Italian telescopes.     

Status of the ROTSE‐III telescope network

ROTSE‐III is a homogeneous worldwide array of 4 robotic telescopes. They were designed to provide optical observations of γ ‐ray burst (GRB) afterglows as close as possible to the start of γ ‐ray emission. ROTSE‐III is fulfilling its potential for GRB science, and provides optical observations for a variety of astrophysical sources in the interim between GRB events. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The visibility of gamma-ray burst afterglows in dusty star-forming regions

Recent observations of the environments of gamma-ray bursts (GRBs) favour massive stars as their progenitors, which are likely to be surrounded by gas and dust. The visibility of the optical and UV emission of a GRB is expected to depend on the characteristics of both the dust and the GRB emission itself. A reasonable distribution of surrounding dust is capable of absorbing all the optical and UV emission of the optical flash and afterglow of a GRB, unless the optical flash has a peak isotropic luminosity L _peak≳10⁴⁹ erg s⁻¹ . This means that dark bursts should exist and these bursts will have to be studied at infrared rather than optical wavelengths. In this paper details will be given about the infrared GRB dust emission. The reprocessed dust emission peaks at a rest-frame wavelength of about 8 μm. Forthcoming space telescopes, in particular the IRAC camera on board the Space Infrared Telescope Facility , could detect this emission out to a redshift of about two. However, an accurate position of the GRB afterglow must be provided for this emission to be identified, because the light curve of the reprocessed dust emission does not vary on time-scales less than several years.     

An imaging optical/UV monitor for X-ray astronomy observatories

The understanding of high-energy astrophysical sources often depends on observations over the entire electromagnetic spectrum. Yet, extensive multifrequency observing campaigns can consume the resources of a large number of telescopes, including ground-based and satellite facilities, and usually involve large, unwieldy consortia of observers. Because most X-ray sources are variable on a short time-scale, there is an additional need to make the multifrequency observations simultaneous. The logistical difficulties involved in coordinating these observations, coupled with the vagaries of the weather at ground-based observing sites, mean that comparatively few such coordinated campaigns are attempted; of those that have been tried the success rate for achieving simultaneity is low.In the present paper we argue that simultaneous X-ray, optical and ultraviolet observations could be achieved more logically, cheaply, and effectively by mounting a small boresighted optical/UV-telescope alongside future X-ray telescopes. A 12 optical/UV monitor could, for instance, be incorporated into X-ray facilities such as the American AXAF or the European XMM missions with minimal impact on the total cost, weight, size, and telemetry requirements. Such a telescope, equipped with a position sensitive photoncounting detector, could provide two-dimensional photometric imaging of stars as faint asB=23.5 in a 1000 s exposure with a resolution that could easily be matched to that of the X-ray telescope. A series of wide- and narrow-band filters could be used to define spectral bands, while wide-field, low-resolution spectroscopy could be provided by a prism. Such an instrument could monitor not only the multifrequency variability of such active sources as quasars, Seyfert galaxies, BL Lac objects, X-ray binaries, cataclysmic variables, RS CVn stars, and flare stars, but also could provide astrometry, broadband colours, low-resolution spectroscopy, and imaging of constant sources and fortuitously observed field objects. Moreover, the concept of providing multifrequency simultaneous coverage of astrophysical objects in an unbiased way allows new phenomena to be discovered. A review is given of the scientific problems that require such a monitor, and some of the design and performance characteristics of a suitable monitor are discussed.     

XTE J1901+014—the first low-mass Fast X-ray transient?

We continue to study the fast X-ray transient XTE J1901+014 discovered in 2002 by the RXTE observatory, whose nature has not yet been established. Based on the XMM-Newton observations of the source in 2006, we have obtained its energy spectrum, light curves, and power spectrum in the energy range 0.5–10 keV, which are in good agreement with our results obtained previously from the data of other observatories. In turn, this suggests that the source’s emission is stable in the quiescent state. The XMM-Newton observations also allowed the source’s localization accuracy to be improved to ≤2″, which subsequently enabled us to search for its optical companion with the RTT-150 and 6-m BTA (Special Astrophysical Observatory) telescopes. Combining optical, X-ray, and infrared observations, we have concluded that the optical companion in the system under study can be either a later-type star at a distance of several kpc or a very distant red giant or an A or F star. Thus, XTE J1901+014 may be the first low-mass fast X-ray transient.