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Using an onboard sunlit solar diffuser (SD) as the primary radiance source, the visible infrared imaging radiometer suite (VIIRS) on the Suomi National Polar-orbiting Partnership satellite regularly performs radiometric calibration of its reflective solar bands (RSBs). The SD bidirectional reflectance distribution function (BRDF) value decreases over time. A numerical degradation factor is used to quantify the degradation and is determined by an onboard SD stability monitor (SDSM), which observes the sun and the sunlit SD at almost the same time. We had shown previously that the BRDF degradation factor was angle-dependent. Consequently, due to that the SDSM and the RSB view the SD at very different angles relative to both the solar and the SD surface normal vectors, directly applying the BRDF degradation factor determined by the SDSM to the VIIRS RSB calibration can result in large systematic errors. We develop a phenomenological model to calculate the BRDF degradation factor for the RSB SD view from the degradation factor for the SDSM SD view. Using the yearly undulations observed in the VIIRS detector gains for the M1–M4 bands calculated with the SD BRDF degradation factor for the SDSM SD view and the difference between the VIIRS detector gains calculated from the SD and the lunar observations, we obtain the model parameter values and thus establish the relation between the BRDF degradation factors for the RSB and the SDSM SD view directions. [ABSTRACT FROM PUBLISHER]

Images of stars adopt shapes far from the ideal Airy pattern due to atmospheric density fluctuations. Hence, diffraction-limited images can only be achieved by telescopes without atmospheric influence, e.g. spatial telescopes, or by using techniques like adaptive optics or lucky imaging. In this paper, we propose a new computational technique based on the evaluation of the COvariancE of Lucky Images (COELI). This technique allows us to discover companions to main stars by taking advantage of the atmospheric fluctuations. We describe the algorithm and we carry out a theoretical analysis of the improvement in contrast. We have used images taken with 2.2-m Calar Alto telescope as a test bed for the technique resulting that, under certain conditions, telescope diffraction limit is clearly reached. [ABSTRACT FROM AUTHOR]

Due to its unique rotation mechanism, a slant-axis mount provides a better adaptability for the extreme site conditions, such as those in Antarctica, compared to traditional equatorial and altazimuth mounts. However, the study devoted to the motion characteristics of slant-axis mount has not been reported yet. In this paper, the advantages and disadvantages of the slant-axis mount are first discussed. The transformation relations between the slant-axis coordinate system and the horizontal and equatorial coordinate systems are then derived, based on this the motion characteristics of slant-axis mount during a normal sidereal tracking are studied, and compared with those of altazimuth mounts. The image rotation behavior of slant-axis mount is also studied, and its influence on the reflector holographic measurement is analyzed. Finally, a pointing model including seven error terms and suitable for slant-axis telescopes is derived. [ABSTRACT FROM AUTHOR]

Context. Red giants are evolved stars that exhibit solar-like oscillations. Although a multitude of stars have been observed with space telescopes, only a handful of red giant stars were targets of spectroscopic asteroseismic observing projects. Aims. We search for solar-like oscillations in the two bright red giant stars γ Psc and θ¹ Tau from a time series of ground-based spectroscopy and determine the frequency of the excess of oscillation power ?max and the mean large frequency separation Δν for both stars. Seismic constraints on the stellar mass and radius will provide robust input for stellar modelling. Methods. The radial velocities of γ Psc and θ¹ Tau were monitored for 120 and 190 days, respectively. Nearly 9000 spectra were obtained. To reach accurate radial velocities, we used simultaneous thorium-argon and iodine-cell calibration of our optical spectra. In addition to the spectroscopy, we acquired interferometric observations of γ Psc for an independent estimate of the radius. We also analysed 22 days of observations of θ¹ Tau with the MOST satellite. Results. The frequency analysis of the radial velocity data of γ Psc revealed an excess of oscillation power around 32 µHz and a large frequency separation of 4.1 ± 0.1 µHz. θ¹ Tau exhibits oscillation power around 90 µHz, with a large frequency separation of 6.9 ± 0.2 µHz. Scaling relations indicate that γ Psc is a star of about 1 M⊙ and 10 R⊙. The object θ¹Tau appears to be a massive star of about 2.7 M⊙ and 10 R⊙. The radial velocities of both stars were found to be modulated on timescales much longer than the oscillation periods. Conclusions. The estimated radii from seismology are in agreement with interferometric observations and also with estimates based on photometric data. While the mass of θ¹ Tau is in agreement with results from dynamical parallaxes, we find a lower mass for γ Psc than is found in the literature. The long periodic variability agrees with the expected timescales of rotational modulation. [ABSTRACT FROM AUTHOR]

Context. Blazars are variable sources on various timescales over a broad energy range spanning from radio to very high energy (>100 GeV, hereafter VHE). Mrk 501 is one of the brightest blazars at TeV energies and has been extensively studied since its first VHE detection in 1996. However, most of the ?-ray studies performed on Mrk 501 during the past years relate to flaring activity, when the source detection and characterization with the available ?-ray instrumentation was easier to perform. Aims. Our goal is to characterize the source ?-ray emission in detail, together with the radio-to-X-ray emission, during the non-flaring (low) activity, which is less often studied than the occasional flaring (high) activity. Methods. We organized a multiwavelength (MW) campaign on Mrk 501 between March and May 2008. This multi-instrument effort included the most sensitive VHE γ-ray instruments in the northern hemisphere, namely the imaging atmospheric Cherenkov telescopes MAGIC and VERITAS, as well as Swift, RXTE, the F-GAMMA, GASP-WEBT, and other collaborations and instruments. This provided extensive energy and temporal coverage of Mrk 501 throughout the entire campaign. Results. Mrk 501 was found to be in a low state of activity during the campaign, with a VHE flux in the range of 10%-20% of the Crab nebula flux. Nevertheless, significant flux variations were detected with various instruments, with a trend of increasing variability with energy and a tentative correlation between the X-ray and VHE fluxes. The broadband spectral energy distribution during the two different emission states of the campaign can be adequately described within the homogeneous one-zone synchrotron self-Compton model, with the (slightly) higher state described by an increase in the electron number density. Conclusions. The one-zone SSC model can adequately describe the broadband spectral energy distribution of the source during the two months covered by the MW campaign. This agrees with previous studies of the broadband emission of this source during flaring and non-flaring states. We report for the first time a tentative X-ray-to-VHE correlation during such a low VHE activity. Although marginally significant, this positive correlation between X-ray and VHE, which has been reported many times during flaring activity, suggests that the mechanisms that dominate the X-ray/VHE emission during non-flaring-activity are not substantially different from those that are responsible for the emission during flaring activity. [ABSTRACT FROM AUTHOR]

The system design of a cryogenic 2-13 GHz feed is considered with emphasis on its application in future wideband radio telescope systems. The feed is based on the so-called Eleven antenna and the design requires careful integration of various sub-designs in order to realize cryogenic operation. The various sub-designs include the electrical design of the Eleven antenna, design of the critical center puck, alternative solutions for integrating the Eleven antenna with low-noise amplifiers (LNAs), mechanical and cryogenic design and tests, and system noise temperature estimation and measurements. A great deal of simulated and measured results are presented throughout this paper, including the electrical, mechanical and cryogenic performance, and an assessment of the system noise temperature. The objective of this work is to present a good feed candidate that is well-suited for VLBI2010 and SKA radio telescopes. Further developments needed to completely fulfill the requirements for these future wideband radio telescopes are also discussed. [ABSTRACT FROM AUTHOR]

The 28 papers in this special issue on antennas for next generation radio telescopes present state-of-the-art work being carried out at more than 20 institutions around the world in ten geographically dispersed countries. [ABSTRACT FROM AUTHOR]

The proposed next-generation radio-telescope, the Square Kilometre Array (SKA), has ~3000 parabolic dish antennas and operates over many gigahertz of bandwidth. This results in a telescope with correlator requirements ~10 000 times more than any current telescope. This will be a challenge using traditional approaches. A method is proposed that will reduce the computational requirements for that part of the SKA where the antenna separation is not too great. This is done by deliberately introducing redundancy so that many of the outputs of the correlator measure the same data. We then show that the sum of these redundant correlations can be obtained efficiently by the use of fast convolution. The degree to which computational requirements are reduced is selectable. This translates directly into a power saving in the correlator and image processing. [ABSTRACT FROM AUTHOR]

Next generation of instruments for radio astronomy will benefit greatly from reflector antenna feeds that demonstrate very wide instantaneous bandwidth and exhibit low noise behavior. Our study focuses on design and measurement of an ultra-wideband inverted conical sinuous antenna and its integration with a low noise pseudo differential amplifier. The self-complementary, frequency independent nature of the planar sinuous geometry results in a nearly constant beam pattern and fixed phase center over more than a 10:1 operating frequency range. In order to eliminate the back-lobe response over such a wide frequency range, we have projected the sinuous pattern onto a cone, and a ground plane is placed directly behind the cone's apex. This inverted, conical geometry assures wide bandwidth operation by locating each sinuous resonator a quarter wavelength above the ground plane. The presence of a ground plane near a self complementary antenna destroys the self complementary nature of the composite structure resulting in frequency dependent impedance variations. We demonstrate, using simulations and measurements, how the return loss can be improved by modifying the sinuous geometry. A physically smaller, laboratory version of the 0.3 to 3 GHz antenna that is truncated to operate from 1 to 3 GHz was fabricated to verify proper LNA-feed integration through careful modeling and measurements. Over this range, a return loss of better than 9 dB is measured while simulations indicate a nearly constant beam pattern. A full decade bandwidth, low noise amplifier was specially designed for noise match to the higher terminal impedance encountered by this antenna yielding an improved sensitivity over what is possible with conventional 50 Ω amplifiers. A measured system noise temperature of less than 100 K is reported. Based on these results, we will increase the bandwidth of the system to 10:1 by simply attaching additional resonators to the sinuous arms. [ABSTRACT FROM AUTHOR]

The evolution of telescope mounts and domes is presented as the astronomical use of the telescope is about to reach its 400th anniversary. The changes in the mechanical designs are traced to requirements generated from the evolution of detectors and instrumentation. [ABSTRACT FROM AUTHOR]

The article offers information on mounting of telescopes. Topics discussed include categorization of telescope mounts in alt-azimuth and equatorial mounts, maintenance of sidereal time by the mounts for observation of Earth rotation, incorporation of goto facility for supporting celestial objects database, description of Dobsonian mount as simplest altazimuth mounting utilized for Newtonian reflectors, and use of equatorial mounts for observation of objects in night sky.

The detection of structures in the sky with optical surface brightnesses fainter than 30 mag arcsec−2 (3σ in 10 × 10 arcsec boxes; r-band) has remained elusive in current photometric deep surveys. Here we show how present-day telescopes of 10 m class can provide broadband imaging 1.5–2 mag deeper than most previous results within a reasonable amount of time (i.e., <10 hr on-source integration). In particular, we illustrate the ability of the 10.4 m Gran Telescopio de Canarias telescope to produce imaging with a limiting surface brightness of 31.5 mag arcsec−2 (3σ in 10 × 10 arcsec boxes; r-band) using 8.1 hr on source. We apply this power to explore the stellar halo of the galaxy UGC 00180, a galaxy analogous to M31 located at ∼150 Mpc, by obtaining a radial profile of surface brightness down to μr ∼ 33 mag arcsec−2. This depth is similar to that obtained using the star-counts techniques for Local Group galaxies, but is achieved at a distance where this technique is unfeasible. We find that the mass of the stellar halo of this galaxy is ∼4 × 109M⊙, i.e., (3 ± 1)% of the total stellar mass of the whole system. This amount of mass in the stellar halo is in agreement with current theoretical expectations for galaxies of this kind. [ABSTRACT FROM AUTHOR]

In this paper, we describe a hexapod-based telescope mount system intended to provide sidereal tracking for the Fly's Eye Camera project—an upcoming moderate, 21″ pixel−1 resolution all-sky survey. By exploiting such a kind of meter-sized telescope mount, we get a device that is both capable of compensating for the apparent rotation of the celestial sphere and the same design can be used independently from the actual geographical location. Our construction is the sole currently operating hexapod telescope mount performing dedicated optical imaging survey with a sub-arcsecond tracking precision. [ABSTRACT FROM AUTHOR]

Detection of the cosmological neutral hydrogen signal from the Epoch of Reionization (EoR) and estimation of its basic physical parameters are principal scientific aims of many current low-frequency radio telescopes. Here we describe the Cosmological H i Power Spectrum Estimator (CHIPS), an algorithm developed and implemented with data from the Murchison Widefield Array, to compute the two-dimensional and spherically-averaged power spectrum of brightness temperature fluctuations. The principal motivations for CHIPS are the application of realistic instrumental and foreground models to form the optimal estimator, thereby maximizing the likelihood of unbiased signal estimation, and allowing a full covariant understanding of the outputs. CHIPS employs an inverse-covariance weighting of the data through the maximum likelihood estimator, thereby allowing use of the full parameter space for signal estimation (“foreground suppression”). We describe the motivation for the algorithm, implementation, application to real and simulated data, and early outputs. Upon application to a set of 3 hr of data, we set a 2σ upper limit on the EoR dimensionless power at Mpc−1 of mK2 in the redshift range z = [6.2–6.6], consistent with previous estimates. [ABSTRACT FROM AUTHOR]