Showing papers in "Experimental Astronomy in 2001"

The New Multichannel Radiospectrograph ARTEMIS-IV/HECATE, of the University of Athens

Abstract: We present the new solar radiospectrograph of the University of Athens operating at the Thermopylae Station since 1996. Observations cover the frequency range from 110 to 688 MHz. The radiospectrograph has a 7-meter parabolic antenna and two receivers operating in parallel. One is a sweep frequency receiver and the other a multichannel acousto-optical receiver. The data acquisition system consists of a front-end VME based subsystem and a Sun Sparc-5 workstation connected through Ethernet. The two subsystems are operated using the VxWorks real-time package. The daily operation is fully automated: pointing of the antenna to the sun, starting and stopping the obser- vations at pre-set times, data acquisition, data compression by 'silence suppression', and archiving on DAT tapes. The instrument can be used either by itself to study the onset and evolution of solar radio bursts or in conjunction with other instruments including the Nancay Decametric Array and the WIND/WAVES RAD1 and RAD2 low frequency receivers to study associated interplanetary phenomena.

A Seeing Monitor for Solar and Other Extended Object Observations

Abstract: In this article I describe a site survey facility, which measures the signals of the solar equivalent of a Differential Image Motion Monitor (S-DIMM) and of a six element linear array of solar scintillometers. Combining the S-DIMM ro andscintillometer σI observations allows the determination of the fractions of the seeing in the free atmosphere and in the ground/lake layer. From the scintillometer array observations Cn2(h) is determined for heights corresponding to the first ∼500 m along the line-of-sight. With minor changes this seeing monitor can also be used for other extended objects like the Moon and planets.

The New Preamplification System for the UTR-2 Radio Telescope

Abstract: The paper describes the preamplification antenna system (PAS) ofthe decameter radio telescope UTR-2. The new PAS have the continuous frequency range 8 to 40 MHz. The dynamic range is: 54 dbm at low frequencies and 30 dbm at high frequencies. To extendthe dynamic range the signal spectrum is divided and summed up with the aid of frequency selective devices, each consisting oftwo identical antimetric bandpass filters of even order and twohybrid adders. Within each of the three bands, the signal is amplified in multistage amplifiers with a deep linear frequencyindependent negative feedback in each stage. Part One presents thegoals, underlying ideas and structure of the PAS. Part Twodescribes details of the design, specific parameters and some observational results.

OPTIMA: A Photon Counting High-Speed Photometer

Abstract: OPTIMA is a small, versatile high-speed photometer which is primarily intended for time resolved observations of young high energy pulsars at optical wavelengths. The detector system consists of eight fiber fed photon counters based on avalanche photodiodes, a GPS timing receiver, an integrating CCD camera to ensure the correct pointing of the telescope and a computerized control unit. Since January 1999 OPTIMA proves its scientific potential by measuring a very detailed light-curve of the Crab Pulsar as well as by observing cataclysmic variable stars on very short timescales. In this article we describe the design of the detector system focussing on the photon counting units and the software control which correlates the detected photons with the GPS timing signal.

Optimal Baffle Design in a Cassegrain Telescope

Abstract: An exact analytical procedure is described for the optimum design of baffles for any Cassegrain telescope with conicoid type mirrors. Here, `optimum' means the least obstruction coefficient given perfect blocking of direct light by baffles. The corresponding algorithm is based on the ray-tracing formulas in a two-mirror telescope with arbitrary position of the aperture stop. The optimal configuration of baffles is unique. The dependence of the obstruction coefficient upon the telescope characteristics is studied. The optical system can be designed in such a way that the obstruction is equal to a predefined value. As examples, the Hubble Space Telescope and the Ritchey–Chretien system with the obstruction coefficient 0.25 are considered.

The EVN-MarkIV VLBI Data Processor

Abstract: A functional description is given of the new 16-station MarkIV VLBI data processor for the European VLBI Network. The data processor can operate in many configurations including sensitive continuum modes, line modes with unprecedented spectral resolution, and wide field imaging and pulsar gating modes. The EVN-MarkIV processor was developed by an international European/U.S. consortium. Several similar processors are deployed in both Europe and in the U.S.

Using laser beacons for daytime adaptive optics

Abstract: This article examines the use of Laser Beacons for daytime astronomical observations. There are two potential applications: the diffraction limited observation of (1) the structure in the solar corona at all wavelengths, and (2) non-solar astronomical objects in the thermal infrared part of the spectrum. We examine the brightness of the Laser Beacon required as well as the limitations imposed by the daytime sky brightness and sky/telescope thermal emission on the observable magnitude limits. For both applications the use of Laser Beacon adaptive optics in daytime results in important research opportunities.

1-Gbps VLBI, The First Detection of Fringes

Abstract: We report on the detection of VLBI fringes from quasars by a new VLBI system operating at 1 Gbps (1024 Mbits-per-second). Newly developed 1024 Msps (mega sample-per-second) AD samplers and 1024 Mbps recorders were used for the observations. A correlator with external buffers was used for the 1024 Mbps correlation processing of the tapes data.Our new VLBI system enabled 1024 Mbps VLBI, and this allowed the sampling of a 512 MHz bandwidth from a radio telescope receiver. This is the highest sampling speed ever used for VLBI, and the widest bandwidth used for VLBI observation. Initial sensitivity as evaluated by SNR comparison with earlier VLBI systems produced results to matched the expanded bandwidth. In our first observations, simultaneous optical fibre linked real-time VLBI observations were made to check the validity of data and precisely detemine the clock offsets among the radio telescopes.

Multi-Conjugate Adaptive Optics with Two Deformable Mirrors – Requirements and Performance

Abstract: In order to increase the corrected field of view of an adaptive optics (AO) system, several deformable mirrors (DM) have to be placed in the conjugate planes of the dominant turbulent layers (multi-conjugate adaptive optics,MCAO (Beckers, 1988)).The performance of MCAO systems depends on the quality of thewavefront sensing ofthe individual layers and on the number of corrected modes in eachindividual layer as in single layer AO systems. In addition, the increase in corrected field of view depends on the number of guide stars providing information about theturbulence over a sufficiently large area in each turbulent layer. In this article, we investigate these points and provide formulae for calculating the increased field of view with a new approach using the spatial correlation functions of the appliedpolynomials (e.g. Zernike). We also present a new scheme of measuring the individual wavefront distortion of each of the dominantlayers with a Shack-Hartmann-Curvature Sensor using gradientinformation as well as scintillation. An example for the performance of a two layer MCAO system is given for the 3.5-m telescope of the Calar Alto Observatory, Spain, using ameasured Cn 2-profile. The corrected field of view in K-band(2.2 μm) can be as large as 3 arcmin with a Strehl ratio above 60%.

CFH12K: Optimizing 12 MIT/LL CCID20 CCDs for a Direct Imaging Application

Abstract: CFH12K is a 12 k by 8 k wide field imaging camera for the CFHT prime focus. The mosaic consists of twelve MIT Lincoln laboratories 2 k by 4 k thinned backside illuminated CCID20 devices. The devices’ operating parameters have been optimized to ensure the best data quality for use in broad and narrow band filter imaging mode. Adaptation to the CFHT prime focus environment included modifications to reduce the scattered light seen by the CFH12K. Science data taken by the camera has proven the success of CFHT’s new capability for 42 by 28 arcmin2 imaging with high resolution subarcsecond seeing.

Degradation due to Quantization Noise in Radio Astronomy Phased Arrays

Abstract: In this paper, a model is develped for determining the probability distribution of the output of a digital ader in case of 2-, 3- and 4-level quantization before summation. This probability distribution is then used to determine the efficiency of a system which determines the total power of the signal at the output of the digital adder. The sumsignal is also used in a VLBI correlation mode where the sumsignal is correlated with the signal from a VLBI partner. The effeciency, expressed in so-called Degradation Factors, is determined for 1- and 2-bit correlation and 2-, 3- and 4-level quantization before summation. Furthermore, the application of the calculated values (Total Power Detector Efficiencies and Degradation Factors) is illustrated.

Gamma Ray and Hadron generated Čerenkov Photon Spectra at Various Observation Altitudes

Abstract: We study the propagation of Cerenkov photons generated by Very HighEnergy γ-rays and hadrons in the atmosphere. The photon productionheight distributions are estimated from semi-empirical methods andcompared with those derived by standard simulation techniques. Incidentspectra at various observation altitudes are then derived after applyingwavelength dependent corrections due to photon attenuation in theatmosphere during the propagation of photons from the height of productionto the height of observation. These are generated both for γ-ray and hadron primaries of various energies. The derivedproduction height distributions agree very well with those generated bythe simulation package `CORSIKA' at all energies and for both γ-ray and proton primaries. The incident photon spectra are found to beboth altitude and primary energy dependent. The peak ofthe incident spectrum shifts towards the shorter wavelength withincreasing altitude of observation for a given primary. Also the peak ofthe photon spectrum shifts towards the shorter wavelength withincreasing energy of the primary at a given altitude. The fraction of the UVcomponent in the incident Cerenkov spectrum is estimated both forγ-ray and hadronic primaries at various observation altitudes andenergies. Hadron generated Cerenkov spectra are marginally richer in UVlight and the difference increases slightly at higher altitudes. The fraction of the UV to the visible light in the Cerenkov spectrum could be a usefulparameter to separate γ-rays from cosmic ray background only if onecan measure this fraction very accurately.

Speeding Up Image Reconstruction Methods in Coded Mask γ Cameras Using Neural Networks: Application to the EM Algorithm

Abstract: When using γ-ray coded-mask cameras, one does not get a direct image as in classical optical cameras but the correlation of the mask response with the source. Therefore the data must be mathematically treated in order to reconstruct the original sky sources. Generally this reconstruction is based on linear methods, such as correlating the detector plane with a reconstruction array, or non-linear ones such as iterative or maximization methods (i.e. the EM algorithm). The latter have a better performance but they increase the computational complexity by taking a lot of time to reconstruct an image. Here we present a method for speeding up such kind of algorithms by making use of a neural network with a back-propagation learning rule.

Optimizing CCDs for spectrographs

Abstract: For a spectrograph giving a fixed format spectrum,the quantum efficiency (QE) can be optimized for the different wavelengthsacross the CCD. It is shown that a slight modification of the conventionalsingle layer anti-reflection coating can give major improvements in QE forsuch instruments, while at the same time minimizing problems with fringingand stray light from the CCD.

Array Configurations That Tile the Plane

Abstract: Future radiotelescopes need ever increasing sensitivity. With the limits of receiver sensitivity being reached this can only be achieved by increasing collecting area. The increase in antenna numbers and the multi-resolution capabilities of these telescopes, such as the SKA, means a significant departure from existing designs is needed. In this article, some fundamental designs for antenna arrays that uniformly sample the UV plane are presented. The SKA is used to provide target specifications. The designs presented allow asymmetric configurations providing more freedom for site selection. They also allow a direct tradeoff between field of view and correlator complexity.

TACTIC Imaging γ-Ray Telescope: A Simulation Study of the Effect of Switched-off Pixels on Its Quality Factor

Abstract: The 349-pixel Cerenkov Light Imaging Camera of the γ-raytelescope array, TACTIC, has been recently commissioned at Mt.Abu, India. This telescope is being used for gamma-ray sourceobservations at photon energies above 0.5 TeV. Comprehensivesimulation studies, using the CORSIKA air-shower code, are inprogress to predict and optimize the performance of the TACTIC.In this short note, we study the expected performance of theImaging Element of the TACTIC under conditions of a pixelswitch-off, forced either by the well-known star-rotationphenomenon or by a defective instrumentation channel. Thechanges in the image parameters, and the resulting telescopequality factor, are studied after simulating two representativecases of pixel shut-down.