Imaging With The Mauritius Radio Telescope Challenges and Results

The Mauritius Radio Telescope (MRT) has been built with the main objective of surveying the southern sky at meter wavelengths. MRT is a Fourier synthesis, T-shaped non-coplanar array. It consists of a2048 m long East-West arm with 1024 fixed helices and a 880 m long South arm with 15 trolleys. Each trolley has four helices. A 512 channel, 2-bit 3-level complex correlation receiver is used to process the data from the EW and S group outputs. At least 60 days of observing are required for obtaining the Fourier components of the brightness distribution of the sky required to complete the survey. The MRT survey will be one of the most extensive survey at low frequencies providing a moderately deep radio catalog reaching a source density of about 2 × 10⁴ sr^-1 over most of the sky south of δ=-10^°with an angular resolution of 4' × 4.6' sec (δ) and a limiting flux density of 200 mJy (3 σ level) at 151 MHz. This paper will describe the telescope, the observations carried out so far, challenges of imaging with the data acquired over a period exceeding four years with a non-coplanar array, and summarises the results obtained so far. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Mauritius Radiotelescope

The Mauritius Radiotelescope (MRT) is a T-shaped array of helical antennas with a 2048 m EW arm and a 890 m South arm. The primary objective of the telescope is to produce a sky survey in the declination zone -15° to -65° with a point source sensitivity of 200 mJy and an angular resolution of 4'×4.6'cosec(z) at 151.6 MHz, z being the zenith angle . This paper describes the telescope and the present status     

Limitations Of The Tangent Plane Approximation For Wide-field Imaging Using The Mauritius Radio Telescope

The Mauritius Radio Telescope (MRT) is a Fourier synthesis, non-coplanar T array. The primary objective of the telescope is to survey the southern sky at 151.5 MHz in the declination range -70^° to -10^°. Due to non-coplanarity, wide-field imaging and deconvolution of wide field images made using MRT are challenging problems in applications of radio interferometric techniques. This paper discusses the usefulness and limitations of the tangent plane approximation in transforming the measured visibilities to wide field images and in estimating the point spread function (PSF) required for the deconvolution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Detection, excision and statistics of interference at the Mauritius radio telescope

A technique to detect man-made interference in the visibility data of the Mauritius Radio Telescope (MRT) has been developed. This technique is based on the understanding that the interference is generally ‘spiky’ in nature and has Fourier components beyond the maximum frequency which can arise from the radio sky and can therefore be identified. We take the sum of magnitudes of visibilities on all the baselines measured at a given time to improve detectability. This is then high-pass filtered to get a time series from which the contribution of the sky is removed. Interference is detected in the high-pass data using an iterative scheme. In each iteration, interference with amplitudes beyond a certain threshold is detected. These points are then removed from the original time series and the resulting data are high-pass filtered and the process repeated. We have also studied the statistics of the strength, numbers, time of occurrence and duration of the interference at the MRT. The statistics indicate that most often the interference excision can be carried out while post-integrating the visibilities by giving a zero weight to the interference points.     

A digital correlation receiver for the GEETEE radio telescope

A 128-channel digital correlation receiver has been built for the GEETEE 1, the low-frequency radio telescope situated at Gauribidanur, South India, (latitude 13°36′12′′ N). The receiver uses a modified doublesideband (DSB) technique. The quadrature samples required for a DSB system are obtained by sampling the digitized intermediate frequency (I.F.) signals by two clocks which are separated in time by one quarter of the period of the I.F. The visibilities required for one-dimensional synthesis are measured using one-bit correlators. A technique to measure amplitude information for the signal using a threshold detector and a one-bit correlator has been developed. The receiver has been successfully used for continuum, spectral-line and pulsar observations. The antenna system of GEETEE and its configuration for one dimensional synthesis are also described in this paper This telescope is jointly operated by the Indian Institute of Astrophysics, Bangalore and the Raman Research Institute, Bangalore.     

RHESSI Observations of the Size Scales of Solar Hard X-ray Sources

The Reuven Ramaty High-Energy Solar Spectroscopic Imager RHESSI telescope produces hard X-ray images by Fourier imaging techniques that are capable of determining the sizes and shapes of sources with spatial scales in the range ∼ 2′′–180′′. Applying the method of Unpixelized Forward Fitting to RHESSI modulation profiles from simple flares, we have identified the presence of `halo' sources whose size scale (∼ 40′′) greatly exceeds the `core' sizes (≤ 6′′–14′′). Although such `core-halo' structures have been observed at radio wavelengths using a similar technique, the radio and hard X-ray phenomena may be different. These observations raise questions about the nature of these `halos'. Among the possibilities are that they are albedo sources, thin-target loops, or unidentified diffuse structures. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1022484822851     

Quantitative Morphology Of C/1995 O1 (Hale–Bopp) In February – April 1997

Extensive widefield CCD direct imaging of C/1995 O1 (Hale-Bopp) at UBVRI was carried out at Hoher List Observatory with the 1.06 m telescope (field of view 20′ × 20′) and at Potsdam Observatory with the 0.70 m telescope (field of view 8′ × 8′). The corresponding spatial resolution is 850–1000 km pix^-1and 525–590 km pix^-1, respectively. The data covers 25 nights from February 20 to April 21, 1997. In order to quantify the various features in the apparent inner coma we introduce a new tomographic method that minimizes the morphological bias caused by image processing. The tomographic analysis leads to quantitative maps refering to the position and intensity of the dust ejections for each image frame. Variability and periodicity within the inner coma can be thoroughly deduced due to various sets of consecutive nights in the observation period mentioned above. The results are compared with applications of adaptive Laplace filtering. This revised version was published online in July 2006 with corrections to the Cover Date.     

A synthesis map of the sky at 34.5 MHz

This paper describes a wide-field survey made at 34.5 MHz using GEETEE,1 the low frequency telescope at Gauribidanur (latitude 13°36′12′′N). This telescope was used in the transit mode and by per forming 1-D synthesis along the north-south direction the entire observable sky was mapped in a single day. This minimized the problems that hinder wide-field low-frequency mapping. This survey covers the declination range of-50° to + 70° (- 33° to +61° without aliasing) and the complete 24 hours of right ascension. The synthesized beam has a resolution of 26′ x 42′ sec (δ- 14°. 1). The sensitivity of the survey is 5 Jy/beam (1σ). Special care has been taken to ensure that the antenna responds to all angular scale structures and is suitable for studies of both point sources and extended objects This telescope is jointly operated by the Indian Institute of Astrophysics, Bangalore and the Roman Research Institute, Bangalore.     

Scientific program construction principles and time allocation scheme for the World Space Observatory—Ultraviolet mission

We present scientific program construction principles and a time allocation scheme developed for the World Space Observatory—Ultraviolet (WSO-UV) mission, which is an international space observatory for observation in UV spectral range 100–300 nm. The WSO-UV consists of a 1.7 m aperture telescope with instrumentation designed to carry out high resolution spectroscopy, long-slit low resolution spectroscopy and direct sky imaging. The WSO-UV Ground Segment is under development by Spain and Russia. They will coordinate the Mission and Science Operations and provide the satellite tracking stations for the project.     

Searches for HH objects in star formation regions. VII. Herbig-Haro objects in the region of the nebula GM 2–41

Five new Herbig-Haro objects (HH 1036–1040) have been discovered in the neighborhood of the nebula GM 2–41 in a region with an area of 14′ × 14′, at the center of the HII region DR 15 located in the southern periphery of the Cyg OB2 association. Four of them have a complex structure typical of HH flows. Hydrogen molecular emission is detected in the object HH 1036 using archived images from the Spitzer telescope. Two new infrared nebulae illuminated by very red young stellar objects are also found.     

The GEM project: An international collaboration to survey galactic radiation emission

The GEM (Galactic Emission Mapping) project is an international collaboration established with the aim of surveying the full sky at long wavelengths with a multi-frequency radio telescope. A total of 745 hours of observation at 408 MHz were completed from an Equatorial site in Colombia. The observations cover the celestial band 0 ^h <<24 ^h , and –24° 22<<+35° 37. Preliminary results of this partial survey will be discussed. A review of the instrumental setup and a 10° resolution sky map at 408 MHz is presented.Presented by S. Torres at the UN/ESA Workshop on Basic Space Sciences: From Small Telescopes to Space Missions, Colombo, Sri Lanka 11–14 January 1996     

Investigation of the stellar population and determination of interstellar extinction toward the Chandra Bulge Field based on RTT-150 data

We have investigated the stellar population of the Chandra Bulge Field (CBF) 35′ × 35′ in area using the Russian-Turkish RTT-150 telescope with the goal of constructing an interstellar extinction map and determining the extinction law. The optical extinction has been determined from the positions of red clump giants (a group of red giants with the same luminosity and color) on the color-magnitude diagram in different parts of the field. This has allowed an interstellar extinction map of the field under consideration to be constructed with a resolution of 1′ × 1′. Based on the results of our analysis, we have also shown that the extinction law in the investigated field differs significantly from the standard one, most likely because the dust properties in the Galactic bulge differ from those in the Galactic disk. The derived extinction law confirms the measurements in the outer parts of the Galactic bulge.     

Exploring morphological correlations among H2CO, 12CO, MSX and continuum mappings

There are relatively few H₂CO mappings of large-area giant molecular cloud (GMCs). H₂CO absorption lines are good tracers for low-temperature molecular clouds towards star formation regions. Thus, the aim of the study was to identify H₂CO distributions in ambient molecular clouds. We investigated morphologic relations among 6-cm continuum brightness temperature (CBT) data and H₂CO (1₁₁−1₁₀; Nanshan 25-m radio telescope), ¹²CO (1–0; 1.2-m CfA telescope) and midcourse space experiment (MSX) data, and considered the impact of background components on foreground clouds. We report simultaneous 6-cm H₂CO absorption lines and H110α radio recombination line observations and give several large-area mappings at 4.8 GHz toward W49 (50′×50′), W3 (70′×90′), DR21/W75 (60′×90′) and NGC2024/NGC2023 (50′×100′) GMCs. By superimposing H₂CO and ¹²CO contours onto the MSX color map, we can compare correlations. The resolution for H₂CO, ¹²CO and MSX data was ∼10′, ∼8′ and ∼18.3″, respectively. Comparison of H₂CO and ¹²CO contours, 8.28-μm MSX colorscale and CBT data revealed great morphological correlation in the large area, although there are some discrepancies between ¹²CO and H₂CO peaks in small areas. The NGC2024/NGC2023 GMC is a large area of HII regions with a high CBT, but a H₂CO cloud to the north is possible against the cosmic microwave background. A statistical diagram shows that 85.21% of H₂CO absorption lines are distributed in the intensity range from −1.0 to 0 Jy and the ΔV range from 1.206 to 5 km s⁻¹.     

Construction of Wide Field Cryogenic Telescope

We constructed a wide field cryogenic telescope (WFCT) containing a whole Ritchey-Chrétien system and a focal plane array in a cryostat for near infrared observations to cover the field of view of 0.4°. The telescope has a primary mirror of 220 mm and an engineering grade 256 × 256 InSb array. The optical components such as two mirrors, filters, spiders, and radiation shield tube are cooled down to 180 K as well as the InSb array to 35 K by a mechanical refrigerator. We show the results of the background surface brightnesses and the limiting magnitudes at 3.3 and 3.67 μm measured at Sutherland, South Africa. We describe the on-going upgrade of this instrument, equipped with a 1024 × 1024 ALADDIN InSb science grade array. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Evolutionary Status of Early-type Galaxies in Abell 2390

We explore the evolution of the early-type galaxy population in the rich cluster Abell 2390 at z=0.23. For this purpose, we have obtained spectroscopic data of 51 elliptical and lenticular galaxies with MOSCA at the 3.5 m telescope on Calar Alto Observatory. As our investigation spans both a broad range in luminosity (–22.3≤M_B≤–:19.3)and a wide field of view (10′×10′), the environmental dependence of different formation scenarios can be analysed in detail as a function of radius from the cluster center. In this paper, we present first results on the Faber-Jackson relation and, for a subsample of 14 galaxies with morphological and structural parameters from HST, we also investigate the evolution of the Kormendy relation and the Fundamental Plane. We find a mild luminosity evolution of the early-type galaxies in Abell 2390: our objects are on average brighter by m _B∼0.4 mag. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Performance of MEFOS, the ESO Multi-Object Fibre Spectrograph

We are describing a new multi-fibre positioner, MEFOS, that was in generaluse at the La Silla Observatory, and implemented at the prime focus of theESO 3.6 m telescope. It is an arm positioner using 29 arms in a one degreefield. Each arm is equipped with an individual viewing system for accuratesetting and carries two spectroscopic fibres, one for the astronomical objectand the other one for the sky recording needed for sky subtraction. Thespectral fibres intercept 2.5 arcsec on the sky and run from the prime focusto the Cassegrain, where the B&C spectrograph is located. Afterdescribing the observational procedure, we present the first scientificresults.     

Brazilian Decimetric Array (Phase-I)

An East – West, one-dimensional radio interferometer array consisting of five parabolic dish antennas has been set up at Cachoeira Paulista (longitude 45°0′20″ W, latitude 22°41′19″ S) for observations of the Sun and some of the strong sidereal sources by the Instituto Nacional de Pesquisas Espaciais (INPE), Brazil. This is Phase-I of the proposed Brazilian Decimetric Array and can be operated at any frequency in the range 1.2 – 1.7 GHz. The instrument has been in operation since November 2004 onwards at 1.6 GHz. The angular and temporal resolutions at this frequency are ∼3′ and 100 ms, respectively. Details of the array, analog/digital receiver system, and a preliminary East – West one-dimensional solar image at the 1.6 GHz are presented in this paper.     

The Ooty synthesis radio telescope: First results

A 4-km synthesis radio telescope has recently been commissioned at Ootacamund, India for operation at 327 MHz. It consists of the Ooty Radio Telescope (530 m × 30 m) and 7 small antennas which are distributed over an area of about 4 km × 2 km. It has a coverage of about ± 40‡ in declination δ. The beam-width is about 40 arcsec × 90 arcsec at δ = 0‡ and about 40 arcsec × 50 arcsec at δ = 40‡. The sensitivity attained for a 5:1 signal-to-noise ratio is about 15 m Jy after a 10-hour integration. The observational programmes undertaken and some of the results obtained recently are summarized. The radio halo around the edge-on spiral NGC 4631 is found to have a larger scale-height at 327 MHz than is known at higher frequencies. Mapping of interesting radio galaxies at 327 MHz is being carried out; preliminary results for 0511-305 (∼2 Mpc) and 1333-337 (∼750 kpc) are summarized. The very-steep-spectrum radio source in the Abell cluster A85 is found to be resolved; since it has no obvious optical counterpart, it is conceivable that it is a remnant of past activity of a galaxy that has drifted away in about 10⁹ years.     

SciMeasure Wavefront Sensor Cameras and their Application in the Palomar Adaptive Optics System

SciMeasure, in collaboration with Emory University and the Jet Propulsion Laboratory, has developed a very versatile CCD controller for use in adaptive optics, optical interferometry, and other applications requiring high-speed readout rates and/or low read noise. The overall architecture of this controller system will be discussed and its performance using both EEV CCD39 and MIT/LL CCID-19 detectors will be presented. This controller is used in the adaptive optics system, developed by JPL, for the 200′′ Hale telescope at Palomar Mountain. Early diffraction-limited science results, recently achieved by the AO system, are presented. We gratefully acknowledge the financial support of NASA through SBIR contracts NAS8–97195 and NAS8–98081. This revised version was published online in July 2006 with corrections to the Cover Date.     

A New TeV Source Confirmed in Whipple Archival Data: TeV J2032+41

A re-analysis of data near Cygnus X-3 in 1989–1990 using the Whipple Observatory atmospheric Cherenkov imaging telescope confirms the existence of the TeV J2032 + 4130 source first reported at a conference by the Crimean Astrophysical Observatory and confirmed independently by the HEGRA Collaboration in a referred publication. The significance of the Whipple observations at the a priori HEGRA position is 3.3σ. The peak signal was found at RA = 20 h 32 m, Dec = +41^∘ 33′. This is 0.6^∘ north of Cygnus X-3. The flux level (12% of the level of the Crab Nebula) is intermediate between that reported by the Crimean (100%) and HEGRA (3%) groups.