N. UDAYA SHANKAR
Articles written in Journal of Astrophysics and Astronomy
Volume 11 Issue 3 September 1990 pp 297-310
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
Volume 11 Issue 3 September 1990 pp 311-322
A simple but effective modification to the conventional CLEAN algorithm is suggested. This modification ensures both stability and speed when CLEAN is applied to maps containing a mixture of point sources and extended structures. The method has been successfully applied to the recently-completed sky survey at 34.5 MHz (Dwarakanath & Udaya Shankar 1990). This survey was made using the Gauribidanur T array (GEETEE)1 in 1-D aperture synthesis mode. Since in this case the ‘dirty beam’ (point spread function) cannot be directly computed, a method to obtain this is discussed in detail. The results of this deconvolution procedure have been encouraging in terms of reduced computing time and improved dynamic range in our maps. This algorithm should find wider application in deconvolving maps which have both extended structures and point sources
Volume 11 Issue 3 September 1990 pp 323-410
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
Volume 19 Issue 1-2 June 1998 pp 35-53
A new, meter-wave radio telescope has been built in the north-east of Mauritius, an island in the Indian Ocean, at a latitude of -20.14‡. The Mauritius Radio Telescope (MRT) is a Fourier Synthesis T-shaped array, consisting of a 2048 m long East-West arm and an 880 m long South arm. In the East-West arm 1024 fixed helices are arranged in 32 groups and in the South arm 16 trolleys, with four helices on each, which move on a rail are used. A 512-channel digital complex correlation receiver is used to measure the visibility function. At least 60 days of observing are required for obtaining the visibilities up to 880 m spacing. The Fourier transform of the calibrated visibilities produces a map of the area of the sky under observation with a synthesized beam width 4′ × 4.6′ sec(δ + 20.14‡) at 151.5 MHz.
The primary objective of the telescope is to produce a sky survey in the declination range –70‡ to –10‡ with a point source sensitivity of about 200 mJy (3a level). This will be the southern sky equivalent of the Cambridge 6C survey. In this paper we describe the telescope, discuss the array design and the calibration techniques used, and present a map made using the telescope.
Volume 22 Issue 2-3 June 2001 pp 213-227
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.
Volume 32 Issue 4 December 2011 pp 425-426
Volume 44 All articles Published: 28 March 2023 Article ID 0024 ORIGINAL RESULTS
A key science project for SKA-Low is the interferometer imaging of spatio-temporal fluctuations in the hydrogen ionization fraction and spin temperature as a diagnostic of cosmic dawn (CD) and epoch of reionization (EoR) in the redshift range $6 \lesssim z \lesssim 28$. However, detection of the global CD/EoR signal, which provides the differential HI 21-cm brightness temperature with respect to the background radiation temperature, is outside the current purview of SKA-Low observing in interferometer mode. This global CD/EoR signal can provide the critical zero-spacing measurement vital for establishing the base level of fluctuations measured by SKA-Low interferometers. We present here, a concept to detect the global CD/EoR signal as a supplement to the CD/EoR key science of SKA-Low. This would be enabled by adding a compact array of outrigger antennas interspersed between the core stations of SKA-Low. The autocorrelations of the outrigger antennas would form the measurement set for the global CD/EoR. The visibilities measured between outriggers and SKA stations would be `blind' to the global CD/EoR signal and would provide a Global Sky Model and calibration of the bandpass and mode-coupling in the primary beam of the outriggers. The global signal measurement capabilitywill strengthen the SKA-Low CD/EoR program.
Volume 44, 2023
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