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Pandey, V. N.; Shankar, N. Udaya; Somanah, Radhakhrishna

doi:10.1023/a:1021125814899

Cited by 3 publications

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“…A two‐dimensional (2D) image in α– sin za coordinates is formed by stacking one‐dimensional (1D) images on the meridian at different sidereal times. Images of approximately a steradian (18 h ≤α≤ 24 h, − 70°≤δ≤−10°) of the southern sky, with an rms noise in images of ∼300 mJy beam −1 (1σ), were produced by Pandey & Udaya Shankar (2005). A suite of programs developed in‐house were used to reduce ∼5000 h of the survey data (a quarter of the total ∼ 20 000 h observed over a span of ∼5 yr).…”

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confidence: 99%

Two-dimensional homography-based correction of positional errors in widefield MRT images

Nayak¹,

Daiboo²,

Shankar³

2010

Monthly Notices of the Royal Astronomical Society

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A steradian of the southern sky has been imaged at 151.5 MHz using the Mauritius Radio Telescope (MRT). These images show systematics in the positional errors of sources when compared to source positions in the Molonglo Reference Catalogue (MRC). We have applied two-dimensional homography to correct for systematic positional errors in the image domain and thereby avoid re-processing the visibility data. Positions of bright (above 15σ ) point sources, common to the MRT catalogue and MRC, are used to set up an over-determined system to solve for the homography matrix. After correction, the errors are found to be within 10 per cent of the beamwidth for these bright sources and the systematics are eliminated from the images. This technique will be of relevance to the new generation radio telescopes where, owing to huge data rates, only images after a certain integration would be recorded as opposed to raw visibilities. It is also interesting to note how our investigations cued to possible errors in the array geometry. The analysis of positional errors of sources showed that MRT images are stretched in declination by ∼1 part in 1000. This translates to compression of the baseline scale in the visibility domain. The array geometry was re-estimated using the astrometry principle. The estimates show an error of ∼1 mm m −1 , which results in an error of about half a wavelength at 150 MHz for a 1-km north-south baseline. The estimates also indicate that the east-west arm is inclined by an angle of ∼40 arcsec to the true east-west direction.

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Section: Introductionmentioning

confidence: 99%

Two-dimensional homography-based correction of positional errors in widefield MRT images

Nayak¹,

Daiboo²,

Shankar³

2010

Monthly Notices of the Royal Astronomical Society

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A steradian of the southern sky at 151.5 MHz using the Mauritius Radio Telescope

Pandey

Shankar

2006

Proc. IAU

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Abstract. We present a few results of a low-frequency southern sky survey carried out using the Mauritius Radio Telescope (MRT).Keywords. catalogs, galaxies: clusters: general, surveys, supernova remnantsWe present and discuss wide field images covering ∼ 1.25 sr (18• ) of the sky at 151.5 MHz with an angular resolution of 4 × 4. 6 and an rms noise of 260 mJy beam −1 . A source catalog of ∼ 2800 sources and its comparative study including cross identification with the Molonglo Reference Catalogue at 408 MHz and Culgoora catalogue at 160 MHz is described. The comparison reveals that there are no systematics in the final source list and the positional accuracies agree typically to about 0.07 times the MRT beam width. The procedures developed for flux calibration, which involves scaling different images to a common level, estimation of the primary beam shape of the helix, and recovery of the amplitude information of the signal lost in a 2-bit, 3-level correlator with an AGC are discussed. Our true flux density estimates agree to within 6.3 % with the known flux densities of unresolved sources in the MS 4Jy sample.Due to low-frequency and availability of short spacings, our images (surface brightness sensitivity ∼ 2.1 × 10 −21 W m −2 Hz −1 sr −1 ) are sensitive to extended sources like giant radio sources, relics and supernova remnants (SNRs). Recently observed sub-structures in X-ray temperature maps support the existence of extended shock waves at locations of several known cluster relics. We show a specific example of such extended radio emission from these proposed shocks around the X-ray cluster Abell 3667 in the MRT image. These are steep spectrum sources, and we expect to detect quite a few such sources in the MRT survey.An additional 0.6 sr of the sky (15• ) has also been imaged and its comparison with existing low frequency images to search for new SNR candidates is under progress. As an example of interesting sources in the images, we briefly discuss a few steep spectrum sources, giant radio sources, double sources, fossil galaxies and report the structure of a few resolved SNRs.

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Oozeer

Shankar

2002

Astrophysics and Space Science

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Cited by 3 publications

References 2 publications

Two-dimensional homography-based correction of positional errors in widefield MRT images

Two-dimensional homography-based correction of positional errors in widefield MRT images

A steradian of the southern sky at 151.5 MHz using the Mauritius Radio Telescope

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