Soobash DaibooArvind Nayak

N Udaya ShankarGirish Kumar Beeharry

Mauritius Radio Telescope Raman Research Institute

The Southern Sky at 151.5 MHz:Latest Results

Outline

· The MRT· Brief overview of a steradian of MRT survey (Pandey &

Uday Shankar 2006).· 2-D Homography to correct for positional errors in images.· Re-estimation of array geometry.· Validation of the estimation of array geometry.· Fields to be imaged· Requirements 24-hour total power curve. Field-of-view calibration. Retuning hardware & software environments.· Results of imaging with the new pipeline.

The Mauritius Radio Telescope

- Joint project of UoM, RRI & IIA- Located at Bras d;Eau, Mauritius, (20S,57E)- T-shaped 2 km EW, 0.88 km NS - Non-coplanar EW arm- Helical dipoles: 32 EW groups of 32 & 16 NS movable

groups of 4- Transit instrument with tracking for pulsar obs.- Fourier synthesis: 60 days to complete visible sky- Primary Beam 60o

- 512 complex correlator- 2 bit 3 level correlation- 1024 cross-corr. + 64 auto corr.

The Mauritius Radio Telescope

Pandey & Udaya Shankar (2006) : images and a source catalogue of a steradian (Sr) of sky*. Right Ascension (RA) range covered: 18h to 00h30m. Declination (DEC) range covered: -70° to -10°. Frequency: 151.5 MHz. Integration time: 4 s. Bandwidth: 1 MHz. Angular resolution: 4' X 4'.6 sec(δ+20°.14). RMS noise (1-σ): ~320 mJy/beam. Source catalogue: ~2800 sources. Almost complete uv coverage. Sky coverage of MRT is ~4.8 sterads( 0.38 of Total Sky)

A steradian of the MRT survey: specs

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A steradian of the MRT survey: sources

· ~2500 sources in the steradian of sky imaged by MRT.

· ~2000 sources in the Molonglo Reference Catalogue (MRC).

· ~1700 MRT sources common to MRC.

Most of the ~300 sources in MRC not detected in MRT images are at the ends of the declination range of MRT where attenuation due to helix primary beam is maximum.

A steradian of the MRT survey: sources

A steradian of the MRT survey: Sample Images

· The Molonglo Reference Catalogue (MRC) at 408 MHz covers the sky between Dec -85° to 18°.5.· Used as reference catalogue.· It completely overlaps the region covered by MRT survey.· Sky coverage ~7.5 steradians. · Angular resolution: 2'.62X 2'.86 sec(δ+35°.5).· RMS noise: ~700 mJy/beam.· MRC Source catalogue: ~12000 sources (~2000 in the region of overlap with MRT).· The catalogue is complete at 1 Jy.

2-D Homography correct ions for positional errors in images.

Errors in RA & sin(ZA)

Errors in RA

Errors in sin(ZA)

Positional Errors

MRT contours on MOST (843 MHz) images

· To correct for systematic errors in the positions of sources in the images, phase correction has to be applied to the visibility data.· At MRT the procedure involved in the transfer of visibilities to images is too long.· Possible alternative was to correct for the systematic errors in image domain.· 2-D homography for correction of positional errors.

Positional error correction

2-D Homography

· Relates corresponding points in two images of a same scene by a transformation matrix.- Given a set of points x in P2 and a corresponding set of points x' like P2 space, homography computes the projective transformation that takes x to x'. Here x and x' represent RA & sin(ZA) coordinates from MRT and MRC respectively.The homography sought is a 3 x 3 matrix H such that:

H x = x'

2-D Homography

· 2 independent equations per point.· Minimal solution To estimate affine transformation: 3 points yield an exact solution for H. To estimate projective transformation: 4 points yield an exact solution for H.· More points No exact solution, because measurements are inexact ("noise"). Search for "best" in a least-squares sense.

Block schematic

· The steradian of sky has 400 sources (above 15-σ) common to the MRT and MRC catalogues.· Estimated a single homography matrix from the entire source population

Positional errors before & after correction

Positional errors before & after correction

MRT contours overlaid on MOST images after correction

Re-estimation of array geometry.

· For the new imaging::the cause for positionalerrors?· A look at the positional errors as a function of sin(ZA) indicated:

Estimated errors· About 600 calibration tables used (3 calibrators X 60 allocations X 3 file per alloc).· Over-determined linear system solved by singular value decomposition.

Gradient: 1 mm/mMatches with the 1 part per 1000 seen in both error analysis and homographymatrix.

Top- Phase differences between two calibrators in the NS direction

Bottom- Estimate of error in NS shown against the NS direction

Validation of the new array geometry· We re-imaged half a steradian of the sky in the RA range: 22h to 01h· This required re-generation of: files with new array geometry. New position dependent point spread functions (PSFs) for deconvolution. · 240 X 60 PSFs each of 18° X 15°. A total of ~15 GB of disk space.

Gap to be imaged

Problems in imaging the gap· Only two calibrators (MRC1932-464 and MRC0915-118) with a separation of 10 hours in RA available to image the gap· Includes regions with Galactic plane.· Includes regions which have Galactic plane and Sun in the grating response.· Regions getting fauther from the primary calibrators. Quality factor (QF) taken as average of the QF of data file and QF of calibrator file. This lowers the overall quality of the observed data when imaging regions farther from the calibrator. · This leads to incomplete uv coverage at unacceptable levels.· Ionospheric scintilations has been observed. Calibration values derived many hours before or after the RA being observed do not give the correct calibration· Unpublished images by Pandey and Uday Shankar show an increase in the noise when we image further away from the calibrators even though the sky background is decreasing.· This compelled us to develop a field-of-view calibration scheme.

Validation of the new array geometry· We re-imaged half a steradian of the sky in the RA range: 22h to 01h· This required re-generation of: files with new array geometry. New position dependent point spread functions (PSFs) for deconvolution. · 240 X 60 PSFs each of 18° X 15°. A total of ~15 GB of disk space.

Requirements for imaging the gap

· 24-hour total power curve.· Field-of-view calibration.· Retuning the hardware & software environments.

24-hour total power curve

· In a 2-bit 3-level correlator at MRT, the AGCs maintain a constant signal level to the samplers, even though the brightness distribution of the sky changes.· Amplitude information of the signal from the sky is lost resulting in similar correlation for a weak source in a weak background and a strong source in a correspondingly stronger background.· To obtain the amplitude information due to variation in the background temperature as seen by the EW and NS groups, the self correlations were also measured separately by switching off the AGCs in one EW (E16) and one NS (S15) groups.

Total Power

·Total power estimated from the measured self correlation counts of the EW and NS groups.· Longest stretch is ~12 hours.· Curves obtained by averaging the total powers estimated on different days.

Continuous 24-hour total power curve obtained by linear regression

Field of view calibration· To overcome these problems we devised a scheme where the field being imaged is used to derive the calibration table.· The MRC and MRT catalogues show there are enough sources which can be collectively used for calibration available in each hour and the four declination zones being imaged for field of view calibration. There are ~30 above 10-sigma sources per imaging zone.· The EW array sees 1 radian of the sky in the DEC. Whereas, EWxNS (long baselines) sees ~15°. Bandwidth de-correlation curve for different baselines takes the above aspect into account for a model of the sky.· We flag side lobes of very strong out-of-field sources like Cyg-A, Cas-A· Tested the scheme on the RA 19h00m to 20h00m where we also have the calibrator MRC1932-464. The single-source calibration with a strong source and the field-of-view of calibration with multiple weaker sources show similar calibration tables.

Re-tuning of hardware & software environments

· While field of view calibration was successful it brought in new problems. This required re-tuning of hardware & software Environments.

· 7 x 2 GHz processors PC's in a 100 Mbps NFS network.

· 10 Terabyte of total disk space.

Re-tuning of hardware & software environments

· Re-organisation of MRT data archive into a uniform format. Depending on the DEC zone to be imaged and the relative timing of the observation cycle and the system development at MRT, there were 56 versions of precompiled MARMOSAT which had to be manually chosen. This has been automated and a standard header is generated for facilitating automation.· New calibration routine developed in Matlab.· Takes approx 5 hours to generate calibration table for one hour.· Generating dirty images from 1-hour visibility files on all allocations takes approximately 3 days if run non-stop on 7 computers.· An interprocess communication between the computers based on lock-files ensures that the computers work on unique files.· Integrated environment of new, extended and the old MARMOSAT

The data processing pipeline of MRT

Images with the new calibration scheme

· Imaged approximately 0.5 steradians:

RA 23h00m to 02h00m and DEC range -10° to -70°.

· ~1200 sources at 5-sigma detection level.

· Noise is approx 260 mJy. This is about 20% below the old noise level of 320 mJy. Equivalent to 40% increase in the amount of usable data.

3-hour image of a zone

Zoom in view

Extended source overlaid on MOST image

·Contour levels: 5,7,10,14,22,48,100X RMS.

RMS: ~260 mJy/beam

Comparison with MRC· 725 MRC sources in the same region.· 620 common sources between MRT and MRC

Near Future Work