Daniel Mitchell (SAO / CfA)

For the MWA Consortium

The MWA Consortium

• MIT-Haystack• MIT campus (MKI)• Smithsonian Inst.• Harvard U.• U. Melbourne• Curtin U. of Tech.

Also contributions from individuals/groupsat U. Wisc, U. Wash, CalTech

• U. WA• U. Tasmania• U. Sydney• ANU• Raman Research Inst.• CSIRO

32T Results (Field centered on Pic A)

First 32 tiles: ~ 5% demonstrator

Longer tracks in January 2010

RTS “First light” in December 2009

Outline

Overview of the site Murchison Radio-astronomy Observatory (MRO) RFI Environment

Overview of the signal path Antennas, receivers, correlator

Overview of the processing pipeline Real-time imaging system

Photos: D. Oberoi & D. Mitchell

Boolardy Stn., Western Australia

MRO Radio Quiet Zone 100 – 150 km radius restricted zone Remote. Limited access to site & data Power constraints Remote operation & monitoring

Very low spectral occupancy (time & freq)

Shire of MurchisonPop: ~110 in 41,173 km2

only AU shire without a town

RFI Environment

Redshifted HI: VHF band (80-300 MHz)

• Sources of RFI: Satellite and aircraft signals. Distant transmitters enhanced by

reflections off meteor trails or aircraft. Distant transmitters enhanced by

occasional refractive ducting. Local RFI.

• See Judd Bowman's presentation on Wednesday morning for an more on MRO RFI.

Min/Max over a few days in 2007 (Bowman)

ORBCOMM other satellites

Terrestrial?

Aviation, ... Offset due to the galaxyFM radio RADCAL, DMSP F15, ...

ORBCOMM• 137–138 MHz circular. (~2.5 kHz)

• 26 LEO satellites (3x8 + 2 polar)

• 3 or 4 x ~3 hrs windows per day

• Useful for beam characterization

Satellite trails and approx. power over 24 hrs

MWA Goals

radiation gas-kinetic

SOHO/EIT Pritchard & Loeb 2008

Testori et al. (2001, 2004) / Wooleben et al. (2005)

Chatterjee & Murphy (adapted from Cordes et al. 2003)

EoR via redshifted HI Solar & Heleospherical

Galactic & Extragalactic Transients

MWA Specifications

• Lots of cheap antennas (512 antenna tiles)

• Good snapshot beam (>105 concentrated baselines)

• Full polarisation

• Wide frequency range (31 MHz from 80–300 MHz)

• Wide, steerable FOV (10-50 degrees)

• A few arc-minute resolution (1.5 km: 2.5'-8.5')

• Max BL ~ km (isoplanatic patch size): ~ 2D ionosphere

• Phase 1 RFI plan: go somewhere quiet and flag

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MWA Antenna Tile Beams Have sidelobe structure at -10 to -20 dB

All sky Nulls

Are time variable Dipoles point to zenith Not fixed to RA/Dec Little dynamic control

Are tile dependent Different for each visibility

Require a fix in gridding and during deconvolution

Are relatively simple Small number of components No moving parts

Have resistance to terrestrial RFI Ground plane creates null at horizon Tiles are low to the ground

MWA Signal Path

Simple median-based Flagging

• Current real-time RFI arsenal consists of a simple flagging routine to assist calibration.

• Will run at the start of the RTS, while integrating visibilities for 2 – 8 seconds.

• Flag based on ratio of median and interquartile range (proportional to SNR).

• Initial tests used auto-correlations and generated statistics for each frequency channel.

cross-corr phase after flagging

auto-corrs flags

t

f MWA memo, Wayth 2008

Calibration

Solve for the location of, and gain towards, many point sources.

Use the gain measurements (Jones matrices) to constrain a primary beam model for each antenna tile.

Use the position offsets to constrain a 2D ionospheric phase screen.

Use the ~ wavelength2 dependency of the ionosphere to isolate the models.

Update RFI flags

Imaging

RTS: produce calibrated, regridded, weighted snapshot images 105 BLs, 103 freqs, 4 pol. products per 0.5 sec: O(10) GB/s

too much data to store: real-time processing. 8 second cadence to correct O(1) min ionospheric changes. Deal with the ionosphere in the image plane. Deal with w-terms in the image plane (at the same time). Integrate processed images over 5 – 10 min. Formal self-calibration not possible, but will store all cal. info.

forward modeling, matched filtering, ... Need and have excellent instantaneous uv coverage.

Warped Snapshots (-3.5 hrs to +3.5 hrs)

Co-pol (instrument pp) Cross-pol (instrument pq)Significant

changes in (l',m')

Significantchanges in

polarized gain

MAPS simulation: HA (img center) = -3.5 hrs to +3.5 hrs 2 of 4 FFT snapshot images per freq per cadence All input unpolarized (diffuse & point sources) All polarization comes from the instrument

Warped Snapshots (-3.5 hrs to +3.5 hrs)

The “true” grid is determined for each snapshot.

The grids are further distorted to include estimated ionospheric displacements.

Each pixel is weighted for optimal averaging: sky.gain2/sig2 (weighting is done during gridding).

Regridded Snapshots (-3.5 hrs to +3.5 hrs)

Each weighted snapshot is regridded to a static coordinate system (position & polarisation).

Images are accumulated. Deconvolution needs to

account for the antenna-dependent weights.

Last chance flag RFI in R/T

32T Results (Field centered on Pic A)

First 32 tiles: ~ 5% demonstrator

Longer tracks in January 2010

RTS “First light” in December 2009