Molonglo SKA Prototyping and MNRF

Narrabri AM Meeting, 4 July 2001

Molonglo SKA Prototyping and MNRF

Molonglo

AUSTRALIABrisbane

Darwin

Perth

Canberra

Hobart

Adelaide

Melbourne

Sydney

+


Research Team:Anne Green1, John Bunton2, Duncan Campbell-Wilson1, Lawrence Cram1, Ralph Davison1, Dick Hunstead1, Daniel “Mitch” Mitchell1,3, Andrew Parfitt2, Elaine Sadler1, George “Ñima” Warr1,3

[1] School of Physics, University of Sydney[2] Telecommunications and Industrial Physics, CSIRO[3] Australia Telescope National Facility, CSIRO

Molonglo SKA Prototyping and MNRF


What is the SKA? The next generation radio telescope

The Square Kilometre Array radio telescope:

• Large collecting area for high sensitivity (1 km2)

• Array elements (stations) distributed over a wide area for high resolution (~4000 km)

• For good uv plane coverage, stations can’t be too sparse


Radio telescope sensitivityincreasing logarithmically with time

SKA

GMRTVLA

ATVLA

WSRTArecibo

BonnParkes

Jodrel

Dwingeloo

Reber

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

1930 1940 1950 1960 1970 1980 1990 2000 2010 2020

Date

Lo

g R

ela

tive S

en

sit

ivit

y


SKA will provide higher sensitivity and resolution

HST

VLA

SKA


Radio frequency interference (RFI)must be excised to get high sensitivity


For high resolution array stations are distributed across a continent


Phased array (Netherlands)

1000km(Courtesy NFRA)


Luneberg Lens (ATNF)


Luneberg Lens Focusing Action (ATNF)


Parabolic Reflector Array (SETI Institute, USA)


Aerostatically mounted receiver aboveLarge Adaptive Reflector (Canada)


Large [Arecibo-like] Reflectors (China)


Cylindrical Parabolic CollectorMolonglo (USyd, ATNF, CSIRO)

Molonglo

AUSTRALIABrisbane

Darwin

Perth

Canberra

Hobart

Adelaide

Melbourne

Sydney

+


Molonglo SKA Prototyping and MNRF. Overview

• Molonglo telescope is an E-W array of two collinear cylindrical paraboloids. • total length of 1.6 km (2 x 800m with 15 m gap between them). • Collecting area 18,000 m2 (largest in the Southern Hemisphere).

• We propose to equip the telescope with new feeds, low-noise amplifiers, digital filterbanks and FX correlator as an SKA prototype with continuous frequency coverage in multibeam mode from 300-1420 MHz.

• Allows development and testing of several new technologies.

• Provides a sensitive instrument for exploring the distant universe.

• Funding for this project is currently being sought from the Australian Government's Major National Research Facilities (MNRF) program.

• The Molonglo Telescope has been operated by the University of Sydney since 1965. Major achievements include the Molonglo Reference Catalogue (MRC; 408 MHz) and a sensitive all-sky imaging survey, Sydney University Molonglo Sky Survey (SUMSS; 843 MHz) that will be completed in 2003.


Target Specifications

Parameter 1420 MHz 300 MHz

Frequency Coverage 300–1420 MHz

Bandwidth 250 MHz

Resolution (δ < -30°) 26" x 26" csc|δ| 123" x 123" csc|δ|

Imaging field of view 1.5° x 1.5° csc|δ| 7.7° x 7.7° csc|δ|

UV coverage Fully sampled

Tsys < 50K < 150K

System noise (1σ) 12 hr:

8 min:

11 μJy/beam100 μJy/beam

33 μJy/beam 300 μJy/beam

Polarisation Dual Linear

Correlator I and Q (Full Stokes at 125 MHz bandwidth)

Frequency resolution 120–1 kHz (FXF mode: 240 Hz)

Independent fanbeam 1.3’ x 1.5° 6.2’ x 7.7°

Independent fanbeam offset

±6° ±27°

Sky accessible in < 1 s 180 deg2 1000 deg2


Molonglo SKA Prototyping and MNRF Main science goals:

Low-frequency radio spectrometry (300-1420 MHz)• Selection of objects via their radio spectral shape, e.g.

candidate high-redshift (z>3) galaxies with ultra-steep radio spectra (de Breuck et al. 2000), which allow us to study the formation of galaxies and massive black holes.

Redshifted HI (300-1420 MHz)• HI in absorption against bright continuum sources over a

wide redshift range (z=0 to 3).• HI in emission - evolution of the HI mass function from

z=0 to 0.5. Will be able to detect a bright spiral galaxy (2.5 x 1010 solar masses of HI,V=200 km/s) at z=0.1 in 12 hours.


Selection of high-z radio galaxies by radio spectral index

• de Breuck (PhD 2000) - ultra-steep radio spectra (< -1.3) can select (some) high-z galaxies

• K magnitude is a good initial redshift estimator

• BUT two-point spectral index is crude (finds only ~1 candidate per 15-20 deg2) - we will be >5 times more sensitive and can use detailed radio spectra.


Molonglo able to measure evolution of HI mass function over z=0.03 to 0.26

8

9

10

11

0 0.1 0.2 0.3

Redshift z

log

10 M

lim (

HI)

(M

⊙)

(10 x 12 hr)

Molonglo (12 hr)HIPASS (8 min)

Galaxy with velocity width V = 200 km/s

1.7 deg2 field

H0 = 50 km/s/Mpc, q0 =

0.5


Science Goals ctd…

Low-frequency Galactic recombination lines• Recombination lines of carbon and hydrogen can be

used to probe the partially-ionized ISM and constrain the physical conditions (Anantharamaiah & Kantharia 1999).

Gamma Ray Bursters• Electronic beam steering gives 5% chance of monitoring

instantaneously on alert. Concurrent Pulsars and Source Flux Monitoring • 18 to 400 deg2 accessible around main beam.• Real time dedispersion.Pulsar and SETI Searches (optional 64 fanbeam

system)• Fast high sensitivity search.


Molonglo continuous uv coverage produces excellent image quality


Molonglo Continuum Confusion (10 beams/source) at δ= 60°

0.01

0.1

1

10

0 500 1000 1500

Frequency (MHz)

Flu

x D

en

sity

(m

Jy)

Bock et al 1999SUMSS 843 MHz

Rengelink et al 1997WENSS 325 MHz

Wall 19941420 MHz

beam size:43” x 43” csc||




SKA Technologies

• Multibeaming• Wide instantaneous field of view• Digital Beamforming• FX Correlators• Frequency and Pointing Agility• Wideband linefeeds and LNAs• Cylindrical Antenna Prototype – in particular

addressing• Polarisation purity• Beam variability/stability• Inter-antenna patch coupling

• Adaptive Null steering and Adaptive Noise cancellation


Observing Modes

Wide Field Imaging• Full integration over 12 hours or multiple snapshots.

Spectral Line Observations• FX mode 2048 channels, each 1 - 120 kHz (0.2 - 25 km/s).• FXF mode 1 or 2 channels with 768 complex lags.

Independent Fanbeam Observations• An independent fanbeam can be formed within ±6° (1420 MHz) or ±27°

(300 MHz) of the imaging beam, e.g., for pulsar timing or flux monitoring.

Interleaved Observing [Rapid electronic changes (< 1 s) in frequency & meridian distance]

• Frequency agility spectral index determination. • Meridian angle agility 10,000 deg2 of sky accessible during routine 12h

observations, e.g., source monitoring or calibration.

Optional digital beamformer with 64 fanbeams• Pulsar and SETI search observing.


Signal Path and Antenna Pattern

Cylindrical Parabolic Collectors (Two collinear 778 m x 12 m)

300-1420 MHz Feed and LNA(7,400 feeds, 14,800 LNAs)

Delay line beamforming(Over 9 feeds)

Analog to Digital Converter(1,600 8 bit 250 MHz BW ADCs)

Digital delay beamforming(80 10 m x 10 m patches)

Digital filterbank (160)(Two polarisations @ 250 MHz/patch)

FX Correlator(3,160 baselines, 2,048 channels)

Signal processing & storage(imaging, spectrometer, searching...)

Independent fanbeam(1 within 9 feed field of view)

Digital Beamformer(64 fanbeams within imaging beam)

[Requires extra funding]

Single feed beam

Delay line beam

Independent fanbeam

Imaging beam


Collector

The telescope’s collector consists of two cylindrical paraboloids, 778m x 12m, separated by 15m and aligned east-west (total area 18,000 m2).

The telescope is steered by mechanical rotation of the cylindrical paraboloids about their long axis, and by electronic delays of the feed elements along the arms. The resulting ‘alt-alt’ system can follow fields south of declination -30° for ±6 hours.

The original parabola shape was designed to be accurate for operation at 1.4 GHz. The reflecting mesh was designed for operation at 408 MHz and will need to be replaced for operation above ~1 GHz.

MD=Meridian Distance

Geometry


0

1

2

3

-6 -4 -2 0 2 4 6x (m)

y (m

)

x focus

Molonglo parabola design accurate to > 1400 MHz

Flat mesh tied on supports at points shown

• Mesh supported at 0.6 m (2 ft) intervals in x direction.• Each section gives the same error for a linear fit to a

parabola.• Gives only 0.1 dB loss at 1420 MHz.

Piecewise linear fit to parabola shape


Original 408 MHz mesh needs replacing for operation above ~1 GHz

-1.5

-1.0

-0.5

0.0

-60 -40 -20 0 20 40 60Meridian Distance (deg)

Re

flect

ion

Lo

ss (

dB

)

Original mesh designed for 408 MHz (12mm NS x 25mm

EW)

1420 MHz NS~75K ground

leakage

843 MHz NS


Resurfacing Issues

• Wind loading• Pointing accuracy

Not a major issue if using a mesh rather than a solid surface.

• No noticeable extra loading at Parkes after resurfacing (with porous surface).

• Pointing accuracy degraded at ATCA in windy conditions and low elevations after replacing with solid surface. Possibly due to increased vacuum on lee side of dish.


Beam Shape

0 800-800Distance (m)

Patch positions on reflectors

Synthesised Beam ShapeThe synthesised beam shape for a possible configuration of antenna patches on the telescope is shown.

This configuration has a contiguous patch covering a third of the telescope area for forming 1.3’ beams for pulsar or SETI searches.

The remaining part of the telescope is more sparsely covered (with positions calculated from a simple grading function) to give good imaging resolution.


Wide Band Feeds Required

Single feed covering 300-1420 MHz desirable

• Vivaldi antenna array suitable?• ASTRON THEA operates over

750-1500 MHz• Highest performance at high

frequencies. At lower frequencies:• Higher sky temperature• Increased source counts per

steradian• Larger beam size


Wide band ambient temperature low noise amplifiers required

• ~20K noise temperature • Ambient temperature

operation• Likely to be able to

extend to operate over 300-1400 MHz

• Design simplifications possible if higher input impedance from antenna (designed for 50 input impedance)

• Good starting point for migration to MMIC design

Gain

300 1400850Frequency (MHz)

-30

-15

0

15

30

45

|S| (

dB)

Output Match S22Input Match S11

Prototype design for 400-1200 MHz HEMT based LNA (Ralph Davison)


Beamformer and Correlator

Dual Pol. Line feedDelay line beam

former,1m section

Upconverter, IF,IQ Downconverterand 8-bit Digitiser

Dual Pol. Line feedDelay line beam

former,1m section

Upconverter, IF,IQ Downconverterand 8-bit Digitiser

18 sections per bay

Multi OutputDigital Beamformer

DigitalFilterbank

FAN Beam

Connection to AdjacentBeamformer

FAN Beam

Connection to AdjacentBeamformer

DigitalFilterbank

To Correlator

Analogue

Optical

LO LO

Analog delay line beamforming Accuracy /4

Each polarisationRF 0.3 to 1.4 GHzLO 2.2 to 0.9 GHzIF at 2.5 GHz Quadrature baseband detectionDual 250 MSamples/s 8-bit A/Ds generating a complex 250 MHz signal

Digital BeamformingFine delays accuracy /16Delay corrects for average analog delay errorArbitrary and time varying gradingModifiable beam shape with meridian distance Resources for adaptive null steering

250 MHz complex digital filterbanks 120 kHz frequency channelsSingle FPGA implementation Adaptive noise cancellation on a per channel basis

Beamforming and Digital Filterbanks for one of 44 bays


RFI at Molonglo 200-1500 MHz (Measured 25 June 2001)

-115

-105

-95

-85

-75

0 500 1000 1500

Frequency (MHz)

Me

asu

red

Po

we

r (d

Bm

)

GSM

VHFTV

UHFTV


Molonglo SKA Prototype (Operating by 2004-2005)

• New line feeds: prototypes for SKA cylindrical doublet antennas• New low-noise amplifiers • Resurface telescope to operate at higher frequency • Photonics for LO distribution, signal gathering and beam forming • Wide-band FX correlator • ‘Software telescope’ - computerised control, beam forming and

data acquisition • Automated remote operation, data pipeline

• Result: 300-1400 MHz radio ‘spectrometer’ with >5 times increase in continuum sensitivity, high dynamic range, fully-sampled uv plane. New spectral-line capability for redshifted HI.


Summary

• Science: Studies of the high-redshift universe (high-z galaxies, evolution of HI mass function)

• Technology: prototype for SKA cylindrical antennas, software beamforming, high dynamic range observing with fully-sampled uv plane

• Community use: New national facility that re-opens the radio spectrum below 1.4 GHz, operating by 2005 as a fully-automated remote observing telescope with a data reduction pipeline

Documents

Molonglo SKA Prototyping and MNRF