High-resolution Observations at Long
Wavelengths: First Results from International
LOFAR
Olaf Wucknitz
[email protected]
AG annual meeting, Bonn, 14 September 2010
First results from international LOFAR
• LOFAR
• long baseline issues
• first long-baseline fringes
• first long-baseline images
• The Sun!
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LOFAR
• LOw Frequency ARray
• low frequencies
? LBA: ∼30(10)–80 MHz
? HBA: ∼110–250 MHz
• 30–40 stations in Netherlands
• additional stations in Germany, France, England, Sweden,
Italy?, Poland?, . . .
• wide field of view, several beams
• good survey speed
• full synthesis imaging at low frequencies with high resolution
• long baselines: subarcsec resolution, useful for lens surveys
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Zooming in: The Cosmos at High Resolution
Why long baselines?
fringe-spacing θ = λ /L ≈ resolution
λ /m freq/MHz
30
10
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1 km 30 km 300 km 1000 km
1.◦7
3.04
2100
6.002
10
30
340
1.01
6.009
2.001
3.8
80
130
2600
2.006
0.0077
2.5
120
8.06
1700
1.007
0.0052
1.9
160
6.04
1300
1.003
0.0039
1.4
220
4.07
9.004
0.0094
0.0028
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German LOFAR stations
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LBA at Effelsberg
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LBA details at Unterweilenbach
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HBA + LBA in Tautenburg
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Long baseline issues
• sources resolved
weak signal
• less calibrators
• have to average coherently in time and frequency
? have to correct for delays (∂φ /∂ν)
? have to correct for rates (∂φ /∂t)
need fringe fitting
∗ non-dispersive: clocks, positions, troposphere
∗ dispersive: ionosphere
• larger (differential) ionospheric delays
• larger clock offsets
• differential Faraday rotation
• sparse uv coverage
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First long baseline fringes (NL-Ef, Aug 2009, single sb)
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Multi-band: more sensitivity and higher resolution delay
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Fringes to Tautenburg and Unterweilenbach (Jan 2010)
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‘Results’ of fringe analysis
• long baseline fringes found
• clock offsets in some stations X
• confusion in LBA polarisation labels X
• strong 8 MHz ripple X
• 63 MHz LBA resonance
• strong differential Faraday rotation X
• time for imaging!
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LBA long-baseline map: some details
• 3C196, LBA, 31 / 160 subbands, 44–59 / 30–80 MHz (ripple!)
• bandwidth 6 MHz / 48 MHz
• D2010 16704 6 h on 12/13 Feb 2010
• 5 NL + 3 DE stations (Effelsberg, Unterweilenbach, Tautenburg)
• corrected for 1 µsec and 17 µsec constant delays
• RR and LL from XX/XY/YX/YY using geometric model
• (self-)calibrated and imaged LL/RR in difmap
• MFS with/without spectral index correction
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UV coverage with long and short baselines
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First long baseline maps of 3C196
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[ Lonsdale & Morison (1980) ]
MTRLI (MERLIN) observations of 3C196 at 408 MHz
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Nicer maps: short and long baselines
NL only, 3500 × 2200 beam
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NL+DE, 1.005 × 0.009 beam
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[ Wucknitz + Lonsdale & Morison (1980) ]
LOFAR LBA vs. MERLIN 408 MHz
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First interferometric Solar observations
• initiated by J. Anderson, F. Breitling, G. Mann, A. Polatidis, C.
Vocks, O. Wucknitz (alphabetical order)
• 8 ten-minute scans on 9th June 2010, 9:48 – 15:50 UT
• 4 with LBA and HBA each
• phase/pointing centre near Sun and calibrator sources
• LBA
? difficult, other sources dominating: 3C123, CygA, CasA, . . .
? situation unclear
• HBA
? much clearer signal, Sun dominating on short baselines
? self-calibration possible because of compact component
• only short baselines used!
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The very first map
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Variability: 30 sec in 1 sec steps (8 subbands)
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Summary
• long baseline LOFAR works!
• but no pipeline yet
• fringe analysis revealed a number of technical
problems (mostly solved now)
• Sun can be observed and resolved with LOFAR!
• to do
? full fringe-fitting and calibration
? not independent of polarisation calibration
(differential Faraday rotation)
? Sun as function of time and frequency
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Contents
1
2
3
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7
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9
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titlepage
First results from international LOFAR
LOFAR
Zooming in: The Cosmos at High Resolution
German LOFAR stations
LBA at Effelsberg
LBA details at Unterweilenbach
HBA + LBA in Tautenburg
Long baseline issues
First long baseline fringes (NL-Ef, Aug 2009, single sb)
Multi-band: more sensitivity and higher resolution delay
Fringes to Tautenburg and Unterweilenbach (Jan 2010)
Results of fringe analysis
LBA long-baseline map: some details
UV coverage with long and short baselines
First long baseline maps of 3C196
MTRLI (MERLIN) observations of 3C196 at 408 MHz
Nicer maps: short and long baselines
introduction
summary
contents
back
forward
previous
next
fullscreen
23
18
19
20
21
22
23
titlepage
LOFAR LBA vs. MERLIN 408 MHz
First interferometric Solar observations
The very first map
Variability: 30 sec in 1 sec steps (8 subbands)
Summary
Contents
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