Transcript Kein Folientitel

Phase Correction of VLBI with WVRs
Alan Roy
Ute Teuber
Helge Rottmann
Reinhard Keller
Pushing VLBI to the Highest Resolution:
Some Applications
8687
GHz
M
M 87 Jet Collimation
230 RG
Krichbaum et al. 2005
Sgr A* Event Horizon
Falcke, Melia, Agol (2000)
GR ray tracing
1/r2 emissivity gas
Maximally rotating black hole
Include interstellar scattering
Convolve to resolution of ideal
VLBI array at 1.3 mm and 0.6 mm
(Modelling results)
500 GHz
Rschwarzschild
Krichbaum
et al.GHz
2005
230
Rschwarzschild
Event horizon shadow diameter should be 9.2 Rs = 27 μas in Sgr A*
Pushing VLBI to the Highest Resolution:
Demo at 230 GHz
First trans-Atlantic fringes at 1.3 mm
Signal to noise ratio
500 GHz
230 GHz
8
6
Pico Veleta – Plateau de Bure
4
2 days
2003 April 13
HHT – Pico Veleta
4.2 G (record longest baseline)  30 μas resolution
Sources: 3C 454.3 7 Jy SNR = 7.3
0716+714 3.5 Jy SNR = 6.4?
Krichbaum, Graham, Alef et al., EVN Symp, (2004)
Pushing VLBI to the Highest Resolution:
Coherence Loss due to Troposphere
VLBI phase time series
Coherence Function
360°
7 min
Pico Veleta – Onsala baseline
Source: BL Lac
Frequency: 86 GHz
The Scanning 18-26 GHz WVR for Effelsberg
Front-end opened
Ethernet data acquisition system
Temperature regulation modules
March 16th, 2004
Control unit
The Scanning 18-26 GHz WVR for Effelsberg
 = 18.5 GHz to 26.0 GHz
D = 900 MHz
Channels = 24
Treceiver = 200 K
sweep period = 6 s
Features
 Uncooled
(reduce cost)
 Scanning
(fewer parts, better stability)
 Robust implementation





(weather-proof, temperature stabilized)
Noise injection for gain stabilization
Beam matched to Effelsberg near-field beam
TCP/IP communication
Web-based data access
Improved version of prototype by Alan Rogers
WVR Performance Requirements
Phase Correction
Aim:
coherence = 0.9
requires   / 20 (0.18 mm rms at  = 3.4 mm) after correction
Need: thermal noise  14 mK in 3 s
Measured: 12 mK = 0.05 mm
Need: gain stability 3.9 x 10-4 in 300 s Measured: 2.7 x 10-4
Opacity Measurement
Aim:
correct visibility amplitude to 1 % (1 )
Need: thermal noise  2.7 K
Need: absolute calibration  14 % (1 )
Measured: 12 mK
Measured: 5 %
Typical Water Line Spectrum
Storm, 2003 Jul 24, 1500 UT
WVR Path Data from 3 mm VLBI, April 2004
210
150
120
path length
90
90°
60
elevation
30
0
Elevation
Path length / mm
180
18
24
45°
30
Time / UT hours
36
42
0°
VLBI Phase Correction Demo
NRAO 150
Pico Veleta - Effelsberg
86 GHz VLBI
2004 April 17
No phase correction
VLBI phase
WVR phase
EB phase correction
path
3.4 mm
Coherence function before & after
EB+PV phase correction
420 s
● Path rms reduced 1.0 mm to 0.34 mm
● Coherent SNR rose 2.1 x
VLBI Phase Correction Demo
NRAO 150
Pico Veleta - Effelsberg
86 GHz VLBI
2004 April 17
No phase correction
VLBI phase
WVR phase
EB phase correction
path
3.4 mm
Coherence function before & after
● Path rms reduced 0.85 mm to 0.57 mm
● Coherent SNR rose 1.7 x
420 s
VLBI Phase Correction Demo
NRAO 150
Pico Veleta - Effelsberg
86 GHz VLBI
2004 April 17
Before phase correction at EB
VLBI phase
WVR phase
After phase correction at EB
path
3.4 mm
Coherence function before & after
● Path rms saturated at 0.95 mm
● Coherent SNR decrease 7.5 x
420 s
VLBI Phase Correction Demo
Improvement factor
Coherence function after phase correction at EB divided by CF before phase correction
NRAO 150
Pico Veleta - Effelsberg
86 GHz VLBI
2004 April 17
2.0
1.0
0.0
0s
120 s
240 s
360 s
Coherent integration time
● Coherence improves for most scans
Phase Referencing Errors due to Troposphere
εZTD
ZTD
Absolute Calibration for Astrometry & Geodesy
Conclusion
 WVR corrections from Effelsberg improve phase
coherence during high-frequency VLBI
 Tropospheric delay measured to 10 mm accuracy for astrometry
 WVR data recorded at Effelsberg for every EVN session since 2004
See under http://www.mpifr-bonn.mpg.de/staff/aroy/wvr.html
 Ready to equip high-frequency VLBI array with WVRs
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Oct 9-11, 2006, Wettzell, Germany
http://www.wettzell.ifag.de
http://www.mpifr-bonn.mpg.de/staff/aroy/wvr.html
Scattered Cumulus, 2003 Jul 28, 1300 UT