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Phase Correction of VLBI with WVRs

This paper discusses the use of Water Vapor Radiometers (WVRs) for phase correction in Very Long Baseline Interferometry (VLBI) at high frequencies, specifically at 230 GHz. It explores the improvement in coherence and imaging resolution achieved through WVR-based phase correction. The paper also covers topics such as interstellar scattering, black hole modeling, and the performance requirements of WVRs. The results from VLBI experiments and measurements are presented, demonstrating the effectiveness of WVRs in achieving high-resolution VLBI imaging at 230 GHz.

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Phase Correction of VLBI with WVRs

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  1. Phase Correction of VLBI with WVRs Alan Roy Ute Teuber Helge Rottmann Reinhard Keller

  2. 230 RG M 87 Krichbaum et al. 2005 Pushing VLBI to the Highest Resolution: Some Applications 86 GHz M 87 Jet Collimation 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 230 GHz Rschwarzschild Rschwarzschild Event horizon shadow diameter should be 9.2 Rs = 27 μas in Sgr A*

  3. Pushing VLBI to the Highest Resolution: Demo at 230 GHz First trans-Atlantic fringes at 1.3 mm 500 GHz 230 GHz 8 6 Pico Veleta – Plateau de Bure Signal to noise ratio 4 2 days 2003 April 13 HHT – Pico Veleta 4.2 G (record longest baseline) 30 μas resolution Sources: 3C 454.3 7 JySNR = 7.3 0716+714 3.5 JySNR = 6.4? Krichbaum, Graham, Alef et al., EVN Symp, (2004)

  4. 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

  5. The Scanning 18-26 GHz WVR for Effelsberg Front-end opened March 16th, 2004 Ethernet data acquisition system Temperature regulation modules Control unit

  6. 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

  7. 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 Measured: 12 mK Need: absolute calibration  14 % (1 ) Measured: 5 %

  8. Typical Water Line Spectrum

  9. Storm, 2003 Jul 24, 1500 UT

  10. WVR Path Data from 3 mm VLBI, April 2004 210 180 150 120 path length Path length / mm 90 90° 60 elevation Elevation 45° 30 0 0° 18 24 30 36 42 Time / UT hours

  11. 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 ● Path rms reduced 1.0 mm to 0.34 mm ● Coherent SNR rose 2.1 x 420 s

  12. 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

  13. 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

  14. VLBI Phase Correction Demo 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 Improvement factor 1.0 0.0 0 s 120 s 240 s 360 s Coherent integration time ● Coherence improves for most scans

  15. Phase Referencing Errors due to Troposphere εZTD ZTD

  16. Absolute Calibration for Astrometry & Geodesy

  17. 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 Advertisement RadioNet Workshop on Measurement of Atmospheric Water Vapour Theory, Techniques, Astronomical and Geodetic Applications Oct 9-11, 2006, Wettzell, Germany http://www.wettzell.ifag.de http://www.mpifr-bonn.mpg.de/staff/aroy/wvr.html

  18. Scattered Cumulus, 2003 Jul 28, 1300 UT

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