Removing Source Structure in DiFX Visibility Data
Abstract
Source structure introduces a systematic error in VLBI observations on a per-source basis. We have developed a software tool that takes models of the brightness distribution to correct the interferometric phases of the DiFX correlator output. We have tested our method by applying it to observational VGOS data. Here we present our results concerning the effect on imaging the corrected data, the impact on closure delays, and a full geodetic analysis of group delay databases.
Full text
Removing Source Structure in DiFX Visibility Data Removing Source Structure in DiFX Visibility Data Frédéric Jaron TU Wien, MPIfR 10th International VLBI Technology Workshop Gothenburg, Sweden October 23, 2025
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 2 / 35
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 3 / 35 Credit: NASA Not a point source! Remove structure from correlator output
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 4 / 35 CDDIS CDDIS Control file from correlator report Control file from correlator report Processing Pipeline Processing Pipeline Not limited to VGOS! Not limited to VGOS!
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 5 / 35 Testing our Level-1 Source Structure Correction Method Testing our Level-1 Source Structure Correction Method Applying the Method to Three Operational VGOS 24-h Sessions Applying the Method to Three Operational VGOS 24-h Sessions Session 1: vo2187 Session 1: vo2187 Session 2: vo2244 Session 2: vo2244 Session 3: vo3124 Session 3: vo3124 IVS VLBI level 1A correlator output data were obtained from the NASA Crustal Dynamics Data Information System (CDDIS). 2022 2023 July 6-7 Sept. 1-2 May 4-5 Images from Pérez-Díez et al. (2024, A&A) Images from Pérez-Díez et al. (2024, A&A)
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 6 / 35 Three sources OJ287 Pérez-Díez et al. 2024, A&A, 688, A151 Testing our Level-1 Source Structure Correction Method Testing our Level-1 Source Structure Correction Method Images obtained from analysis of vo2187 data Credit: Victor Pérez-Díez, Iván Martí-Vidal
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 7 / 35 Modeling the Source Structure with Gaussian Functions Modeling the Source Structure with Gaussian Functions Example: 3C418 in VGOS Band A
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 8 / 35 Source Structure Configuration Table Source Structure Configuration Table ●Variable number of Gaussian elliptical components per band (frequency range) ●Peak flux density ●Position ●Extension ●Position angle ●Spectral index + curvature (not used here) ●Variable number of Gaussian elliptical components per band (frequency range) ●Peak flux density ●Position ●Extension ●Position angle ●Spectral index + curvature (not used here) 3C 418 We use these models to correct the level-1 visibility data. We use these models to correct the level-1 visibility data.
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 9 / 35 Imaging the Corrected Data Imaging the Corrected Data
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 16 / 35 vo2187 vo2187 Same session as for imaging Same session as for imaging
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 17 / 35 vo2187 vo2187 Same session as for imaging Same session as for imaging
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 18 / 35 vo2187 vo2187 Same session as for imaging Same session as for imaging
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 19 / 35 Testing our Level-1 Source Structure Correction Method Testing our Level-1 Source Structure Correction Method Applying the Method to Three Operational VGOS 24-h Sessions Applying the Method to Three Operational VGOS 24-h Sessions Session 1: vo2187 Session 1: vo2187 Session 2: vo2244 Session 2: vo2244 Session 3: vo3124 Session 3: vo3124 IVS VLBI level 1A correlator output data were obtained from the NASA Crustal Dynamics Data Information System (CDDIS). 2022 2023 July 6-7 Sept. 1-2 May 4-5 Images from Pérez-Díez et al. (2024, A&A) Images from Pérez-Díez et al. (2024, A&A)
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 20 / 35 Statistical Distribution of Closure Delays Statistical Distribution of Closure Delays Two month later Two month later
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 21 / 35 Session: vo2244 Source: 3C418 Time-seriesTwo months later
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 22 / 35 Session: vo2244 Source: OJ287 Time-seriesTwo months later
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 23 / 35 vo2244 vo2244 Session two months later Session two months later
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 24 / 35 vo2244 vo2244 Session two months later Session two months later
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 25 / 35 Statistical Distribution of Closure Delays Statistical Distribution of Closure Delays Ten months later Ten months later
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 32 / 35 Estimated source positions relative to VIE2023-VG Estimated source positions relative to VIE2023-VG Magnitudes of differences between original and corrected are in agreement with extension of the source structure. Krásná et al. (2025, A&A, 693, A16)
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 33 / 35 Differences between original and corrected relative positions have similar values across sessions for the same sources. Differences between original and corrected relative positions have similar values across sessions for the same sources.
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 34 / 35 Only for session vo2187 here! Only for session vo2187 here! By setting the reference point for our correction tool accordingly, it is possible to compensate for the discrepancy in source position. By setting the reference point for our correction tool accordingly, it is possible to compensate for the discrepancy in source position. Adjusting the reference point Adjusting the reference point
F. Jaron (TU Wien) 10th International VLBI Technology Workshop, October 23, 2025 35 / 35 Conclusions 1. We have developed a tool that corrects DiFX visibility data for source structure. 2. Sources are modeled with elliptical Gaussian components. 3. Images from the corrected data appear more compact than the original ones, but not all of the structure is removed. 4. Closure delays are significantly improved for sources with extended structure. 5. After a full geodetic analysis, post-fit residuals seem slightly improved. 6. A shift in the estimated position can be accounted for by adjusting the reference point. Thank you! Jaron et al., under review by Radio Science