Triple-twin flux density monitoring at VGOS frequencies

 Triple-twin flux density monitoring at VGOS frequencies First results and challenges 1 Alva Kinman1, R. Haas1, K. le Bail1, J. Yang1, E. Varenius, S. Garcia Espada2, R. Bolaño González2, A. Neidhardt3 1 Chalmers University of Technology 2 Norwegian Mapping Authority 3 Technical University of Munich Outline ▪Background ▪Observations and analysis ▪Comparison of Onsala and Ny-Ålesund telescopes ▪Comparison of Onsala and Wettzell telescopes ▪Conclusions 2 Motivation: Flux monitoring for geodesy ▪We would like to observe as many scans as possible during geodetic VLBI sessions → better estimation of troposphere and geodetic parameters ▪In legacy VLBI: SNR-based scheduling using a flux density catalog ▪VLBI Global Observing System (VGOS): SNR based scheduling with interpolated flux densities ▪We would like a flux density catalog for the VGOS frequencies as well! Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions 3  Flux monitoring with the Onsala Twin Telescopes ▪Onsala Twin Telescopes (OTT) used as a single baseline interferometer ▪Two years of regular flux monitoring ▪Results form these experiments are presented in Varenius et al. 2022, Kinman et al. 2025 (EVGA) ▪Estimated flux density uncertainty of 5% 4 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions VGOS twin telescopes ▪Three pairs of VGOS twin telescopes: ▪Ny-Ålesund, Svalbard (Norway) ▪Onsala, Sweden ▪Wettzell, Germany ▪Two 13 m-antennas separated by 70-85 m 5 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions Ny-Ålesund Geodetic Earth Observatory Onsala Space Observatory Geodetic Observatory Wettzell Flux monitoring with three twin telescope pairs ▪More observing time ▪More sources available ▪Could investigate correlation amplitudes on long baselines 6 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions Observations ▪Three short experiments: ft5177, ft5181, fm5264 ▪Sources: ▪Onsala flux monitoring catalog: ICRF3 defining sources + a handful AGN of astronomical interest ▪Flux calibrators 3C286, 3C147, 3C295 ▪Scheduled using VieSched++ 7 ft5177 Duration: 1h Antennas: Nn, Ns Oe, Ow 26 Jun 2025 30 Jun 2025 22 Sep 2025 ft5181 Duration: 1h Antennas: Nn, Ns Oe, Ow Wn, Ws fm5264 Duration: 3h Antennas: (Nn) Oe, Ow Wn, Ws Band Frequency A 3.000 - 3.480 GHz B 5.240 - 5.720 GHz C 6.360 - 6.840 GHz D 10.200 – 10.680 GHz Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions Data processing - Correlation ▪Correlated at Onsala Space Observatory using DiFX 2.8.1 ▪High spectral resolution (320 channels) to enable flagging of phase-calibration spikes on the short baselines 8 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions Data processing – Calibration in CASA Data was processed in Common Astronomy Software Applications (CASA) using a pipeline: ▪A priori gain calibration with Tsys and (flat) gain curves ▪Automatic flagging of unwanted radiation ▪Instrumental fringe fitting ▪Bandpass calibration ▪Averaging of visibilities within each scan and sub-band ▪Additional amplitude calibration using flux calibrator sources 9 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions  Wettzell vs Onsala twins – Flux density ratios 16 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions •Much larger scatter than for the Ny-Ålesund twins! Sources: 36 Sources: 65  Wettzell vs Onsala twins – azimuth dependence 17 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions Wettzell vs Onsala twins – Sky plots 18 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions How large pointing offset is needed to explain this? ▪Most severely affected sources would have a pointing offset of ~ 400’’ = 0.11° 19 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions Possible explanations ▪Weather was stable during all experiments ▪No azimuth dependence in the system temperatures ▪Known issues at Wettzell: ▪Ws: Two failed LNAs → reduced sensitivity in H polarization ▪Wn: Amplifier saturation in down-conversion stage, band D These should decrease sensitivity in all directions ▪This just in: Update in Wn’s tracking code was using the wrong year! 20 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions Conclusions ▪The Onsala Twins and Ny-Ålesund Twins produce consistent flux densities, except at the highest elevations ▪The Wettzell baseline shows strong direction-dependent amplitude variations, needing further investigation ▪Pointing errors could be an explanation ▪Flux monitoring experiments could be used for pointing quality tests! 21 Background Obs & Analysis Results: Ny -Ålesund Results: Wettzell Conclusions Thank you for your attention! Bonus slides 22 Scale factors ▪Compares measured flux with expected flux of a calibrator source ▪Correcting for instrumental amplitude errors, e.g. incorrect noise diode temperatures, scaling errors from bandpass calibration ▪We expect all flux calibrators to give the same scale factor! 23   24 Extra azimuth and elevation plots  25 Tsys for Wettzell, fm5264