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L Harveysmith - One of the best experts on this subject based on the ideXlab platform.
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field sources near the southern sky Calibrator pks b1934 638 effect on spectral line observations with ska mid and its precursors
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: I Heywood, E Lenc, Paolo Serra, B Hugo, K W Bannister, M E Bell, A P Chippendale, L HarveysmithAbstract:Accurate instrumental bandpass corrections are essential for the reliable interpretation of spectral lines from targeted and survey-mode observations with radio interferometers. Bandpass correction is typically performed by comparing measurements of a strong Calibrator source to an assumed model, typically an isolated point source. The wide field-of-view and high sensitivity of modern interferometers means that additional sources are often detected in observations of Calibrators. This can introduce errors into bandpass corrections and subsequently the target data if not properly accounted for. Focusing on the standard Calibrator PKS B1934-638, we perform simulations to asses this effect by constructing a wide-field sky model. The cases of ASKAP (0.7-1.9 GHz), MeerKAT (UHF: 0.58-1.05 GHz; L-band: 0.87-1.67 GHz) and Band 2 (0.95-1.76 GHz) of SKA-MID are examined. The use of a central point source model during bandpass calibration is found to impart amplitude errors into spectra measured by the precursor instruments at the ~0.2-0.5% level dropping to ~0.01% in the case of SKA-MID. This manifests itself as ripples in the source spectrum, the behaviour of which is coupled to the distribution of the array baselines, the solution interval, the primary beam size, the hour-angle of the calibration scan, as well as the weights used when imaging the target. Calibration pipelines should routinely employ complete field models for standard Calibrators to remove this potentially destructive contaminant from the data, a recommendation we validate by comparing our simulation results to a MeerKAT scan of PKS B1934-638, calibrated with and without our expanded sky model.
I Heywood - One of the best experts on this subject based on the ideXlab platform.
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field sources near the southern sky Calibrator pks b1934 638 effect on spectral line observations with ska mid and its precursors
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: I Heywood, E Lenc, Paolo Serra, B Hugo, K W Bannister, M E Bell, A P Chippendale, L HarveysmithAbstract:Accurate instrumental bandpass corrections are essential for the reliable interpretation of spectral lines from targeted and survey-mode observations with radio interferometers. Bandpass correction is typically performed by comparing measurements of a strong Calibrator source to an assumed model, typically an isolated point source. The wide field-of-view and high sensitivity of modern interferometers means that additional sources are often detected in observations of Calibrators. This can introduce errors into bandpass corrections and subsequently the target data if not properly accounted for. Focusing on the standard Calibrator PKS B1934-638, we perform simulations to asses this effect by constructing a wide-field sky model. The cases of ASKAP (0.7-1.9 GHz), MeerKAT (UHF: 0.58-1.05 GHz; L-band: 0.87-1.67 GHz) and Band 2 (0.95-1.76 GHz) of SKA-MID are examined. The use of a central point source model during bandpass calibration is found to impart amplitude errors into spectra measured by the precursor instruments at the ~0.2-0.5% level dropping to ~0.01% in the case of SKA-MID. This manifests itself as ripples in the source spectrum, the behaviour of which is coupled to the distribution of the array baselines, the solution interval, the primary beam size, the hour-angle of the calibration scan, as well as the weights used when imaging the target. Calibration pipelines should routinely employ complete field models for standard Calibrators to remove this potentially destructive contaminant from the data, a recommendation we validate by comparing our simulation results to a MeerKAT scan of PKS B1934-638, calibrated with and without our expanded sky model.
T. Carozzi - One of the best experts on this subject based on the ideXlab platform.
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lbcs the lofar long baseline Calibrator survey
Astronomy and Astrophysics, 2016Co-Authors: N. Jackson, J. Moldón, A. T. Deller, O. Wucknitz, T. Carozzi, A Tagore, E Varenius, L K Morabito, J. ConwayAbstract:We outline the LOFAR Long-Baseline Calibrator Survey (LBCS), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines >1000 km. Suitable sources must contain significant correlated flux density (>∼50−100 mJy) at frequencies around 110−190 MHz on scales of a few hundred milliarcseconds. At least for the 200−300-km international baselines, we find around 1 suitable Calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include Calibrator source lists and fringe-rate and delay maps of wide areas – typically a few degrees – around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200−600 km, with good Calibrators on the longest baselines appearing only at the rate of 0.5 per sq. deg. Coherence times decrease from 1−3 min on 200-km baselines to about 1 min on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 min, is seen on the DE609 baseline, which at 227 km is close to being the shortest. We see median coherence times of between 80 and 110 s on the four longest baselines (580−600 km), and about 2 min for the other baselines. The success of phase transfer from Calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 deg. Although source structures cannot be measured in these observations, we deduce that phase transfer is affected if the Calibrator source structure is not known. We give suggestions for calibration strategies and choice of Calibrator sources, and describe the access to the online catalogue and data products.
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LBCS: The LOFAR Long-Baseline Calibrator Survey
Astronomy and Astrophysics - A&A, 2016Co-Authors: N. Jackson, A Tagore, Javier, Moldon, Adam Deller, E Varenius, C Conway, A Kapinska, E Orrù, M Brentjens, T. CarozziAbstract:We outline LBCS (the LOFAR Long-Baseline Calibrator Survey), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines >1000 km. Suitable sources must contain significant correlated flux density (50 − 100 mJy) at frequencies around 110–190 MHz on scales of a few hundred milliarcseconds. At least for the 200–300-km international baselines, we find around 1 suitable Calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include Calibrator source lists and fringe-rate and delay maps of wide areas – typically a few degrees – around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200–600 km, with good Calibrators on the longest baselines appearing only at the rate of 0.5 per square degree. Coherence times decrease from 1–3 minutes on 200-km baselines to about 1 minute on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 minutes, is seen on the DE609 baseline, which at 227 km is close to being the shortest. We see median coherence times of between 80 and 110 seconds on the four longest baselines (580–600 km), and about 2 minutes for the other baselines. The success of phase transfer from Calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 degree. Although source structures cannot be measured in these observations, we deduce that phase transfer is affected if the Calibrator source structure is not known. We give suggestions for calibration strategies and choice of Calibrator sources, and describe the access to the online catalogue and data products.
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The LOFAR long baseline snapshot Calibrator survey
Astronomy and Astrophysics - A&A, 2015Co-Authors: J. Moldón, A. T. Deller, O. Wucknitz, N. Jackson, A. Drabent, T. Carozzi, J. Conway, A. D. Kapińska, P. Mckean, L. MorabitoAbstract:Aims. An efficient means of locating Calibrator sources for International LOFAR is developed and used to determine the average density of usable Calibrator sources on the sky for subarcsecond observations at 140 MHz. Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full International LOFAR array. Sources were pre-selected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay Calibrators. Results. Over 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory Calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the density of Calibrators on the sky that are sufficiently bright to calibrate dispersive and non-dispersive delays for the International LOFAR using existing methods is 1.0 per square degree. Conclusions. The observed density of satisfactory delay Calibrator sources means that observations with International LOFAR should be possible at virtually any point in the sky, provided that a fast and efficient search using the methodology described here is conducted prior to the observation to identify the best Calibrator.
N. Jackson - One of the best experts on this subject based on the ideXlab platform.
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lbcs the lofar long baseline Calibrator survey
Astronomy and Astrophysics, 2016Co-Authors: N. Jackson, J. Moldón, A. T. Deller, O. Wucknitz, T. Carozzi, A Tagore, E Varenius, L K Morabito, J. ConwayAbstract:We outline the LOFAR Long-Baseline Calibrator Survey (LBCS), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines >1000 km. Suitable sources must contain significant correlated flux density (>∼50−100 mJy) at frequencies around 110−190 MHz on scales of a few hundred milliarcseconds. At least for the 200−300-km international baselines, we find around 1 suitable Calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include Calibrator source lists and fringe-rate and delay maps of wide areas – typically a few degrees – around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200−600 km, with good Calibrators on the longest baselines appearing only at the rate of 0.5 per sq. deg. Coherence times decrease from 1−3 min on 200-km baselines to about 1 min on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 min, is seen on the DE609 baseline, which at 227 km is close to being the shortest. We see median coherence times of between 80 and 110 s on the four longest baselines (580−600 km), and about 2 min for the other baselines. The success of phase transfer from Calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 deg. Although source structures cannot be measured in these observations, we deduce that phase transfer is affected if the Calibrator source structure is not known. We give suggestions for calibration strategies and choice of Calibrator sources, and describe the access to the online catalogue and data products.
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LBCS: The LOFAR Long-Baseline Calibrator Survey
Astronomy and Astrophysics - A&A, 2016Co-Authors: N. Jackson, A Tagore, Javier, Moldon, Adam Deller, E Varenius, C Conway, A Kapinska, E Orrù, M Brentjens, T. CarozziAbstract:We outline LBCS (the LOFAR Long-Baseline Calibrator Survey), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines >1000 km. Suitable sources must contain significant correlated flux density (50 − 100 mJy) at frequencies around 110–190 MHz on scales of a few hundred milliarcseconds. At least for the 200–300-km international baselines, we find around 1 suitable Calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include Calibrator source lists and fringe-rate and delay maps of wide areas – typically a few degrees – around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200–600 km, with good Calibrators on the longest baselines appearing only at the rate of 0.5 per square degree. Coherence times decrease from 1–3 minutes on 200-km baselines to about 1 minute on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 minutes, is seen on the DE609 baseline, which at 227 km is close to being the shortest. We see median coherence times of between 80 and 110 seconds on the four longest baselines (580–600 km), and about 2 minutes for the other baselines. The success of phase transfer from Calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 degree. Although source structures cannot be measured in these observations, we deduce that phase transfer is affected if the Calibrator source structure is not known. We give suggestions for calibration strategies and choice of Calibrator sources, and describe the access to the online catalogue and data products.
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The LOFAR long baseline snapshot Calibrator survey
Astronomy and Astrophysics - A&A, 2015Co-Authors: J. Moldón, A. T. Deller, O. Wucknitz, N. Jackson, A. Drabent, T. Carozzi, J. Conway, A. D. Kapińska, P. Mckean, L. MorabitoAbstract:Aims. An efficient means of locating Calibrator sources for International LOFAR is developed and used to determine the average density of usable Calibrator sources on the sky for subarcsecond observations at 140 MHz. Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full International LOFAR array. Sources were pre-selected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay Calibrators. Results. Over 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory Calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the density of Calibrators on the sky that are sufficiently bright to calibrate dispersive and non-dispersive delays for the International LOFAR using existing methods is 1.0 per square degree. Conclusions. The observed density of satisfactory delay Calibrator sources means that observations with International LOFAR should be possible at virtually any point in the sky, provided that a fast and efficient search using the methodology described here is conducted prior to the observation to identify the best Calibrator.
J. Moldón - One of the best experts on this subject based on the ideXlab platform.
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lbcs the lofar long baseline Calibrator survey
Astronomy and Astrophysics, 2016Co-Authors: N. Jackson, J. Moldón, A. T. Deller, O. Wucknitz, T. Carozzi, A Tagore, E Varenius, L K Morabito, J. ConwayAbstract:We outline the LOFAR Long-Baseline Calibrator Survey (LBCS), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines >1000 km. Suitable sources must contain significant correlated flux density (>∼50−100 mJy) at frequencies around 110−190 MHz on scales of a few hundred milliarcseconds. At least for the 200−300-km international baselines, we find around 1 suitable Calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include Calibrator source lists and fringe-rate and delay maps of wide areas – typically a few degrees – around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200−600 km, with good Calibrators on the longest baselines appearing only at the rate of 0.5 per sq. deg. Coherence times decrease from 1−3 min on 200-km baselines to about 1 min on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 min, is seen on the DE609 baseline, which at 227 km is close to being the shortest. We see median coherence times of between 80 and 110 s on the four longest baselines (580−600 km), and about 2 min for the other baselines. The success of phase transfer from Calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 deg. Although source structures cannot be measured in these observations, we deduce that phase transfer is affected if the Calibrator source structure is not known. We give suggestions for calibration strategies and choice of Calibrator sources, and describe the access to the online catalogue and data products.
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The LOFAR long baseline snapshot Calibrator survey
Astronomy and Astrophysics - A&A, 2015Co-Authors: J. Moldón, A. T. Deller, O. Wucknitz, N. Jackson, A. Drabent, T. Carozzi, J. Conway, A. D. Kapińska, P. Mckean, L. MorabitoAbstract:Aims. An efficient means of locating Calibrator sources for International LOFAR is developed and used to determine the average density of usable Calibrator sources on the sky for subarcsecond observations at 140 MHz. Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full International LOFAR array. Sources were pre-selected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay Calibrators. Results. Over 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory Calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the density of Calibrators on the sky that are sufficiently bright to calibrate dispersive and non-dispersive delays for the International LOFAR using existing methods is 1.0 per square degree. Conclusions. The observed density of satisfactory delay Calibrator sources means that observations with International LOFAR should be possible at virtually any point in the sky, provided that a fast and efficient search using the methodology described here is conducted prior to the observation to identify the best Calibrator.
