Atacama Large Millimeter/submillimeter Array (ALMA)
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Publications making use of these data must include the following statement
in the acknowledgement:
This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00006.E.
ALMA is a partnership of ESO (representing its member states), NSF (USA) and
NINS (Japan), together with NRC (Canada) and MOST and ASIAA (Taiwan),
and KASI (Republic of Korea), in cooperation with the Republic of Chile.
The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ."
In addition, publications from NA authors must include the standard NRAO
acknowledgement:
"The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."
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This Readme file contains a description of the VY CMa Band 9
ALMA 2017 High Frequency Long Baseline Campaign data, scripts and data products.
There is one ALMA execution block (ASDM) observed in a mixed
continuum TDM (2 SPWs) and high sprectral resolution (FDM) (6 SPWs)
spectral resolution mode. Spectral windows include continuum and
H2O and SiO masers.
The array included 49 antennas in a configuration with baseline lengths
between ~113 and 13894 m.
The resolution achieved is ~12 x 11 milli arcseconds.
The RMS achieved in the delivered images are:
~2.0 mJy/beam (continuum)
0.1 Jy/beam / 0.44km/s bin on the 658-GHz water maser line image
(in line-free regions) raising to ~0.8Jy/beam measured away
from signal but in strong emission channels
Additional information can be found in the comments section
included in this README.
The Calibrated Data and Reference Images provided here were produced using
CASA version 5.6.1-8
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COMMENTS:
Data were obtained on 2017 November 3rd as a part of the high frequency
long baseline EOC (Extention and Optimization of Capability) in Band 9.
The array included 49 antennas in a configuration with baseline lengths
between ~113 and 13894 m. The PWV was 0.56 mm.
Note that data was taken for a new imaging capaility commissioning, so several
experimental functions were tested: band-to-band (B2B) phase referecing with
differentail gain calibration (DGC), 90deg-Walsh phase switch for the Band~9
sideband separation.
The total observing time was 47 minutes, and the on-source time for VY CMa
was 344 s. The LO1 frequencies for the target and the phase calibrator are
669 (Band 9) and 149 (Band 4) GHz, respectively. The bright QSO, J0522-3627,
28 deg away from the target, was used as the DGC source, as well as for
bandpass and flux-scale calibration.
The correlator was configured to have eight spectral windows in Band 9 with
a bandwidth of 1.875 GHz each using the 90-deg Walsh phase switching to
separate upper side band and lower side band. Spectral window 33 covers
the 658 GHz H2O maser. Spectral window 39 covers the 676~GHz SiO maser
line. Those two spectral windows have the frequency resolution of
15.625 kHz.
The line-free continuum emission image using the averaged eight spectral
windows with Briggs weighting (robust = 2) has the synthesized beam size of
12 × 11 mas and the image RMS noise is 1.5 mJy/beam. In the initial map,
there is a bright and compact component (VY CMa) that cannot be resolved even
with the longest baselines, so that the self-calibration can be applied to
the data (such a compact component is ideal for performing self-calibration).
In order to obtain the visibility data free from the atmospheric and
instrumental phase errors, phase and amplitude self-calibrations were performed
for the line-free continuum channels with the solution interval of 16 s
to remove residual phase offsets between the spectral windows.
The self-calibration solutions were then applied to all target data, and the
bright maser peak, which has a higher S/N, was used for further self-calibration,
now making and applying the solutions with an interval of 4 s to all data.
Improvements were seen on the continuum peak, which increased from 42 to
135 mJy/beam, while the image rms noise was largely unchanged at 1.5 mJy/beam.
This indicates that stochastic phase errors remained even after B2B phase
referencing, which gave an image coherence loss of ~70%. The final
self-calibrated image was synthesized with the eight spectral windows'
line-free emission.
In the course of the data reduction, there were steps in quite unsusual
processes to deal with B2B phase referencing and troubles related to
the above functions such as the 90-deg Walsh phase switching.
One antenna (DA54) was found to lack the first DGC sequence in spectral
windows 41 and 43 and required extra attention.
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VYCMa_B9_B2B_Scripts.tgz contains the calibration and imaging scripts:
* uid___A002_Xc660ef_X2d26.ms.scriptForCalibration_noplots.py
* VYCMa_scriptForSelfcalAndImaging.py
VYCMa_B9_B2B_UncalibratedData.tgz contains the raw data:
One ASDM datasets:
* uid___A002_Xc660ef_X2d26
VYCMa_B9_B2B_CalibratedData.tgz contains the calibrated uv-data:
* vycma.cont.ms
* vycma.line.ms
To re-create these calibrated datasets you would need to execute the calibration
scripts (e.g uid___A002_Xc660ef_X2d26.ms.scriptForCalibration_noplots.py) provided
in VYCMa_B9_B2B_Scripts.tgz
VYCMa_Band9_ReferenceImages.tgz contains the line and continuum
self-calibrated image products and primary beams:
VYCMa_sci.spw29_31_33_35_37_39_41_43.cont.tt0.I.manual-selfcal-robust.image.fits
VYCMa_sci.spw29_31_33_35_37_39_41_43.cont.tt0.I.manual-selfcal-robust.pb.fits
VYCMa_sci.spw39.cube.I.manual-selfcal-29SiO-J16-15-676GHz-robust.image.fit
VYCMa_sci.spw39.cube.I.manual-selfcal-29SiO-J16-15-676GHz-robust.pb.fits
VYCMa_sci.spw33.cube.I.manual-selfcal-H20-658GHz-robust.image.fits
VYCMa_sci.spw33.cube.I.manual-selfcal-H20-658GHz-robust.pb.fits
The reference images can be re-created by executing the imaging script
(VYCMa_scriptForSelfcalAndImaging.py).
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HOW TO EXECUTE THE CALIBRATION SCRIPTS
(1) Put an ALMA ASDM and the corresponding calibration script
in the same directory
(2) In CASA 5.6.1-8 e.g.
execfile('uid___A002_Xc660ef_X2d26.ms.scriptForCalibration_noplots.py')
HOW TO EXECUTE THE IMAGING SCRIPT WITH SELF-CALIBRATION
(1) Put vycma.cont.ms and vycma.line.ms, and the imaging script
in the same directory
(2) In CASA 5.6.1-8
execfile('VYCMa_scriptForSelfcalAndImaging.py')