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| == Appendix: Obtaining Data: 3C 391 Example ==
| | #REDIRECT [[Obtaining Data: 3C 391 Example]] |
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| For the purposes of the summer school tutorials, a small number of initial processing steps had been applied. Here we describe in more detail the series of steps that one is likely to have to conduct to obtain a data set similar to what was used for the summer school tutorials, using the 3C 391 data set as an example.
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| The original test data were taken on 2010 April 24, and are stored as file TDEM0001_sb1218006_1.55310.33439732639 .
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| === Acquiring data from the Archive ===
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| The data are publicly available from the [https://archive.nrao.edu/archive/archiveproject.jsp NRAO archive], under Project (Proposal) Name TDEM0001. When submitting an archive query with this project name, the archive lists two separate files; one taken on 2010-Apr-15, and the other on 2010-Apr-24. You will want to download the second file (TDEM0001_sb1218006_1.55310.33439732639; file size 39.79 GB). This file contains data at the full 1-s time resolution. You can select to download an SDM file from the archive, a measurement set, or an AIPS UVFITS file. In the latter two cases, you can opt for spectral or temporal averaging of the data. Spectral averaging prior to bandpass calibration is discouraged, since it can cause phase decorrelation. Whether or not to temporal average will depend on the array configuration, field of view, and acceptable level of time-average smearing. In this example, we opt not to perform any spectral or temporal averaging, and later show how this may be done after the fact by running the task [[split]] on the measurement set.
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| Since the purpose of this tutorial is to demonstrate the steps involved in obtaining one's data from the archive, we will download the data in as unprocessed a format as possible, namely an SDM-BDF dataset (all files). To create a single file (rather than a directory) for downloading, we check the "Create MS or SDM tar file" box, and also check the box labelled "Apply flags generated during observing", to remove data known to be bad (for instance, antennas which are off source during slewing operations).
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| Having entered your email address at the top of the form and selected the required data set, we now request to "Download checked files". When the data have been copied to the relevant directory, you will receive an email. Follow its instructions to download the tarred SDM file.
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| === Converting to a measurement set ===
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| <source lang="python">
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| importevla(asdm='TDEM0001_sb1218006_1.55310.33439732639',vis='3c391_mosaic_fullres.ms',flagzero=True,shadow=True)
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| </source>
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| === Initial flagging ===
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| Depending upon the science goals and the details of the observation, averaging in time, frequency, or both may be possible at this stage. For instance, the 3C 391 data used in the summer school tutorial were acquired with a 1-second sampling in the D configuration. Given the size of 3C 391 itself, and the fact that there are no other strong nearby sources, it makes sense to average these data in time (and possibly in frequency as well, although that was not done for the summer school). For the summer school tutorial itself, we also restricted ourselves to just a single spectral window, even though the observations were acquired with two spectral windows. Thus,
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| <source lang="python">
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| split(vis='3c391_ctm_mosaic.ms',outputvis='3c391_ctm_mosaic_10s.ms',datacolumn='data',
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| spw='0',width=1,timebin='10s')
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| </source>
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| results in a single spectral window, with an unchanged frequency resolution, averaged to 10-second sampling.
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