EVLA Continuum Tutorial 3C391: Difference between revisions
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A command line call to image and deconvolve the dataset would be: | A command line call to image and deconvolve the dataset would be: | ||
<source lang="python"> | <source lang="python"> | ||
clean(vis='3c391_ctm_mosaic_spw0.ms',imagename='3c391_ctm_spw0_IQUV',cell=['2.0arcsec','2.0arcsec'],imsize=[ | clean(vis='3c391_ctm_mosaic_spw0.ms',imagename='3c391_ctm_spw0_IQUV',cell=['2.0arcsec','2.0arcsec'],imsize=[576,576], | ||
mode='mfs',niter=10000,threshold='1.0mJy',stokes='IQUV',psfmode='clarkstokes', | mode='mfs',niter=10000,threshold='1.0mJy',stokes='IQUV',psfmode='clarkstokes', | ||
weighting='briggs',imagermode='mosaic',multiscale=[1,3,10,30,100],mask=[130,145,345,365]) | weighting='briggs',imagermode='mosaic',multiscale=[1,3,10,30,100],mask=[130,145,345,365]) | ||
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Since our fractional bandwidth is non-zero (128 MHz at a central frequency of 4.6 GHz), there is a slight improvement in uv-coverage if we do not average all channels together. Our dataset consists of multiple spectral channels, whose data we wish to combine into a single image, gridding each channel separately. We therefore set ''mode='mfs'''. | Since our fractional bandwidth is non-zero (128 MHz at a central frequency of 4.6 GHz), there is a slight improvement in uv-coverage if we do not average all channels together. Our dataset consists of multiple spectral channels, whose data we wish to combine into a single image, gridding each channel separately. We therefore set ''mode='mfs'''. | ||
We need to select the appropriate pixel size to use. Using plotms to look at the newly-calibrated, target-only data set: | |||
<source lang="python"> | |||
plotms(vis='3c391_ctm_mosaic_spw0.ms') | |||
</source> | |||
we select the axes tab on the left hand side, and select UVDist_L as the x-axis. This gives the plot shown below | |||
[[Image:3c391 ctm spw0 uvplt.jpg|3C391 4600MHz Amplitude vs UV Distance in wavelengths]] | |||
The maximum baseline shown corresponds to about 16,000 wavelengths, i.e. an angular scale of 12 arcseconds (<math>\lambda/D=1/16000</math>). Since we wish to have a number of pixels across a resolution element, we then select a pixel size of 2 arcseconds in both co-ordinates by setting ''cell=['2.0arcsec','2.0arcsec']''. The supernova remnant is known to have a diameter of order 9 arcmin, which corresponds to about 270 pixels. To prevent image artifacts arising from aliasing, we wish to keep the emission region to roughly the inner quarter of the image. The Fourier transform is most efficient if the number of pixels on a side is a composite number divisible by 2 and 3 and/or 5. We choose 576, which is <math>2^6\times3^2</math>, and is close to <math>2\times270</math>. We therefore set ''imsize=[576,576]''. | |||
Since 3C 391 has diffuse, extended emission which is being resolved out by the interferometer owing to a lack of short spacings, a naturally-weighted image would show large-scale patchiness in the noise. In order to suppress this effect, we select Briggs weighting (intermediate between natural and uniform weighting), with a default robust factor of 0 (setting ''weighting='briggs'''). | Since 3C 391 has diffuse, extended emission which is being resolved out by the interferometer owing to a lack of short spacings, a naturally-weighted image would show large-scale patchiness in the noise. In order to suppress this effect, we select Briggs weighting (intermediate between natural and uniform weighting), with a default robust factor of 0 (setting ''weighting='briggs'''). | ||
Revision as of 16:35, 5 May 2010
Overview
This article describes the calibration and imaging of a multiple-pointing EVLA continuum dataset on the supernova remnant 3C 391. The data were taken in OSRO1 mode, with 128 MHz of bandwidth in each of two widely spaced spectral windows, centered at 4.6 and 7.5 GHz, and were set up for full polarization calibration. To generate the full data reduction script described here, use the script extractor.
Getting hold of your data
The data are publicly available from the NRAO archive, under Project name TDEM0001. 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 (file size 39.79 GB). This file contains data at the full 1-s time resolution.
Loading in your data
importevla(asdm='TDEM0001_sb1218006_1.55310.33439732639',vis='3c391_mosaic_fullres.ms',flagzero=True,shadow=True)
Examining your data
Before starting the calibration process, we want to get some basic information about the data set. To examine the observing conditions during the observing run, and to find out any known problems with the data, download the observer log. Simply fill in the known observing date (in our case 2010-Apr-24) as both the Start and Stop date, and click on the "Show Logs" button. The relevant log is labeled with the project code, TDEM0001, and can be downloaded as a PDF file.
To get a listing of the individual scans comprising the observation, the frequency setup, source list, and antenna locations, use the task listobs.
listobs(vis='3c391_ctm_mosaic_10s.ms',verbose=T)
listobs should now produce the following output in the casa logger:
2010-04-27 19:16:49 INFO listobs::::casa ########################################## 2010-04-27 19:16:49 INFO listobs::::casa ##### Begin Task: listobs ##### 2010-04-27 19:16:49 INFO listobs::::casa 2010-04-27 19:16:49 INFO listobs::ms::summary ================================================================================ 2010-04-27 19:16:49 INFO listobs::ms::summary+ MeasurementSet Name: /export/home/hamal/jmiller/TDEM0001_sb1218006/3c391_mosaic_fullres.ms MS Version 2 2010-04-27 19:16:49 INFO listobs::ms::summary+ ================================================================================ 2010-04-27 19:16:49 INFO listobs::ms::summary+ Observer: Dr. James Miller-Jones Project: T.B.D. 2010-04-27 19:16:49 INFO listobs::ms::summary+ Observation: EVLA 2010-04-27 19:16:49 INFO listobs::ms::summary Data records: 18666050 Total integration time = 28716 seconds 2010-04-27 19:16:49 INFO listobs::ms::summary+ Observed from 24-Apr-2010/08:01:34.5 to 24-Apr-2010/16:00:10.5 (UTC) 2010-04-27 19:17:47 INFO listobs::ms::summary 2010-04-27 19:17:47 INFO listobs::ms::summary+ ObservationID = 0 ArrayID = 0 2010-04-27 19:17:47 INFO listobs::ms::summary+ Date Timerange (UTC) Scan FldId FieldName nVis Int(s) SpwIds 2010-04-27 19:17:47 INFO listobs::ms::summary+ 24-Apr-2010/08:01:34.5 - 08:02:28.5 1 0 J1331+3030 35750 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:02:29.5 - 08:09:27.5 2 0 J1331+3030 272350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:09:28.5 - 08:16:26.5 3 0 J1331+3030 272350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:16:27.5 - 08:24:25.5 4 1 J1822-0938 311350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:24:26.5 - 08:29:44.5 5 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:29:45.5 - 08:34:43.5 6 3 3C391 C2 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:34:44.5 - 08:39:42.5 7 4 3C391 C3 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:39:43.5 - 08:44:41.5 8 5 3C391 C4 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:44:42.5 - 08:49:40.5 9 6 3C391 C5 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:49:41.5 - 08:54:40.5 10 7 3C391 C6 195000 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:54:41.5 - 08:59:39.5 11 8 3C391 C7 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 08:59:40.5 - 09:01:29.5 12 1 J1822-0938 71500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:01:30.5 - 09:06:48.5 13 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:06:49.5 - 09:11:47.5 14 3 3C391 C2 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:11:48.5 - 09:16:46.5 15 4 3C391 C3 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:16:47.5 - 09:21:45.5 16 5 3C391 C4 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:21:46.5 - 09:26:44.5 17 6 3C391 C5 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:26:45.5 - 09:31:44.5 18 7 3C391 C6 195000 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:31:45.5 - 09:36:43.5 19 8 3C391 C7 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:36:44.5 - 09:38:32.5 20 1 J1822-0938 70850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:38:33.5 - 09:43:52.5 21 2 3C391 C1 208000 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:43:53.5 - 09:48:51.5 22 3 3C391 C2 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:48:52.5 - 09:53:50.5 23 4 3C391 C3 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:53:51.5 - 09:58:49.5 24 5 3C391 C4 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 09:58:50.5 - 10:03:48.5 25 6 3C391 C5 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:03:49.5 - 10:08:47.5 26 7 3C391 C6 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:08:48.5 - 10:13:47.5 27 8 3C391 C7 195000 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:13:48.5 - 10:15:36.5 28 1 J1822-0938 70850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:15:37.5 - 10:20:55.5 29 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:20:56.5 - 10:25:55.5 30 3 3C391 C2 195000 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:25:56.5 - 10:30:54.5 31 4 3C391 C3 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:30:55.5 - 10:35:53.5 32 5 3C391 C4 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:35:54.5 - 10:40:52.5 33 6 3C391 C5 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:40:53.5 - 10:45:51.5 34 7 3C391 C6 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:45:52.5 - 10:50:51.5 35 8 3C391 C7 195000 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:50:52.5 - 10:52:40.5 36 1 J1822-0938 70850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:52:41.5 - 10:57:39.5 37 0 J1331+3030 194350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 10:57:40.5 - 11:02:39.5 38 1 J1822-0938 195000 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:02:40.5 - 11:07:58.5 39 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:07:59.5 - 11:12:47.5 40 3 3C391 C2 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:12:48.5 - 11:17:36.5 41 4 3C391 C3 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:17:37.5 - 11:22:25.5 42 5 3C391 C4 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:22:26.5 - 11:27:15.5 43 6 3C391 C5 188500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:27:16.5 - 11:32:04.5 44 7 3C391 C6 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:32:05.5 - 11:36:53.5 45 8 3C391 C7 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:36:54.5 - 11:38:43.5 46 1 J1822-0938 71500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:38:44.5 - 11:44:02.5 47 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:44:03.5 - 11:48:51.5 48 3 3C391 C2 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:48:52.5 - 11:53:40.5 49 4 3C391 C3 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:53:41.5 - 11:58:29.5 50 5 3C391 C4 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 11:58:30.5 - 12:03:19.5 51 6 3C391 C5 188500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:03:20.5 - 12:08:08.5 52 7 3C391 C6 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:08:09.5 - 12:12:57.5 53 8 3C391 C7 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:12:58.5 - 12:14:47.5 54 1 J1822-0938 71500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:14:48.5 - 12:20:06.5 55 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:20:07.5 - 12:24:55.5 56 3 3C391 C2 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:24:56.5 - 12:29:44.5 57 4 3C391 C3 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:29:45.5 - 12:34:34.5 58 5 3C391 C4 188500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:34:35.5 - 12:39:23.5 59 6 3C391 C5 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:39:24.5 - 12:44:12.5 60 7 3C391 C6 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:44:13.5 - 12:49:01.5 61 8 3C391 C7 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:49:02.5 - 12:50:51.5 62 1 J1822-0938 71500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:50:52.5 - 12:56:10.5 63 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 12:56:11.5 - 13:00:59.5 64 3 3C391 C2 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:01:00.5 - 13:05:48.5 65 4 3C391 C3 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:05:49.5 - 13:10:38.5 66 5 3C391 C4 188500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:10:39.5 - 13:15:27.5 67 6 3C391 C5 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:15:28.5 - 13:20:16.5 68 7 3C391 C6 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:20:17.5 - 13:25:05.5 69 8 3C391 C7 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:25:06.5 - 13:26:55.5 70 1 J1822-0938 71500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:26:56.5 - 13:32:14.5 71 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:32:15.5 - 13:37:03.5 72 3 3C391 C2 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:37:04.5 - 13:41:52.5 73 4 3C391 C3 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:41:53.5 - 13:46:42.5 74 5 3C391 C4 188500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:46:43.5 - 13:51:31.5 75 6 3C391 C5 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:51:32.5 - 13:56:20.5 76 7 3C391 C6 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 13:56:21.5 - 14:01:09.5 77 8 3C391 C7 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:01:10.5 - 14:02:59.5 78 1 J1822-0938 71500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:03:00.5 - 14:08:18.5 79 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:08:19.5 - 14:13:07.5 80 3 3C391 C2 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:13:08.5 - 14:17:57.5 81 4 3C391 C3 188500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:17:58.5 - 14:22:46.5 82 5 3C391 C4 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:22:47.5 - 14:27:35.5 83 6 3C391 C5 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:27:36.5 - 14:32:24.5 84 7 3C391 C6 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:32:25.5 - 14:37:13.5 85 8 3C391 C7 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:37:14.5 - 14:39:03.5 86 1 J1822-0938 71500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:39:04.5 - 14:44:22.5 87 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:44:23.5 - 14:49:11.5 88 3 3C391 C2 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:49:12.5 - 14:54:01.5 89 4 3C391 C3 188500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:54:02.5 - 14:58:50.5 90 5 3C391 C4 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 14:58:51.5 - 15:03:39.5 91 6 3C391 C5 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:03:40.5 - 15:08:28.5 92 7 3C391 C6 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:08:29.5 - 15:13:17.5 93 8 3C391 C7 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:13:18.5 - 15:15:07.5 94 1 J1822-0938 71500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:15:08.5 - 15:20:26.5 95 2 3C391 C1 207350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:20:27.5 - 15:25:15.5 96 3 3C391 C2 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:25:16.5 - 15:30:05.5 97 4 3C391 C3 188500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:30:06.5 - 15:34:54.5 98 5 3C391 C4 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:34:55.5 - 15:39:43.5 99 6 3C391 C5 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:39:44.5 - 15:44:32.5 100 7 3C391 C6 187850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:44:33.5 - 15:49:22.5 101 8 3C391 C7 188500 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:49:23.5 - 15:51:11.5 102 1 J1822-0938 70850 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary+ 15:51:12.5 - 16:00:10.5 103 9 J0319+4130 350350 1 [0, 1] 2010-04-27 19:17:47 INFO listobs::ms::summary (nVis = Total number of time/baseline visibilities per scan) 2010-04-27 19:17:51 INFO listobs::ms::summary Fields: 10 2010-04-27 19:17:51 INFO listobs::ms::summary+ ID Code Name RA Decl Epoch SrcId nVis 2010-04-27 19:17:51 INFO listobs::ms::summary+ 0 N J1331+3030 13:31:08.2880 +30.30.32.9589 J2000 0 774800 2010-04-27 19:17:51 INFO listobs::ms::summary+ 1 J J1822-0938 18:22:28.7042 -09.38.56.8350 J2000 1 1361750 2010-04-27 19:17:51 INFO listobs::ms::summary+ 2 NONE 3C391 C1 18:49:24.2440 -00.55.40.5800 J2000 2 2488850 2010-04-27 19:17:51 INFO listobs::ms::summary+ 3 NONE 3C391 C2 18:49:29.1490 -00.57.48.0000 J2000 3 2280850 2010-04-27 19:17:51 INFO listobs::ms::summary+ 4 NONE 3C391 C3 18:49:19.3390 -00.57.48.0000 J2000 4 2282150 2010-04-27 19:17:51 INFO listobs::ms::summary+ 5 NONE 3C391 C4 18:49:14.4340 -00.55.40.5800 J2000 5 2282150 2010-04-27 19:17:51 INFO listobs::ms::summary+ 6 NONE 3C391 C5 18:49:19.3390 -00.53.33.1600 J2000 6 2281500 2010-04-27 19:17:51 INFO listobs::ms::summary+ 7 NONE 3C391 C6 18:49:29.1490 -00.53.33.1600 J2000 7 2281500 2010-04-27 19:17:51 INFO listobs::ms::summary+ 8 NONE 3C391 C7 18:49:34.0540 -00.55.40.5800 J2000 8 2282150 2010-04-27 19:17:51 INFO listobs::ms::summary+ 9 Z J0319+4130 03:19:48.1601 +41.30.42.1030 J2000 9 350350 2010-04-27 19:17:51 INFO listobs::ms::summary+ (nVis = Total number of time/baseline visibilities per field) 2010-04-27 19:17:54 INFO listobs::ms::summary Spectral Windows: (2 unique spectral windows and 1 unique polarization setups) 2010-04-27 19:17:54 INFO listobs::ms::summary+ SpwID #Chans Frame Ch1(MHz) ChanWid(kHz)TotBW(kHz) Ref(MHz) Corrs 2010-04-27 19:17:54 INFO listobs::ms::summary+ 0 64 TOPO 4536 2000 128000 4536 RR RL LR LL 2010-04-27 19:17:54 INFO listobs::ms::summary+ 1 64 TOPO 7436 2000 128000 7436 RR RL LR LL 2010-04-27 19:17:54 INFO listobs::ms::summary Sources: 20 2010-04-27 19:17:54 INFO listobs::ms::summary+ ID Name SpwId RestFreq(MHz) SysVel(km/s) 2010-04-27 19:17:54 INFO listobs::ms::summary+ 0 J1331+3030 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 0 J1331+3030 1 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 1 J1822-0938 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 1 J1822-0938 1 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 2 3C391 C1 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 2 3C391 C1 1 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 3 3C391 C2 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 3 3C391 C2 1 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 4 3C391 C3 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 4 3C391 C3 1 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 5 3C391 C4 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 5 3C391 C4 1 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 6 3C391 C5 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 6 3C391 C5 1 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 7 3C391 C6 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 7 3C391 C6 1 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 8 3C391 C7 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 8 3C391 C7 1 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 9 J0319+4130 0 - - 2010-04-27 19:17:54 INFO listobs::ms::summary+ 9 J0319+4130 1 - - 2010-04-27 19:18:02 INFO listobs::ms::summary Antennas: 26: 2010-04-27 19:18:02 INFO listobs::ms::summary+ ID Name Station Diam. Long. Lat. 2010-04-27 19:18:02 INFO listobs::ms::summary+ 0 ea01 W09 25.0 m -107.37.25.2 +33.53.51.0 2010-04-27 19:18:02 INFO listobs::ms::summary+ 1 ea02 E02 25.0 m -107.37.04.4 +33.54.01.1 2010-04-27 19:18:02 INFO listobs::ms::summary+ 2 ea03 E09 25.0 m -107.36.45.1 +33.53.53.6 2010-04-27 19:18:02 INFO listobs::ms::summary+ 3 ea04 W01 25.0 m -107.37.05.9 +33.54.00.5 2010-04-27 19:18:02 INFO listobs::ms::summary+ 4 ea05 W08 25.0 m -107.37.21.6 +33.53.53.0 2010-04-27 19:18:02 INFO listobs::ms::summary+ 5 ea07 N06 25.0 m -107.37.06.9 +33.54.10.3 2010-04-27 19:18:02 INFO listobs::ms::summary+ 6 ea08 N01 25.0 m -107.37.06.0 +33.54.01.8 2010-04-27 19:18:02 INFO listobs::ms::summary+ 7 ea09 E06 25.0 m -107.36.55.6 +33.53.57.7 2010-04-27 19:18:02 INFO listobs::ms::summary+ 8 ea11 E04 25.0 m -107.37.00.8 +33.53.59.7 2010-04-27 19:18:02 INFO listobs::ms::summary+ 9 ea12 E08 25.0 m -107.36.48.9 +33.53.55.1 2010-04-27 19:18:02 INFO listobs::ms::summary+ 10 ea13 N07 25.0 m -107.37.07.2 +33.54.12.9 2010-04-27 19:18:02 INFO listobs::ms::summary+ 11 ea14 E05 25.0 m -107.36.58.4 +33.53.58.8 2010-04-27 19:18:02 INFO listobs::ms::summary+ 12 ea15 W06 25.0 m -107.37.15.6 +33.53.56.4 2010-04-27 19:18:02 INFO listobs::ms::summary+ 13 ea16 W02 25.0 m -107.37.07.5 +33.54.00.9 2010-04-27 19:18:02 INFO listobs::ms::summary+ 14 ea17 W07 25.0 m -107.37.18.4 +33.53.54.8 2010-04-27 19:18:02 INFO listobs::ms::summary+ 15 ea18 N09 25.0 m -107.37.07.8 +33.54.19.0 2010-04-27 19:18:02 INFO listobs::ms::summary+ 16 ea19 W04 25.0 m -107.37.10.8 +33.53.59.1 2010-04-27 19:18:02 INFO listobs::ms::summary+ 17 ea20 N05 25.0 m -107.37.06.7 +33.54.08.0 2010-04-27 19:18:02 INFO listobs::ms::summary+ 18 ea21 E01 25.0 m -107.37.05.7 +33.53.59.2 2010-04-27 19:18:02 INFO listobs::ms::summary+ 19 ea22 N04 25.0 m -107.37.06.5 +33.54.06.1 2010-04-27 19:18:02 INFO listobs::ms::summary+ 20 ea23 E07 25.0 m -107.36.52.4 +33.53.56.5 2010-04-27 19:18:02 INFO listobs::ms::summary+ 21 ea24 W05 25.0 m -107.37.13.0 +33.53.57.8 2010-04-27 19:18:02 INFO listobs::ms::summary+ 22 ea25 N02 25.0 m -107.37.06.2 +33.54.03.5 2010-04-27 19:18:02 INFO listobs::ms::summary+ 23 ea26 W03 25.0 m -107.37.08.9 +33.54.00.1 2010-04-27 19:18:02 INFO listobs::ms::summary+ 24 ea27 E03 25.0 m -107.37.02.8 +33.54.00.5 2010-04-27 19:18:02 INFO listobs::ms::summary+ 25 ea28 N08 25.0 m -107.37.07.5 +33.54.15.8 2010-04-27 19:18:02 INFO listobs::::casa 2010-04-27 19:18:02 INFO listobs::::casa ##### End Task: listobs ##### 2010-04-27 19:18:02 INFO listobs::::casa ##########################################
Both to get a sense of the array, as well as identify an antenna for later use in calibration, use the task plotants. In general, for calibration purposes, one would like to select an antenna that is close to the center of the array (and that is not listed in the operator's log as having had problems!).
plotants(vis='3c391_ctm_mosaic_10s.ms',figfile='3c391_ctm_mosaic_antenna_layout.png')
Examining and Editing the Data
It is always a good idea, particularly with a new system like the EVLA, to examine the data. Moreover, from the observer's log, we already know that one antenna will need to be flagged because it does not have a C-band receiver. Start by flagging data known to be bad, then examine the data.
In its current operation, it is common to insert a dummy scan as the first scan. (From the listobs output above, one may have noticed that the first scan is only 10 seconds long.) This first scan can safely be deleted.
flagdata(vis=msname,flagbackup=T,mode='manualflag',selectdata=T,scan='1')
- flagbackup=T : A comment is warranted on the setting of flagbackup (here set to T or True). If set to True, flagdata will save a copy of the existing set of flags before entering any new flags. The setting of flagbackup is therefore a matter of some taste. One could choose not to save any flags or only save "major" flags, or one could save every flag. (One of the authors of this document was glad that flagbackup was set to True as he recently ran flagdata with a typo in one of the entries.)
- mode='manualflag' : Specific data are going to be selected to be edited.
- selectdata=T : In order to select the specific data to be flagged, selectdata has to be set to True. Once selectdata is set to True, then the various data selection options become visible (use inp flagdata to see the possible options). In this case, scan='1' is chosen to select only the first scan. Note that scan expects an entry in the form of a string. (scan=1 would generate an error.)
If satisfied with the inputs, run this task. The initial display in the logger will include
########################################## ##### Begin Task: flagdata ##### flagdata::::casa attached MS [...] Saving current flags to manualflag_2 before applying new flags Creating new backup flag file called manualflag_2
which indicates that, among other things, the flags that existed in the data set prior to this run will be saved to another file called manualflag_2. Should one ever desire to revert to the data prior to this run, the task flagmanager could be used.
From the observer's log, we know that antenna 13 does not have a C band receiver, so it should be flagged as well. The parameters are similar as before.
flagdata(vis=msname,flagbackup=T,mode='manualflag',selectdata=T,antenna='ea13')
- antenna='ea13' : Once again, this parameter requires a string output. Also note that antenna='ea13' and 'antenna='13' are not the same antenna. (Check the output from listobs above.)
flagdata(vis='3c391_ctm_mosaic_10s.ms',mode='quack',quackinterval=20.0,quackmode='beg')
Calibrating the data
Bandpass calibration
Gain calibration
Polarization calibration
Solving for the leakage terms
Solving for the R-L polarization angle
Applying the calibration
Imaging and self-calibration
Now that you have split off your target data into a separate measurement set with all the calibration applied, it's time to make an image. CASA has a single task, clean which both Fourier transforms the data and deconvolves the resulting image.
A command line call to image and deconvolve the dataset would be:
clean(vis='3c391_ctm_mosaic_spw0.ms',imagename='3c391_ctm_spw0_IQUV',cell=['2.0arcsec','2.0arcsec'],imsize=[576,576],
mode='mfs',niter=10000,threshold='1.0mJy',stokes='IQUV',psfmode='clarkstokes',
weighting='briggs',imagermode='mosaic',multiscale=[1,3,10,30,100],mask=[130,145,345,365])
To image the dataset in all four polarizations (total intensity I, linear polarizations Q and U, and circular polarization V), we set stokes='IQUV', which makes separate images in each polarization. We also set psfmode='clarkstokes, which uses the clark clean algorithm to deconvolve each Stokes plane separately. This makes the polarization image more independent of the total intensity.
Since our fractional bandwidth is non-zero (128 MHz at a central frequency of 4.6 GHz), there is a slight improvement in uv-coverage if we do not average all channels together. Our dataset consists of multiple spectral channels, whose data we wish to combine into a single image, gridding each channel separately. We therefore set mode='mfs'.
We need to select the appropriate pixel size to use. Using plotms to look at the newly-calibrated, target-only data set:
plotms(vis='3c391_ctm_mosaic_spw0.ms')
we select the axes tab on the left hand side, and select UVDist_L as the x-axis. This gives the plot shown below
The maximum baseline shown corresponds to about 16,000 wavelengths, i.e. an angular scale of 12 arcseconds ([math]\displaystyle{ \lambda/D=1/16000 }[/math]). Since we wish to have a number of pixels across a resolution element, we then select a pixel size of 2 arcseconds in both co-ordinates by setting cell=['2.0arcsec','2.0arcsec']. The supernova remnant is known to have a diameter of order 9 arcmin, which corresponds to about 270 pixels. To prevent image artifacts arising from aliasing, we wish to keep the emission region to roughly the inner quarter of the image. The Fourier transform is most efficient if the number of pixels on a side is a composite number divisible by 2 and 3 and/or 5. We choose 576, which is [math]\displaystyle{ 2^6\times3^2 }[/math], and is close to [math]\displaystyle{ 2\times270 }[/math]. We therefore set imsize=[576,576].
Since 3C 391 has diffuse, extended emission which is being resolved out by the interferometer owing to a lack of short spacings, a naturally-weighted image would show large-scale patchiness in the noise. In order to suppress this effect, we select Briggs weighting (intermediate between natural and uniform weighting), with a default robust factor of 0 (setting weighting='briggs').
Our data consists of a 7-pointing mosaic, since the supernova remnant fills almost the full primary beam at 4.6 GHz. We therefore wish to perform a joint deconvolution on all 7 fields, which we do simply by setting imagermode='mosaic', and using the default mosaic parameters.
Because the supernova remnant contains both diffuse, extended structure on large spatial scales, and also finer filamentary structure on smaller scales, we use the multiscale clean algorithm, which removes clean components which are circular Gaussians as well as delta functions. We set a logarithmically-spaced range of spatial scales to use for the widths of the Gaussian components, which we specify in pixels. To do this, we set multiscale=[1,3,10,30,100].
To prevent the clean algorithm finding spurious components on noise peaks, especially at the larger scales in the multiscale clean, we set a region within which the task will search for clean components, using mask=[130,145,345,365]. To prevent overcleaning, we wish to stop the deconvolution when we have reached a residual flux level of 1 mJy (setting threshold='1.0mJy'), or when we have generated 10000 clean components (setting niter=10000).
clean will make several output files, all named with the prefix given as imagename. These include:
- .image - the final restored image, with the clean components convolved with a restoring beam and added to the remaining residuals at the end of the imaging process
- .flux - the effective response of the telescope (the primary beam)
- .flux.pbcoverage - the effective response of the full mosaic image
- .mask - the areas where you have permitted imager to find clean components
- .model - the sum of all the clean components, which has been stored as the model_data column in the measurement set
- .psf - the dirty beam, which is being deconvolved from the true sky brightness during the clean process
- .residual - what is left at the end of the deconvolution process; this is useful to diagnose whether or not to clean more deeply
After the imaging and deconvolution process has finished, you can use the viewer to look at your image.
viewer('3c391_ctm_spw0_IQUV.image')
This will bring up a viewer window containing the image, which should look similar to that shown below. The tape deck buttons that you see under the image can be used to step through the different Stokes parameters (I,Q,U,V). You can adjust the color scale and zoom in to a selected region by assigning mouse buttons to the icons immediately above the image (hover over the icons to get a description of what they do).
