|
Tag: Redirect target changed |
| (13 intermediate revisions by 5 users not shown) |
| Line 1: |
Line 1: |
| The first few tutorials provided us with a spectral line data cube and a continuum image. Next we'd like to
| | #REDIRECT [[First_Look_at_Image_Analysis_CASA_6.6.1]] |
| understand some of the properties of the images that we have produced.
| |
| | |
| In case you don't have the images from the first tutorials handy, let's start by copying the image files from the working_data directory to our current directory:
| |
| | |
| <source lang="python">
| |
| # In CASA | |
| os.system("rm -rf sis14_twhya_cont.image")
| |
| os.system("cp -r ../working_data/sis14_twhya_cont.image .")
| |
| os.system("rm -rf sis14_twhya_n2hp.image")
| |
| os.system("cp -r ../working_data/sis14_twhya_n2hp.image .")
| |
| </source>
| |
| | |
| Recall that we orient ourselves with measurement sets (UV data) using the {{listobs}} task.
| |
| For images, an analogous way to get basic header information is to use the {{imhead}} task.
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| imhead("sis14_twhya_cont.image")
| |
| imhead("sis14_twhya_n2hp.image")
| |
| </source>
| |
| | |
| CASA prints the image header information to the logger:
| |
| <pre style="background-color: #fffacd;">
| |
| imhead ##########################################
| |
| imhead ##### Begin Task: imhead #####
| |
| imhead imhead(imagename="sis14_twhya_cont.image",mode="summary",hdkey="",hdvalue="",verbose=False)
| |
| ImageMetaData
| |
| ImageMetaData Image name : sis14_twhya_cont.image
| |
| ImageMetaData Object name : TW Hya
| |
| ImageMetaData Image type : PagedImage
| |
| ImageMetaData Image quantity : Intensity
| |
| ImageMetaData Pixel mask(s) : None
| |
| ImageMetaData Region(s) : None
| |
| ImageMetaData Image units : Jy/beam
| |
| ImageMetaData Restoring Beam : 0.789582 arcsec, 0.551785 arcsec, -58.4646 deg
| |
| ImageMetaData
| |
| ImageMetaData Direction reference : J2000
| |
| ImageMetaData Spectral reference : LSRK (-> TOPO)
| |
| ImageMetaData Velocity type : RADIO
| |
| ImageMetaData Rest frequency : 3.72637e+11 Hz
| |
| ImageMetaData Pointing center : 11:01:51.796000 -34.42.17.366000
| |
| ImageMetaData Telescope : ALMA
| |
| ImageMetaData Observer : cqi
| |
| ImageMetaData Date observation : 2012/11/19/07:37:00
| |
| ImageMetaData Telescope position: [2.22506e+06m, -5.44006e+06m, -2.48168e+06m] (ITRF)
| |
| ImageMetaData
| |
| ImageMetaData Axis Coord Type Name Proj Shape Tile Coord value at pixel Coord incr Units
| |
| ImageMetaData ----------------------------------------------------------------------------------------------------
| |
| ImageMetaData 0 0 Direction Right Ascension SIN 256 256 11:01:51.796 128.00 -1.000000e-01 arcsec
| |
| ImageMetaData 1 0 Direction Declination SIN 256 256 -34.42.17.366 128.00 1.000000e-01 arcsec
| |
| ImageMetaData 2 1 Stokes Stokes 1 1 I
| |
| ImageMetaData 3 2 Spectral Frequency 1 1 3.72637e+11 0.00 2.35055441345e+08 Hz
| |
| ImageMetaData Velocity 0 0.00 -1.891126e+02 km/s
| |
| imhead ##### End Task: imhead #####
| |
| imhead ##########################################
| |
| | |
| imhead ##########################################
| |
| imhead ##### Begin Task: imhead #####
| |
| imhead imhead(imagename="sis14_twhya_n2hp.image",mode="summary",hdkey="",hdvalue="",verbose=False)
| |
| ImageMetaData
| |
| ImageMetaData Image name : sis14_twhya_n2hp.image
| |
| ImageMetaData Object name : TW Hya
| |
| ImageMetaData Image type : PagedImage
| |
| ImageMetaData Image quantity : Intensity
| |
| ImageMetaData Pixel mask(s) : mask0
| |
| ImageMetaData Region(s) : None
| |
| ImageMetaData Image units : Jy/beam
| |
| ImageMetaData Restoring Beam : 0.709488 arcsec, 0.469052 arcsec, -53.779 deg
| |
| ImageMetaData
| |
| ImageMetaData Direction reference : J2000
| |
| ImageMetaData Spectral reference : LSRK
| |
| ImageMetaData Velocity type : RADIO
| |
| ImageMetaData Rest frequency : 3.72672e+11 Hz
| |
| ImageMetaData Pointing center : 11:01:51.796000 -34.42.17.366000
| |
| ImageMetaData Telescope : ALMA
| |
| ImageMetaData Observer : cqi
| |
| ImageMetaData Date observation : 2012/11/19/07:56:27
| |
| ImageMetaData Telescope position: [2.22506e+06m, -5.44006e+06m, -2.48168e+06m] (ITRF)
| |
| ImageMetaData
| |
| ImageMetaData Axis Coord Type Name Proj Shape Tile Coord value at pixel Coord incr Units
| |
| ImageMetaData --------------------------------------------------------------------------------------------------
| |
| ImageMetaData 0 0 Direction Right Ascension SIN 250 125 11:01:51.796 125.00 -8.000000e-02 arcsec
| |
| ImageMetaData 1 0 Direction Declination SIN 250 50 -34.42.17.366 125.00 8.000000e-02 arcsec
| |
| ImageMetaData 2 1 Stokes Stokes 1 1 I
| |
| ImageMetaData 3 2 Spectral Frequency 15 5 3.72672e+11 0.00 -6.21550826e+05 Hz
| |
| ImageMetaData Velocity 0 0.00 5.000000e-01 km/s
| |
| imhead ##### End Task: imhead #####
| |
| imhead ##########################################
| |
| </pre>
| |
| | |
| == Statistics ==
| |
| | |
| Often we'd like to measure basic image
| |
| statistics and fluxes. Recall you can do these operations pretty easily in the viewer, by
| |
| dragging out a box and then right-double clicking inside it to get statistics for that region. You can also get statistics using the {{imstat}} task.
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| my_stats = imstat("sis14_twhya_n2hp.image", chans="0~4")
| |
| </source>
| |
| | |
| The {{imstat}} task returns a python dictionary, and you can examine the contents as follows:
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| print my_stats
| |
| print my_stats['rms']
| |
| </source>
| |
| | |
| It's useful to see that the RMS is about 20 mJy/beam in the
| |
| line cube.
| |
| | |
| You can also just print the dictionary to the terminal screen:
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| imstat("sis14_twhya_n2hp.image", chans="0~4")
| |
| </source>
| |
| | |
| This task is useful for measuring basic source characteristics. For example,
| |
| calculate the statistics for a box encompassing the disk - you see that the
| |
| integrated flux is about 1.5 Jy.
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| imstat("sis14_twhya_cont.image", box="100,100,150,150")
| |
| my_stats = imstat("sis14_twhya_cont.image", box="100,100,150,150")
| |
| print my_stats['flux']
| |
| </source>
| |
| | |
| Alternatively, a box defined off the disk will give image noise statistics.
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| imstat("sis14_twhya_cont.image", box="25,150,225,200")
| |
| </source>
| |
| | |
| == Moments ==
| |
| | |
| For the spectral line cube, it's very useful to "collapse" the cube in
| |
| various ways to analyze the emission. The {{immoments}} task lets you do
| |
| this.
| |
| | |
| The first few "moments" are defined as follows:
| |
| <pre>
| |
| moments = -1 : mean value of the spectral coordinate
| |
| moments = 0 : integrated value of the spectral coordinate
| |
| moments = 1 : intensity weighted coordinate; traditionally used to get 'velocity fields'
| |
| moments = 2 : intensity weighted dispersion of the coordinate; traditionally used to get "velocity dispersion"
| |
| </pre>
| |
| | |
| A more complete description of image moments is given [http://casa.nrao.edu/docs/CasaRef/image.moments.html here in the CASA reference manual].
| |
| | |
| <figure id="Imaging-tutorial-analysis-mom0.png">
| |
| [[File:Imaging-tutorial-analysis-mom0.png|thumb|<caption>The zero moment of the N2H+ image.</caption>]]
| |
| </figure>
| |
| | |
| Let's make a moment 0 image clipped at ~1 sigma:
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| os.system("rm -rf sis14_twhya_n2hp.mom0")
| |
| immoments("sis14_twhya_n2hp.image",
| |
| outfile="sis14_twhya_n2hp.mom0",
| |
| includepix=[20e-3,100],
| |
| chans="4~12",
| |
| moments=0)
| |
| </source>
| |
| | |
| Make a moment 1 image clipped at ~2 sigma:
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| os.system("rm -rf sis14_twhya_n2hp.mom1")
| |
| immoments("sis14_twhya_n2hp.image",
| |
| outfile="sis14_twhya_n2hp.mom1",
| |
| includepix=[40e-3,100],
| |
| chans="4~12",
| |
| moments=1)
| |
| </source>
| |
| | |
| At this point we have a few really neat things to see: first the
| |
| line shows a hole in the middle. Overlay it on the dust (continuum)
| |
| disk using the viewer and you can notice that they align but with the N2H+
| |
| existing only outside a certain radius - in this case the "snow line".
| |
| Also have a look at the velocity field to see the rotation of the
| |
| disk.
| |
| | |
| The {{imview}} task has some command-line scripting capability. For example, here we show how to overlay the line moment
| |
| 0 (as a contour plot) on the continuum.
| |
| | |
| <figure id="Imaging-tutorial-analysis-overlay.png">
| |
| [[File:Imaging-tutorial-analysis-overlay.png|thumb|<caption>The N2H+ moment zero map as contours, overlaid on the continuum emission of TW Hydra. The plot demonstrates the "show line".</caption>]]
| |
| </figure>
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| imview(raster={'file': 'sis14_twhya_cont.image',
| |
| 'range': [-0.01,0.5]},
| |
| contour={'file': 'sis14_twhya_n2hp.mom0',
| |
| 'levels': [0.5,0.6,0.7,0.8] })
| |
| </source>
| |
| | |
| == Export FITS images ==
| |
| | |
| CASA is great (of course) but you will ultimately want to export
| |
| your data to a common format to analyze in other programs, share
| |
| with other astronomers, or archive. It's easy to export images from
| |
| CASA's image format to FITS images via the {{exportfits}} command.
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| exportfits(imagename="sis14_twhya_cont.image",
| |
| fitsimage="twhya_cont.fits",
| |
| overwrite=True)
| |
| </source>
| |
| | |
| For the cube we want to specify additionally that the frequency axis
| |
| will be written out as velocity.
| |
| | |
| <source lang="python">
| |
| # In CASA
| |
| exportfits(imagename="sis14_twhya_n2hp.image",
| |
| fitsimage="twhya_n2hp.fits",
| |
| velocity=True,
| |
| overwrite=True)
| |
| </source>
| |
