Difference between revisions of "N891 simdata (CASA 3.3)"

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[[Category: Simulations]]
 
[[Category: Simulations]]
  
''A new version of this page exists for CASA 3.3: [[N891 simdata (CASA 3.3)]].''
+
''Old version: [[N891 simdata2]].''
 +
 
 +
To create a script of the Python code on this page see [[Extracting scripts from these tutorials]].
  
 
== Nearby edge-on spiral ==
 
== Nearby edge-on spiral ==
 
Roughly modeled after NGC891
 
Roughly modeled after NGC891
  
{{Under Construction}} - mostly correct, but probably not very thoroughly explained. Updated for CASA 3.1 (simdata2->simdata)
+
Updated for CASA 3.3
  
* model origin: Milky Way 13CO from the [http://www.bu.edu/galacticring/ Galactic Ring Survey] on the 14m [http://www.astro.umass.edu/~fcrao/ FCRAO]
+
* Model origin: Milky Way 13CO from the [http://www.bu.edu/galacticring/ Galactic Ring Survey] on the 14m [http://www.astro.umass.edu/~fcrao/ FCRAO]
  
* I binned the cube to coarser velocity resolution in order to speed the simulation. the fits file is [ftp://ftp.cv.nrao.edu/NRAO-staff/rindebet/grs-12kms.fits grs-12kms.fits]
+
* The cube is being binned to a coarser velocity resolution in order to speed the simulation. The fits file is [ftp://ftp.cv.nrao.edu/NRAO-staff/rindebet/grs-12kms.fits grs-12kms.fits]
  
 +
<source lang="python">
 +
# In CASA
 +
# Initializing sim_observe
 +
# Laying down some basic ground rules
 +
default 'sim_observe'
 +
project = 'n891d'
 +
skymodel = 'grs-12kms.fits'
 +
</source>
  
* units: K - first convert to flux surface brightness
+
* Units: K - first convert to flux surface brightness: Jy/Sr = 2x10<sup>23</sup> k T / &lambda;<sup>2</sup>, <!-- <math>\frac{Jy}{Sr} = \frac{2\times 10^{23} k T}{\pi D^2 \Omega}</math>, where <math>\Omega</math> is the beam solid angle --> = 4x10<sup>8</sup>T at 110GHz.
Jy/Sr = 2x10<sup>23</sup> k T / &lambda;<sup>2</sup>,
 
<!-- <math>\frac{Jy}{Sr} = \frac{2\times 10^{23} k T}{\pi D^2 \Omega}</math>, where <math>\Omega</math> is the beam solid angle -->  
 
= 4x10<sup>8</sup>T at 110GHz.
 
  
now we need to decide if this model data will work at the desired pixel scale
+
<source lang="python">
* the GRS resolution of 40" at ~10kpc is 0.04" at 10Mpc, so we should be able to do a simulation of observing at ~0.1-0.2".  The resolution plot ([[File:Beamsummary.png|100px]]) indicates that for ALMA at 100GHz, configuration 20 is appropriate.
+
# In CASA
* if we intend to set <tt>cell=0.04arcsec</tt> in <tt>simdata</tt>, then the cube needs to be multiplied by
+
# Setting the new frequency of the central channel
4x10<sup>8</sup> * (.04/206265)<sup>2</sup> = 1.4x10<sup>-5</sup> to obtain Jy/pixel.  The cube peaks at ~20K, so we can perform the simulation with <tt>inbright=3e-4</tt>, which should yield a peak of ~1mJy/bm.
+
incenter = '110.1777GHz'
 +
</source>
  
will we be dominated by the noise in the input model?
+
* Now we need to decide if this model data will work at the desired pixel scale
* input noise ~150mK or S/N~20, so at our scaled intensity, ~0.05 mJy/bm. The [http://www.eso.org/sci/facilities/alma/observing/tools/etc/ exposure time calculator] says that ALMA will achieve 2.5mJy/bm in 2 hours for the input 212m/s channel width (0.075MHz), so the noise in the input model should not affect our results. 
+
* The GRS resolution of 40" at ~10kpc is 0.04" at 10Mpc, so we should be able to do a simulation of observing at ~0.1-0.2". The resolution plot (See Figure 1) indicates that for ALMA at 100GHz, configuration 20 is appropriate.
* We do have a sensitivity issue though - if we decrease the spectral resolution by a factor of 6 (bin the input channels in some other program - simdata will know how to do that in the future but not yet), and plan for 3 8-hr tracks, then the sensitivity calculator suggests that we'll get <0.25mJy rms, or S/N>10 per beam.  Rather than simulate 3 days of observing, I'll increase inbright by sqrt(3) and simulate one 8 hour track.
+
[[Image:Beamsummary.png|thumb|Figure 1: Resolution plot.]]
 +
* If we intend to set <tt>incell=0.2arcsec</tt> in <tt>sim_observe</tt>, then the cube needs to be multiplied by 4x10<sup>8</sup> * (.04/206265)<sup>2</sup> = 1.4x10<sup>-5</sup> to obtain Jy/pixel. The cube peaks at ~20K, so we can perform the simulation with <tt>inbright=3e-4</tt>, which should yield a peak of ~1mJy/bm.
  
setup:
+
* Will we be dominated by the noise in the input model? Input noise ~150mK or S/N~20, so at our scaled intensity, ~0.05 mJy/bm. The [http://almascience.eso.org/call-for-proposals/sensitivity-calculator ALMA Sensitivity Calculator] says that ALMA will achieve 2.5mJy/bm in 2 hours for the input 212m/s channel width (0.075MHz), so the noise in the input model should not affect our results.
* the ALMA 12m primary beam is 50" so we'd space a mosaic by 25", but the model cube has 326x357 pixels, or 13 arcsec with our small pixels. That's a lot smaller than the primary beam, so it doesn't matter much what output image size we ask for.
 
* there are 659 channels in the input cube, but as noted above we want to bin those to 109 channels of 1.2 km/s each.
 
  
here are the simdata inputs :
+
<source lang="python">
 +
# In CASA
 +
# Setting the new channel width
 +
inwidth = '0.075MHz'
 +
</source>
  
taskname          = "simdata"
+
* We do have a sensitivity issue though - if we decrease the spectral resolution by a factor of 6 (bin the input channels in some other program - sim_observe will know how to do that in the future but not yet), and plan for 3 8-hr tracks, then the sensitivity calculator suggests that we'll get <0.25mJy rms, or S/N>10 per beam.  Rather than simulate 3 days of observing, I'll increase inbright by sqrt(3) and simulate one 8 hour track.  
project            = "n891d"
 
  
modifymodel = T
+
[[File:N891.coord.png|thumb|Figure 2: here's the cube with the <tt>sim_observe</tt>'s scaling and World Coordinate System]]<br>
  
skymodel        = "grs-12kms.fits"
+
<source lang="python">
 +
# In CASA
 +
# Scaling the surface brightness
 +
inbright = '1.4e-4'
 +
</source>
  
incenter          =  "110.1777GHz"
+
* the ALMA 12m primary beam is 50" so we'd space a mosaic by 25", but the model cube has 326x357 pixels, or 13 arcsec with our small pixelsThat's a lot smaller than the primary beam, so it doesn't matter much what output image size we ask for.   
 
 
inwidth          =  "-.468MHz"
 
 
 
inbright          =  "1.4e-4"
 
 
 
indirection          = 'J2000 7h00m34 -23d03m00'
 
 
 
incell              =  "0.2arcsec"
 
 
 
setpointings = T
 
 
 
integration        = "300s"
 
 
 
pointingspacing    =  "25arcsec"
 
  
 +
<source lang="python">
 +
# In CASA
 +
# Finish up the image model, and setting up the pointing
 +
indirection = 'J2000 7h00m34 -23d03m00'
 +
incell = '0.2arcsec'
 +
setpointings = True
 +
integration = '300s'
 +
pointingspacing = '25arcsec'
 
mapsize = '60arcsec'
 
mapsize = '60arcsec'
 +
</source>
  
graphics = "both"
+
There are 659 channels in the input cube, but as noted above we want to bin those to 109 channels of 1.2 km/s each.
  
 +
<source lang="python">
 +
# In CASA
 +
# Finish up the rest of the settings for this run of sim_observe
 +
graphics = 'both'
 
verbose = True
 
verbose = True
 
 
overwrite = True
 
overwrite = True
 
+
observe = True
antennalist       = "alma;0.5arcsec"
+
antennalist = 'alma;0.5arcsec'
 
+
totaltime = '3600s'
predict=T
+
sim_observe()  # Run sim_observe to create the simulated data we need
 
+
default 'sim_analyze'  
totaltime          = "3600s"
+
project = 'n891d'
 
 
 
image=T
 
image=T
 +
vis = project+'.alma_0.5arcsec.ms'
 +
sim_analyze()  # All other default settings are OK in sim_analyze
 +
</source>
  
  
 
+
[[File:N891.grs-24-cube.coord18-59-59.976-40d00m01.972.png]]<br>
here's the cube with the <tt>simdata</tt>'s scaling and World Coordinate System:
+
Figure 3: a spectral profile in the box marked in green
[[File:N891.coord.png]]<br>
+
<br>
and a spectral profile in the box marked in green<br>
 
[[File:N891.grs-24-cube.coord18-59-59.976-40d00m01.972.png]]
 
 
 
Sample results:
 
 
{| style="border:1px solid #3366FF; " cellspacing=2
 
{| style="border:1px solid #3366FF; " cellspacing=2
|Input:<br> [[File:N891d.skymodel.png|300px]]
+
|Input:<br> [[File:N891d.alma_0.5arcsec.skymodel.png|300px]]
|Predict:<br> [[File:N891d.predict.png|300px]]
+
|Predict:<br> [[File:N891d.alma_0.5arcsec.observe.png|300px]]
 
|-
 
|-
|Image:<br> [[File:N891d.image.png|300px]]
+
|Image:<br> [[File:N891d.alma_0.5arcsec.image.png|300px]]
 
|Analyze:<br> [[File:N891d.analysis.png|300px]]
 
|Analyze:<br> [[File:N891d.analysis.png|300px]]
 
|}
 
|}
 +
Figure 4: Sample results
 +
{{Checked 3.3.0}}

Latest revision as of 10:19, 22 November 2011

Simulating Observations in CASA

Old version: N891 simdata2.

To create a script of the Python code on this page see Extracting scripts from these tutorials.

Nearby edge-on spiral

Roughly modeled after NGC891

Updated for CASA 3.3

  • The cube is being binned to a coarser velocity resolution in order to speed the simulation. The fits file is grs-12kms.fits
# In CASA
# Initializing sim_observe
# Laying down some basic ground rules
default 'sim_observe'
project = 'n891d'
skymodel = 'grs-12kms.fits'
  • Units: K - first convert to flux surface brightness: Jy/Sr = 2x1023 k T / λ2, = 4x108T at 110GHz.
# In CASA
# Setting the new frequency of the central channel
incenter = '110.1777GHz'
  • Now we need to decide if this model data will work at the desired pixel scale
  • The GRS resolution of 40" at ~10kpc is 0.04" at 10Mpc, so we should be able to do a simulation of observing at ~0.1-0.2". The resolution plot (See Figure 1) indicates that for ALMA at 100GHz, configuration 20 is appropriate.
Figure 1: Resolution plot.
  • If we intend to set incell=0.2arcsec in sim_observe, then the cube needs to be multiplied by 4x108 * (.04/206265)2 = 1.4x10-5 to obtain Jy/pixel. The cube peaks at ~20K, so we can perform the simulation with inbright=3e-4, which should yield a peak of ~1mJy/bm.
  • Will we be dominated by the noise in the input model? Input noise ~150mK or S/N~20, so at our scaled intensity, ~0.05 mJy/bm. The ALMA Sensitivity Calculator says that ALMA will achieve 2.5mJy/bm in 2 hours for the input 212m/s channel width (0.075MHz), so the noise in the input model should not affect our results.
# In CASA
# Setting the new channel width
inwidth = '0.075MHz'
  • We do have a sensitivity issue though - if we decrease the spectral resolution by a factor of 6 (bin the input channels in some other program - sim_observe will know how to do that in the future but not yet), and plan for 3 8-hr tracks, then the sensitivity calculator suggests that we'll get <0.25mJy rms, or S/N>10 per beam. Rather than simulate 3 days of observing, I'll increase inbright by sqrt(3) and simulate one 8 hour track.
Figure 2: here's the cube with the sim_observe's scaling and World Coordinate System


# In CASA
# Scaling the surface brightness
inbright = '1.4e-4'
  • the ALMA 12m primary beam is 50" so we'd space a mosaic by 25", but the model cube has 326x357 pixels, or 13 arcsec with our small pixels. That's a lot smaller than the primary beam, so it doesn't matter much what output image size we ask for.
# In CASA
# Finish up the image model, and setting up the pointing
indirection = 'J2000 7h00m34 -23d03m00'
incell = '0.2arcsec'
setpointings = True
integration = '300s'
pointingspacing = '25arcsec'
mapsize = '60arcsec'

There are 659 channels in the input cube, but as noted above we want to bin those to 109 channels of 1.2 km/s each.

# In CASA
# Finish up the rest of the settings for this run of sim_observe
graphics = 'both'
verbose = True
overwrite = True
observe = True
antennalist = 'alma;0.5arcsec'
totaltime = '3600s'
sim_observe()  # Run sim_observe to create the simulated data we need
default 'sim_analyze'  
project = 'n891d'
image=T
vis = project+'.alma_0.5arcsec.ms'
sim_analyze()  # All other default settings are OK in sim_analyze


N891.grs-24-cube.coord18-59-59.976-40d00m01.972.png
Figure 3: a spectral profile in the box marked in green

Input:
N891d.alma 0.5arcsec.skymodel.png
Predict:
N891d.alma 0.5arcsec.observe.png
Image:
N891d.alma 0.5arcsec.image.png
Analyze:
N891d.analysis.png

Figure 4: Sample results

Last checked on CASA Version 3.3.0.