N891 simdata (CASA 3.3): Difference between revisions

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Roughly modeled after NGC891
Roughly modeled after NGC891


{{Under Construction}} - mostly correct, but probably not very thoroughly explained. Updated for CASA 3.3
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]
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# Laying down some basic ground rules
# Laying down some basic ground rules
default 'sim_observe'
default 'sim_observe'
project = "n891d"
project = 'n891d'
skymodel = "grs-12kms.fits"
skymodel = 'grs-12kms.fits'
</source>
</source>


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# In CASA
# In CASA
# Setting the new frequency of the central channel
# Setting the new frequency of the central channel
incenter = "110.1777GHz"
incenter = '110.1777GHz'
</source>
</source>


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* 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.
* 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.
[[Image:Beamsummary.png|thumb|Figure 1: Resolution plot.]]
[[Image:Beamsummary.png|thumb|Figure 1: Resolution plot.]]
* If we intend to set <tt>incell=0.2arcsec</tt> in <tt>simdata</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.
* 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.


* 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://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.   
* 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.   


<source lang="python">
<source lang="python">
# In CASA
# In CASA
# Setting the new channel width
# Setting the new channel width
inwidth = "0.075MHz"
inwidth = '0.075MHz'
</source>
</source>


* 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.   
* 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.   


[[File:N891.coord.png|thumb|Figure 2: here's the cube with the <tt>simdata</tt>'s scaling and World Coordinate System]]<br>
[[File:N891.coord.png|thumb|Figure 2: here's the cube with the <tt>sim_observe</tt>'s scaling and World Coordinate System]]<br>


<source lang="python">
<source lang="python">
# In CASA
# In CASA
# Scaling the surface brightness
# Scaling the surface brightness
inbright = "1.4e-4"
inbright = '1.4e-4'
</source>
</source>


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# Finish up the image model, and setting up the pointing
# Finish up the image model, and setting up the pointing
indirection = 'J2000 7h00m34 -23d03m00'
indirection = 'J2000 7h00m34 -23d03m00'
incell = "0.2arcsec"
incell = '0.2arcsec'
setpointings = True
setpointings = True
integration = "300s"
integration = '300s'
pointingspacing = "25arcsec"
pointingspacing = '25arcsec'
mapsize = '60arcsec'
mapsize = '60arcsec'
</source>
</source>
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# In CASA
# In CASA
# Finish up the rest of the settings for this run of sim_observe
# Finish up the rest of the settings for this run of sim_observe
graphics = "both"
graphics = 'both'
verbose = True
verbose = True
overwrite = True
overwrite = True
observe = True
observe = True
antennalist = "alma;0.5arcsec"
antennalist = 'alma;0.5arcsec'
totaltime = "3600s"
totaltime = '3600s'
sim_observe()  # Run sim_observe to create the simulated data we need
sim_observe()  # Run sim_observe to create the simulated data we need
default 'sim_analyze' # Initialize sim_analyze  
default 'sim_analyze'   
project = "n891d"
project = 'n891d'
image=T
image=T
vis = project+'/'+project+'.alma_0.5arcsec.ms'
vis = project+'.alma_0.5arcsec.ms'
sim_analyze()  # All other default settings are OK in sim_analyze
sim_analyze()  # All other default settings are OK in sim_analyze
</source>
</source>
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<br>
<br>
{| 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
Figure 4: Sample results
{{Checked 3.3.0}}

Latest revision as of 11:19, 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.