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

From CASA Guides
Line 11: Line 11:
 
{{Under Construction}} - mostly correct, but probably not very thoroughly explained. Updated for CASA 3.3
 
{{Under Construction}} - mostly correct, but probably not very thoroughly explained. 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">
 
<source lang="python">
Line 24: Line 24:
 
</source>
 
</source>
  
Units: K - first convert to flux surface brightness: Jy/Sr = 2x10<sup>23</sup> k T / &lambda;<sup>2</sup>,
+
* 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 -->  
 
<!-- <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.
 
= 4x10<sup>8</sup>T at 110GHz.
Line 35: Line 35:
  
 
* Now we need to decide if this model data will work at the desired pixel scale  
 
* 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.
+
* 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>cell=0.04arcsec</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>cell=0.04arcsec</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.
Line 58: Line 58:
 
setup:
 
setup:
 
* 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.   
 
* 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.   
 +
 
<source lang="python">
 
<source lang="python">
 
# In CASA
 
# In CASA
Line 72: Line 73:
  
 
<source lang="python">
 
<source lang="python">
#In CASA
+
# In CASA
 +
# Finish up the rest of the settings for this run of simdata
 
graphics = "both"
 
graphics = "both"
 
verbose = True
 
verbose = True
Line 85: Line 87:
  
 
here's the cube with the <tt>simdata</tt>'s scaling and World Coordinate System:
 
here's the cube with the <tt>simdata</tt>'s scaling and World Coordinate System:
[[File:N891.coord.png]]<br>
+
[[File:N891.coord.png|Figure 2: here's the cube with the <tt>simdata</tt>'s scaling and World Coordinate System]]<br>
 
and a spectral profile in the box marked in green<br>
 
and a spectral profile in the box marked in green<br>
 
[[File:N891.grs-24-cube.coord18-59-59.976-40d00m01.972.png]]
 
[[File:N891.grs-24-cube.coord18-59-59.976-40d00m01.972.png]]

Revision as of 15:27, 3 October 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

This article is under construction. Watch this space!

- mostly correct, but probably not very thoroughly explained. 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 simdata
# Laying down some basic ground rules
default("simdata")
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 cell=0.04arcsec in simdata, 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 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.

# 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 - 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.
# In CASA
# Scaling the surface brightness
inbright           =  "1.4e-4"

setup:

  • 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 simdata
graphics = "both"
verbose = True
overwrite = True
observe = True
antennalist        =  "alma;0.5arcsec"
totaltime          =  "3600s"
image=T
simdata()


here's the cube with the simdata's scaling and World Coordinate System: Figure 2: here's the cube with the simdata's scaling and World Coordinate System
and a spectral profile in the box marked in green
N891.grs-24-cube.coord18-59-59.976-40d00m01.972.png

Sample results:

Input:
N891d.skymodel.png
Predict:
N891d.predict.png
Image:
N891d.image.png
Analyze:
N891d.analysis.png