Difference between revisions of "Simulation Recipes"

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(Nearby edge-on spiral)
(Nearby edge-on spiral)
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* model data: Milky Way 13CO from the [http://www.bu.edu/galacticring/ Galactic Ring Survey] on the 14m [http://www.astro.umass.edu/~fcrao/ FCRAO]
 
* model data: Milky Way 13CO from the [http://www.bu.edu/galacticring/ Galactic Ring Survey] on the 14m [http://www.astro.umass.edu/~fcrao/ FCRAO]
 
* units: K - first convert to flux surface brightness
 
* units: K - first convert to flux surface brightness
<!-- Jy/Sr = 4x10<sup>23</sup> k T / ( &pi; D<sup>2</sup> &Omega; ), where &Omega; -->
+
Jy/Sr = 2x10<sup>23</sup> k T / &lambda;<sup>2</sup>,
<math>\frac{Jy}{Sr} = \frac{4\times 10^{23} k T}{\pi D^2 \Omega}</math>, where <math>\Omega</math>  
+
<!-- <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 -->
is the beam solid angle.  At 110GHz the factor is 2x10<sup>8</sup>
+
= 4x10<sup>8</sup>T at 110GHz.
  
 
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 ([[File:Beamsummary.png|100px]]) 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 ([[File:Beamsummary.png|100px]]) indicates that for ALMA at 100GHz, configuration 20 is appropriate.
 
* if we intend to set <tt>cell=0.04arcsec</tt> in <tt>simdata</tt>, then the cube needs to be multiplied by  
 
* if we intend to set <tt>cell=0.04arcsec</tt> in <tt>simdata</tt>, then the cube needs to be multiplied by  
2x10<sup>8</sup> * (.04/206265)<sup>2</sup> = 7.9x10<sup>-6</sup> to obtain Jy/pixel.  The cube peaks at ~10K, so we can perform the simulation with <tt>inbright=8e-5</tt>, which should yield a peak of ~1mJy/bm.
+
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?
 
will we be dominated by the noise in the input model?

Revision as of 18:31, 10 January 2010

Simulating Observations in CASA

Massively Under Construction

(i.e. don't believe what you see here yet, or ask me when it doesn't make sense :)

Nearby edge-on spiral

Roughly modeled after NGC891

  • model data: Milky Way 13CO from the Galactic Ring Survey on the 14m FCRAO
  • units: K - first convert to flux surface brightness

Jy/Sr = 2x1023 k T / λ2, = 4x108T at 110GHz.

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 (Beamsummary.png) indicates that for ALMA at 100GHz, configuration 20 is appropriate.
  • 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.
  • 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.

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.
  • 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're the simdata inputs : File:Simdata.n891.txt

CASA <> execfile("simdata.ngc891.txt")
CASA <> go simdata

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