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
{| border="0" cellspacing="0" cellpadding="2" align="center"
+
Jy/Sr = 4x10<sup>23</sup> k T / ( &pi; D<sup>2</sup> &Omega; ), where &Omega;
! Jy
+
<math>\frac{Jy}{Sr} = \frac{4\times 10^{23} k T}{\pi D^2 \Omega}</math>, where <math>\Omega</math>  
|-
+
is the beam solid angle.  At 110GHz the factor is 2x10<sup>8</sup>
| Sr
 
|}
 
 
 
<math>\frac{Jy}{Sr} = \frac{4\times 10^{23} k T}{\pi D^2 \Omega}</math>, where <math>\Omega</math> is the beam solid angle.  At 110GHz the factor is 2x10<sup>8</sup>
 
  
 
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  

Revision as of 16:07, 16 December 2009

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 = 4x1023 k T / ( π D2 Ω ), where Ω [math]\frac{Jy}{Sr} = \frac{4\times 10^{23} k T}{\pi D^2 \Omega}[/math], where [math]\Omega[/math] is the beam solid angle. At 110GHz the factor is 2x108

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". 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

2x108 * (.04/206265)2 = 7.9x10-6 to obtain Jy/pixel. The cube peaks at ~3K, so we can perform the simulation with inbright=2.3e-5, which should yield a peak of ~0.3mJy/bm.

will we be dominated by the noise in the input model?

  • input noise ~150mK or S/N~20, so at our scaled intensity, ~0.015 mJy/bm. The exposure time calculator says that ALMA will achieve 2.6mJy/bm in 2 hours if we keep the input 212m/s channel width (0.075MHz), so the noise in the input model should not affect our results.

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, so we'll keep the same width and nchan.

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 on the channel image above N891.grs-24-cube.coord18-59-59.976-40d00m01.972.png