Simulation Recipes

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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

| border="0" cellspacing="0" cellpadding="2" align="center" ! Jy |- | Sr |}

[math]\displaystyle{ \frac{Jy}{Sr} = \frac{4\times 10^{23} k T}{\pi D^2 \Omega} }[/math], where [math]\displaystyle{ \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.


  • 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