Difference between revisions of "N891 simdata2"

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(Nearby edge-on spiral)
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Roughly modeled after NGC891
 
Roughly modeled after NGC891
  
{{Under Construction}} - mostly correct, but probably not very thoroughly explained.
+
{{Under Construction}} - mostly correct, but probably not very thoroughly explained. Updated for CASA 3.1 (simdata2->simdata)
  
 
* 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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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>simdata2</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  
 
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.
 
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.
  
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* 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.
 
* 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 simdata2 inputs : [[File:N891.simdata2.py]]
+
here're the simdata inputs : [[File:N891.simdata2.py]]
 
  CASA <> execfile("N891.simdata2.py")
 
  CASA <> execfile("N891.simdata2.py")
 
  CASA <> go simdata2
 
  CASA <> go simdata2

Revision as of 11:09, 1 December 2010

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 3.1 (simdata2->simdata)

  • I binned the cube to coarser velocity resolution in order to speed the simulation. the fits file is grs-12kms.fits


  • 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:N891.simdata2.py

CASA <> execfile("N891.simdata2.py")
CASA <> go simdata2

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

Sample results:

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