Simulation Recipes: Difference between revisions
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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 = 2x10<sup>23</sup> k T / λ<sup>2</sup>, | |||
<math>\frac{Jy}{Sr} = \frac{ | <!-- <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 | = 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 | ||
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 22:31, 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 (
) 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:
and a spectral profile in the box marked in green
