# Difference between revisions of "N891 simdata (CASA 3.3)"

Line 15: | Line 15: | ||

* I binned the cube to coarser velocity resolution in order to speed the simulation. the fits file is [ftp://ftp.cv.nrao.edu/NRAO-staff/rindebet/grs-12kms.fits grs-12kms.fits] | * I binned the cube to coarser velocity resolution in order to speed the simulation. the fits file is [ftp://ftp.cv.nrao.edu/NRAO-staff/rindebet/grs-12kms.fits grs-12kms.fits] | ||

+ | <source lang="python"> | ||

+ | # In CASA | ||

+ | # Initializing simdata | ||

+ | # Laying down some basic ground rules | ||

+ | default("simdata") | ||

+ | project = "n891d" | ||

+ | skymodel = "grs-12kms.fits" | ||

+ | </source> | ||

− | * 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>, |

− | Jy/Sr = 2x10<sup>23</sup> k T / λ<sup>2</sup>, | ||

<!-- <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 --> | <!-- <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 --> | ||

= 4x10<sup>8</sup>T at 110GHz. | = 4x10<sup>8</sup>T at 110GHz. | ||

− | + | <source lang="python"> | |

− | * 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 ( | + | # Setting the new frequency of the central channel |

− | * if we intend to set <tt>cell=0.04arcsec</tt> in <tt>simdata</tt>, then the cube needs to be multiplied by | + | incenter = "110.1777GHz" |

+ | </source> | ||

+ | |||

+ | * 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 (See Figure 1) indicates that for ALMA at 100GHz, configuration 20 is appropriate. | ||

+ | [[Image:Beamsummary.png|thumb|Figure 1: Resolution plot.]] | ||

+ | * 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. | ||

Line 35: | Line 48: | ||

here are the simdata inputs : | here are the simdata inputs : | ||

− | + | ||

− | |||

− | |||

− | |||

− | |||

inwidth = "0.075MHz" | inwidth = "0.075MHz" | ||

inbright = "1.4e-4" | inbright = "1.4e-4" |

## Revision as of 14:07, 3 October 2011

↵ **Simulating Observations in CASA**

*Old version: N891 simdata2.*

To create a script of the Python code on this page see Extracting scripts from these tutorials.

## 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 3.3

- model origin: Milky Way 13CO from the Galactic Ring Survey on the 14m FCRAO

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

```
# In CASA
# Initializing simdata
# Laying down some basic ground rules
default("simdata")
project = "n891d"
skymodel = "grs-12kms.fits"
```

- units: K - first convert to flux surface brightness: Jy/Sr = 2x10
^{23}k T / λ^{2},

= 4x10^{8}T at 110GHz.

```
# Setting the new frequency of the central channel
incenter = "110.1777GHz"
```

- 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 (See Figure 1) indicates that for ALMA at 100GHz, configuration 20 is appropriate.

* if we intend to setcell=0.04arcsecinsimdata, then the cube needs to be multiplied by

4x10^{8} * (.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 are the simdata inputs :

inwidth = "0.075MHz" inbright = "1.4e-4" indirection = 'J2000 7h00m34 -23d03m00' incell = "0.2arcsec" setpointings = True integration = "300s" pointingspacing = "25arcsec" mapsize = '60arcsec' graphics = "both" verbose = True overwrite = True observe = True antennalist = "alma;0.5arcsec" totaltime = "3600s" image=T simdata() </source>

here's the cube with the `simdata`'s scaling and World Coordinate System:

and a spectral profile in the box marked in green

Sample results:

Input: |
Predict: |

Image: |
Analyze: |