N891 simdata (CASA 3.3)

↵ 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

The cube is being binned to a 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 = 2x10²³ k T / λ², = 4x10⁸T at 110GHz.

# In CASA
# 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 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.

# In CASA
# Setting the new channel width
inwidth          =  "0.075MHz"

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.

# In CASA
# Scaling the surface brightness
inbright           =  "1.4e-4"

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.

# In CASA
# Finish up the image model, and setting up the pointing
indirection          =  'J2000 7h00m34 -23d03m00'
incell               =  "0.2arcsec"
setpointings = True
integration        =  "300s"
pointingspacing    =  "25arcsec"
mapsize = '60arcsec'

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.

# In CASA
# Finish up the rest of the settings for this run of simdata
graphics = "both"
verbose = True
overwrite = True
observe = True
antennalist        =  "alma;0.5arcsec"
totaltime          =  "3600s"
image=T
simdata()