N891 simdata (CASA 3.4)

From CASA Guides
Revision as of 10:27, 12 June 2012 by Jcrossle (talk | contribs)
Jump to navigationJump to search

Simulating Observations in CASA

Nearby edge-on spiral

Roughly modeled after NGC891

Updated for CASA 3.4

  • The cube is being binned to a coarser velocity resolution in order to speed the simulation. The fits file is grs-12kms.fits
# Initializing simobserve
# Laying down some basic ground rules
default 'simobserve'
project = 'n891d'
skymodel = 'grs-12kms.fits'
  • Units: K - first convert to flux surface brightness: Jy/Sr = 2x1023 k T / λ2, = 4x108T 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.
Figure 1: Resolution plot.
  • If we intend to set incell=0.2arcsec in simobserve, 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 ALMA Sensitivity 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.
# 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 - simobserve 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.
Figure 2: here's the cube with the simobserve's scaling and World Coordinate System

# 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.
# 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.

# Finish up the rest of the settings for this run of simobserve
graphics = 'both'
verbose = True
overwrite = True
obsmode = 'int'
antennalist = 'alma;0.5arcsec'
totaltime = '3600s'
simobserve()  # Run simobserve to create the simulated data we need
default 'simanalyze'  
project = 'n891d'
imsize = [336,360]
vis = project+'.alma_0.5arcsec.ms'
simanalyze()  # All other default settings are OK in simanalyze

Figure 3: a spectral profile in the box marked in green

N891d.alma 0.5arcsec.skymodel.png
N891d.alma 0.5arcsec.observe.png
N891d.alma 0.5arcsec.image.png

Figure 4: Sample results

Last checked on CASA Version 3.4.0.