N891 simdata (CASA 3.4)

↵ Simulating Observations in CASA

• This is an advanced simulation tutorial. New users are recommended to begin with the Simulation Guide for New Users (CASA 3.4).
• This guide is applicable to CASA version 3.4. For older versions of CASA see N891 simdata2.
• To create a script of the Python code on this page see Extracting scripts from these tutorials.

This tutorial simulates ALMA interferometric data for a nearby edge-on spiral galaxy.

Roughly modeled after NGC891

Data

For our model, we will use a data cube of 13CO from the Galactic Ring Survey on the 14-meter FCRAO. To speed the simulation, this FITS file contains the data we want binned to a coarser velocity resolution.

Simulation

# In CASA
# 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.

# 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.

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.

# 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 - 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

# 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 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'
image=T
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

Input:	Predict:
Image:	Analyze:

Figure 4: Sample results

Last checked on CASA Version 3.4.0.