Protoplanetary Disk Simulation - VLA-CASA5.3.0
VLA version: under construction!!!
- This is an advanced simulation tutorial. New users are recommended to begin with reading the CASA Docs documentation pages on "Simulations".
- This guide is applicable to CASA version 5.1.
- To create a script of the Python code on this page see Extracting scripts from these tutorials.
For this CASA Guide we will use a protoplanetary disk model from S. Wolf. Get the data here. Note that this is a modified image from the ALMA Simulation Inputs, where we changed the center frequency 35GHz and the bandwidth to 128MHz. '
Script with Explanation
Set simobserve as the current task and reset all parameters.
# In CASA default("simobserve")
Let's call our project "psimvla". This defines the root prefix for any output files from simobserve.
project = "psimvla"
Review the image coordinate system using task imhead.
# This reports image header parameters in the Log Messages window imhead("ppdisk41_GHz_50pc.fits")
This confirms that the data are set at 41GHz center frequency with a bandwidth of 128MHz. Note that the reference coordinate of the image is at pixel (0,0), i.e. in a corner of the image, not the center. In [] a method is provided on how to derive the image centers through the toolkit. Using that method, or inspecting the image with the viewer shows that the image center is at RA=18h00m00.031s, DEC=-22d59m59.6s.
We'll set skymodel to the FITS file downloaded, above, and leave all skymodel subparameters at their default values. simobserve will create CASA image psimvla.skymodel.
skymodel = "ppdisk41_GHz_50pc.fits"
We will specify the sky position for the center of the observation and set the map size to the size of the model image. The image is very small compared to the VLA primary beam at 35GHz () Since the model image is 2/3 arcseconds across, we should only need one pointing. In this case, pointingspacing and maptype can be left at their default values.
setpointings = True direction = "J2000 18h00m00.031s -22d59m59.6s" mapsize = "0.76arcsec"
We do want to simulate an interferometric observation, so we set obsmode accordingly. We'll set totaltime to a 20-minute snapshot observation.
obsmode = "int" totaltime = "1200s"
We want to use the appropriate antenna configuration for the desired angular resolution. Configuration 20, alma.out20.cfg, is the largest "compact" configuration. A list of configuration files available in CASA 4.1 is available here.
antennalist = "vla.a.cfg"
We do not want to simulate observations with thermal noise in this example, so we set:
thermalnoise = ''
Now run simobserve.
Now that we've simulated the visibility measurements, we want to generate an image from the simulated data. simanalyze makes this easy. We begin by setting project to the same prefix used in simobserve and setting image to True.
default ("simanalyze") project = "psimvla" image = True
We set modelimage to use the input FITS image when cleaning the simulated visibilities. We set the image size to 192 pixels square.
modelimage = "ppdisk41_GHz_50pc.fits" vis = project + ".vla.a.ms" imsize = [192, 192]
Specify the number of iterations for cleaning, with proper threshold and weighting.
niter = 10000 threshold = "1e-7Jy" weighting = "natural"
We'd like to calculate a difference and fidelity image, but we don't need to see the uv coverage.
analyze = True showuv = False showresidual = True showconvolved = True
Plot both to the screen and PNG files with lots of messages.
graphics = "both" verbose = True overwrite = True