Simulating Observations in CASA 3.4

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Introduction

Simulation capability in CASA follows the usual two-layered structure: there is a beginner-level Python task interface called simobserve, which calls methods in the sm C++ tool. The task interface turns a model of the sky (2 to 4 dimensions including frequency and Stokes) into the visibilities that would be measured with ALMA, (E)VLA, CARMA, SMA, ATCA, PdB, etc. simobserve can also add thermal noise (from receiver, atmosphere, and ground) to the visibilities. The task simanalyze will then produce a cleaned image of the model visibilities, compare that image with your input convolved with the synthesized beam, and calculate a fidelity image.

For more advanced users, the sm tool has methods that can be used to add phase delay variations, gain fluctuations and drift, cross-polarization, and (coming soon) bandpass and pointing errors to your simulated data. sm also has more flexibility in adding thermal noise than simobserve.

Note that simobserve and simanalyze are new tasks in CASA 3.3. In earlier versions of CASA the functionality of both tasks was contained in a single task called simdata. simdata is still available in CASA 3.3 but is deprecated and may be removed in a future CASA release.

CASA simulation uses the aatm atmospheric model, a thin wrapper of Juan Pardo's ATM library, to accurately calculate all atmospheric corruption terms (noise, phase delay) accurately as a function of frequency and site characteristics.

Part of CASA's simulation routines are generic ephemeris and geodesy calculations available in Python: see simutil.py.

Note on cleaning: just as is the case for real images, cleaning images produced by simobserve can lead to a spurious decrease in object fluxes and noise on the image ("clean bias"). This is particularly true for observations with poor coverage of the uv-plane, i.e. using telescopes with small numbers of antennas, such as the ALMA Early Science configurations, and/or in short "snapshot" observations. Users should always clean images with care, using a small number of iterations and/or a conservative (3-5sigma) threshold, and boxing bright sources.

Steps to simulation

If you have not already installed CASA, follow these steps: Install CASA.

simobserve and simanalyze can be broken down into "subtasks" that are modular, i.e. as long as you follow a few conventions about filenames, you can run each bit independently and optionally. For example, using simobserve, you can modify the sky model, then predict ACA visibilities, then run again and predict ATCA 12m visibilities. With simanalyze you can image and analyze both the ACA and ATCA measurement sets together. You can run interactive clean yourself, and, minding your filenames, you can run simanalyze just to calculate a difference image and analyze the results.

The major steps involved in simulating data with CASA are:

  1. Modify Model - relabel (scale) the spectral and spatial coordinates and brightness of the sky model image.
  2. Set Pointings - calculate a mosaic of pointings and save in a text file. You could also make the text file yourself.
  3. Predict - Calculate visibilities for a specified array on a specified day.
  4. Corrupt - Corrupt the measurement set with thermal noise, phase noise, cross-polarization, etc.
  5. Image - Image the visibility data with clean.
  6. Analyze - Calculate and display the difference between output and input, and fidelity image.

Simulating ALMA Observations

We will update CASA's simulation tasks and tools as ALMA commissioning proceeds. During this period, we expect the noise properties of the telescope to be increasingly better characterized, and its configurations to be refined. Updates will be placed below, under "ALMA updates", along with an estimate of which version of CASA they will be applied to. Updated configuration files will also be placed here, until they can be incorporated in the next CASA release.

Users should also be aware of the Observation Support Tool (OST) [1]. This is a web-based interface to an ALMA simulator hosted by the University of Manchester, UK. Like the simobserve, it is based on the CASA sm toolkit, but uses different wrapper scripts, and, in particular, has a different treatment of atmospheric effects. Comparisons to the ALMA sensitivity calculator made in March 2011 suggest that both simobserve and the OST give similar noises for observations in bands 3-8, but the OST diverges in bands 9 and 10. In general, however, because the ALMA sensitivity calculator will be used for the technical assessment of ALMA proposals, only values from it, not simobserve or the OST, should be used to estimate exposure times for ALMA Science Goals.

Tutorials, Recipes, and Example images

Simulation Guide for New Users (CASA 3.3) 30Dor ES.png
A fully annotated tutorial that uses a Spitzer SAGE 8 micron continuum image of 30 Doradus and scales it to greater distance.
M51 at z = 0.1 and z = 0.3 (CASA 3.3) M51thumb.png
A fully annotated tutorial that uses a BIMA-SONG cube of a nearby galaxy and scales it to greater distance.
Simulation Guide Component Lists (CASA 3.3) Analyze fits list.jpg
Tutorial for simulating data based on multiple sources (using both a FITS image and a component list).
PPdisk simdata (CASA 3.3) Psimthumb.png
A sky model with a lightly annotated script that simulates a protoplanetary disk. Uses a theoretical model of dust continuum from Sebastian Wolff, scaled to the distance of a nearby star.
N891 simdata (CASA 3.3) N891thumb.png
A sky model, script, and discussion that simulates a nearby edge-on spiral galaxy. Uses a galactic CO cube from the Galactic Ring Survey and places it at 10Mpc. The data are similar to what NGC891 would look like if it were observable from the southern hemisphere.
Einstein-Face (CASA 3.3) Einstein fs cfg8 1hr.gif
A sky model and lightly annotated script that simulates the face of Einstein as seen by ALMA. Uses a non-science image to demonstrate the effects of spatial filtering by ALMA.
ACA Simulation M51c.ALMA 0.5arcsec.skymodel.png
A tutorial for simulating ALMA observations that use multiple configurations or use the

12-meter array in combination with the ALMA Compact Array.

Other example input images
Other example output simulations (scripts to reproduce these are coming)

Technical and Planning

We welcome input on developing the CASA simulator. Contact "rindebet at nrao.edu" if you would like to volunteer your input.