Simulating Observations in CASA 3.3
This guide is applicable to CASA version 3.3.
Simulation capability in CASA follows the usual two-layered structure: there is a beginner-level Python task interface called sim_observe, 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. sim_observe can also add thermal noise (from receiver, atmosphere, and ground) to the visibilities. The task sim_analyze 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 sim_observe.
Note that sim_observe and sim_analyze 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 sim_observe 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.
sim_observe and sim_analyze 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 sim_observe, you can modify the sky model, then predict ACA visibilities, then run again and predict ATCA 12m visibilities. With sim_analyze 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 sim_analyze just to calculate a difference image and analyze the results.
The major steps involved in simulating data with CASA are:
- Modify Model - relabel (scale) the spectral and spatial coordinates and brightness of the sky model image.
- Set Pointings - calculate a mosaic of pointings and save in a text file. You could also make the text file yourself.
- Predict - Calculate visibilities for a specified array on a specified day.
- Corrupt - Corrupt the measurement set with thermal noise, phase noise, cross-polarization, etc.
- Image - Image the visibility data with clean.
- 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) . This is a web-based interface to an ALMA simulator hosted by the University of Manchester, UK. Like the sim_observe, 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 sim_observe 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 sim_observe or the OST, should be used to estimate exposure times for ALMA Science Goals.
No updates currently for CASA 3.3.
Tutorials, Recipes, and Example images
|Simdata New Users Guide|
|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|
|A fully annotated tutorial that uses a BIMA-SONG cube of a nearby galaxy and scales it to greater distance.|
|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.|
|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.|
|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.|
|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.