Simulating Observations in CASA 4.1

• This guide describes steps used to simulate interferometric observations in CASA.

• For an introduction to simulations, with examples and a discussion of their relevance to ALMA observing proposals, start at the Guide to Simulating ALMA Data.

• This guide is applicable to CASA version 4.1. For older versions of CASA please see Simulating Observations in CASA 4.0.

Introduction

There are two main tasks for simulating observations in CASA: simobserve and simanalyze. Starting with a model of sky brightness, simobserve generates the visibilities that would be measured with a telescope such as ALMA, the VLA, CARMA, SMA, or PdB. The simobserve task can add thermal noise to the visibilities. The simobserve task uses the aatm atmospheric model (based on Juan Pardo's ATM library) to simulate real observing conditions and introduce atmospheric "corruption", i.e. noise and phase delay.

Next, the task simanalyze will produce a cleaned image based on the generated visibilities. It can compare the simulated image with your input (convolved with the output clean beam) and then calculate a "fidelity image" that indicates how well the simulated output matches the convolved input image.

Both simobserve and simanalyze can be broken down into a number of steps. The major steps are:

1. simobserve
   1. Modify Model - If desired, you can scale the spatial coordinates, spectral axis, and brightness of the sky model image to simulate a specific type of observation. (For example, if you start with a model of M100 you might wish to scale the axes to simulate an observation of an M100-like galaxy that is 4X more distant.)
   2. Set Pointings - If your simulation contains multiple pointings in a mosaic, calculate the positions of individual pointings. These are saved in a text file. (You could also make such a text file yourself.)
   3. Predict - Calculate the visibilities. You will specify a telescope, antenna configuration, and date for the simulated observation. The Antenna List page describes how to specify the telescope, including new telescopes, in more detail.
   4. Corrupt - Next, "corrupt" the data, for example by introducing with thermal noise or phase noise.
2. simanalyze
   1. Image - Image the visibility data with CASA's clean task.
   2. Analyze - Calculate and display the difference between output and input, and the fidelity image.

The Simulation Guide for New Users tutorial introduces and explains the individual steps in detail. It is possible to run the steps independently and optionally, as long as you follow the simobserve and simanalyze conventions about filenames.

Taskname History

The simulation tasks have been under significant revision and development.

• In CASA 3.3 simobserve and simanalyze were named sim_observe and sim_analyze, respectively.
• In CASA 3.4 and earlier, the functionality of both tasks was contained in task simdata, which is now obsolete.
• In CASA 4.1, a new experimental task simalma has been introduced, which simplifies simulation of 12m interferometric + 7m interferometric + total power ALMA observations. Please report any issue with simalma, and check back here for an upcoming simalma casaguide.

ALMA Simulations and Proposals

Although simobserve can simulate data for many instruments, this guide (and the tutorials linked below) focuses on simulating ALMA observations. ALMA is still being commissioned. We will update CASA's simulation tasks and tools as commissioning proceeds to better reflect the available array configurations and noise characteristics.

Rigorous verification of the noise calculations between simobserve and the ALMA sensitivity calculator are performed periodically. (The ALMA Observation Preparation Tool uses the same algorithm as the ALMA sensitivity calculator.) The most recent such verification was performed during August 2012.

The noise in a synthesis image is dependent on the imaging and deconvolution parameters; therefore small differences between the simple radiometer-equation based ALMA sensitivity calculator and simobserve/simanalyze are to be expected. See NAASC memo 102 for more discussion.

Larger differences between the radiometer equation and a simulation may also result in a simulated image that is limited by dynamic range. It is for this reason that ALMA users should consider running a simulation when applying for telescope time. The results of the simulation can be a strong support to one's observing proposal, as a way to demonstrate the scientific feasibility of the observing plan.

Finally, simobserve will not be able to provide realistic results if the pixel size of the input sky model is comparable to, or larger than, the simulated output clean beam.

Users should 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 simobserve, the OST is based on the CASA sm toolkit. However, the OST and simobserve use different wrapper scripts and employ different treatment of atmospheric effects. Comparisons to the ALMA sensitivity calculator made in March 2011 suggest that both simobserve and the OST give similar noise level for observations in bands 3 through 8. However, the results of the two tools diverge in bands 9 and 10.

Values from simobserve or the OST should not be used to calculate exposure times for ALMA Science Goals. Only values from the ALMA sensitivity calculator should be used for this purpose, as it is the ALMA sensitivity calculator that will be used in the technical assessment of ALMA proposals. However, the results of simulations may be helpful to support a request for more observing time than is required by the sensitivity calculator, in order to obtain better uv-plane coverage.

Tutorials

Simulation Guide for New Users (CASA 4.1)
A fully annotated tutorial that uses a Spitzer SAGE 8 micron continuum image of 30 Doradus and scales it to greater distance. A good place for new users to start.
Protoplanetary Disk Simulation (CASA 4.1)
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. This is another fairly generic simulation - if you're short on time, you probably don't need to go through this one and the New Users guide, but it can be useful to go through multiple examples.
Simulation Guide Component Lists (CASA 4.1)
Tutorial for simulating data based on multiple sources (using both a FITS image and a component list). If you are interested in simulating from a list of simple sources (point, Gaussian, disk), rather than or in addition to a sky model image, then read the considerations here.
Einstein-Face (CASA 4.1)
A sky model and lightly annotated script that simulates the face of Einstein as seen by ALMA. This simulation is particularly useful for those who wish to better understand spatial filtering by an interferometer, but doesn't demonstrate new capabilities of the simulation tasks beyond those described above.
ACA Simulation (CASA 4.1)
A tutorial for simulating ALMA observations that use multiple configurations or use the 12-meter array in combination with the ALMA Compact Array. Of particular interest to those wishing to explore multi-component ALMA observations and their combination and e.g. whether the ACA improves your scientific fidelity.

Example Input Images

Several examples of input model images are showcased here: Sim Inputs. These may be useful for running your own simulation tests, beyond what are presented in the above tutorials.

Advanced CASA Simulation

Under the Hood: The sm Tool

simobserve calls methods in the sm (simulation) tool. For advanced CASA users, the sm tool has methods that can add to simulated data: phase delay variations, gain fluctuations and drift, cross-polarization, and (coming soon) bandpass and pointing errors. sm also has more flexibility than simobserve in adding thermal noise. The tutorials linked from this page describe the simulation of data using the task interface only. To learn more about the sm tool, see the CASA Toolkit Reference Manual.

The sm tool can be used to corrupt your measurement set with thermal noise, phase noise, cross-polarization, etc., in a more advanced way than is done in simobserve. To learn advanced techniques for corrupting a simulated measurement set, see Corrupt.

Ephemeris and Geodesy

Generic ephemeris and geodesy calculations can be done using CASA Python module simutil.py.

Warning: CLEAN Bias

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 (an effect known as "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.

User Feedback

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