Guide To Simulating ALMA Data

Most recently updated for CASA Version 6.6.1 using Python 3.8

Introduction: About this Document

This document describes why and how to use simulated observations to help understand your ALMA data, or to help you plan an ALMA observation. The document covers:

• Why you might want to simulate ALMA observations
• How simulations can help you plan a proposal
• Examples of simulating ALMA data
• Running your own simulations with the ALMA Observation Support Tool
• Simulating ALMA observations with CASA sim tools

Why Simulate ALMA Observations?

Observations made with radio interferometers can be tricky to interpret and analyze. Interferometers do not sample all spatial frequencies on the sky, so the image you generate from an interferometric observation does not necessarily represent the full sky brightness distribution. Specifically, interferometers are not sensitive to diffuse emission, and depending on a number of factors including the placement of the individual antennas, the length of the observation, and the location of the target source on the sky, an interferometric observation will be insensitive to structure on some angular scales. Observers therefore should use care when analyzing and interpreting interferometric images.

Plots of "uv coverage" are often used to illustrate the range of spatial frequencies measured by a specific interferometric observation. A uv coverage plot can be thought of as a mask that is applied to the Fourier transform of the emission pattern on the sky. More densely sampled uv coverage leads to better images because the observation is more sensitive to a wider range of spatial frequencies on the sky.

Several interferometers, including ALMA and the VLA, offer a number of antenna configurations, amounting to a "zoom lens" capability. To aid in interpreting their data, observers may find it helpful to simulate an interferometric observation using a variety of antenna configurations and different models of the sky brightness distribution. Simulations can also be a powerful tool to help new users understand both the power and the limitations of interferometric observations.

A Note Regarding Use of ALMA Simulations in your Proposal

Simulations of ALMA observations are not required for an ALMA proposal; however, a simulation could bolster an ALMA proposal in some cases. For example, simulations can demonstrate the need for specific configurations, or combinations of configurations, to resolve certain structures or meet specific scientific goals.

In Full Operations, ALMA will include a main array of fifty 12 m antennas, and the ALMA Compact Array (ACA) which comprises twelve 7 m antennas and four 12 m antennas used for "total power" observations. At this time, every ALMA observation employs the 12-m main array. Observations with the 7-m Array and Total Power antennas are optional, used to fill in large-scale structure in mapping experiments. The ALMA OT advises users on whether to use the ACA based on two numbers: the required resolution and the maximum angular scale. If you have some uncertainty in the spatial scales for your ALMA target, you may wish to use simulations to justify inclusion of the ACA.

Background Information for Those New to Interferometry

Those new to interferometric observing can gain some intuition on the use of interferometers by exploring several software tools available on the web. For example, the Virtual Radio Interferometer is an interactive java application that allows one to simulate basic observations with MERLIN, ATCA, or WSRT, or ASKAP. The VRI can demonstrate how antenna arrangement affects uv coverage, the synthesized beam (i.e. the "resolution"), and the range of spatial sensitivities. Users can select from a few sample, idealized sky brightness patterns (e.g. a narrow gaussian) or use real sky images, then fiddle interactively with the placement of antennas and the duration of the observation to see the effects of uv coverage on observations.

Sample Simulations

Before getting to the details of using ALMA simulation tools, it is useful to examine a few example simulations. These examples were generated with the CASA simulation software described later in this document. These simulations demonstrate how observing with different ALMA configurations can affect the final image.

A Collection of Point Sources

This example shows a simulated observation of a model with multiple point sources. The "plus" symbols in the model image represent locations of the point sources. The panels in this example and those that follow show the model image and the simulated observation with an "extended" array on the top row. The bottom row shows an observation with a compact configuration, and then a compact configuration with the ACA included. The ellipse on the bottom left of the simulated data shows the synthesized beam size. The angular size of the image is approximately 6x6 arcsec. In general, it is preferable to observe closely-spaced compact sources with an extended configuration, as demonstrated in these simulations. Note that the ACA does not contribute significantly to this observation. It's main contribution comes in observing sources with relatively bright, diffuse emission in addition, perhaps, to compact targets.

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Point Sources and Elliptical Gaussian Brightness Distributions

Expanding on the first example, here we add several elliptical gaussians to the model with the point sources. Again, the "plus" symbols in the model image represent locations of point sources. In this case, the extended configuration completely misses the emission from the larger Gaussian component. Only the compact configuration recovers the flux from the extended Gaussian components. Including the ACA gives a more complete representation of the source flux. To recover both the conpact and extended structure in this model would require combining data from multiple main-array configurations, plus the ACA.

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An M51-like Galaxy

Here we simulate an observation of a galaxy similar in structure to M51, but smaller in angular scale. The extended array observation gives a reasonably good representation of the model image, because most of the apparent emission is on small angular scales. The compact array observation shows a negative "bowl" of emission between the spiral arms because of the missing short-spacing data. By adding the ACA to the compact array observation, the short spacings are recovered and the final flux densities recorded are truer to the model image.

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ALMA Simulation Tools

ALMA observers have two powerful simulation tools available: the CASA tasks (simalma, simobserve, and simanalyze), and the Observation Support Tool (OST). Both of these simulators are built on the CASA sm toolkit, and both are available from links on the ALMA Science Portal.

Note that significant differences may be seen between the noise predicted by the ALMA Sensitivity Calculator and the measured RMS in simulated images. These differences are primarily a result of the RMS measured in an image depending sensitively on the details of how the image is deconvolved. Noise values determined from CASA simulations or the OST should not be used in a proposal. Only values from the ALMA sensitivity calculator should be used to calculate exposure times for ALMA Science Goals, 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. Therefore, if using the results from the simulators in a Technical Justification of an observing proposal, users should discuss any significant discrepancies between the RMS in the simulated image and that determined by the sensitivity calculator.

In addition, users should note that the simulator currently underestimates both the surface brightness and the total flux in single dish maps by as much as 20% because of uncertainties in the primary beam shape. Any simulation that uses the TP array will be affected by this calibration issue.

The OST

The OST is a web-based interface hosted by the EU ARC at the University of Manchester. Users who do not have experience with CASA, and who do not need full control of the simulation and imaging process will usually find the OST a good choice for generating simulated images. The GUI works in a straightforward top-down approach. Full documentation for the OST is available.

The OST determines atmospheric opacity in a slightly different way than the CASA simulation tasks, and so it may return slightly different results from the CASA simulation tasks. The differences are most noticeable for simulations in Bands 9 and 10.

CASA simulation tools

CASA provides a powerful capability to simulate interferometric observations with ALMA, as well as other telescopes. The main CASA tasks for simulation are simobserve and simanalyze. The task simobserve generates a data set with simulated visibilities based on an input model image. The task simanalyze produces a cleaned image based on the simulated visibilities, and it generates some diagnostic images.

CASA also provides the task simalma. This task simplifies the steps needed to simulate ALMA observations that combine data from multiple arrays or multiple configurations. For example, users can simulate observations that combine data from the main 12 m array with data from the ALMA Compact Array using simalma.

These tasks are described here: Simulating Observations in CASA. Usage of simalma is described in the final simulation tutorial on that page.