Guide To Simulating ALMA Data

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THIS PAGE IS UNDER CONSTRUCTION -- Information on CASA simulations is applicable to CASA version 4.2

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 (using CASA 4.2)

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 encompasses 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. The 7-m array and Total Power observations 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 XX" by XX" (get this from Juergen).

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. The angular scale is XX" by XX". Need to work with Juergen to complete this description.

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A Simplified Model of a Proto-planetary Disk
This simulation shows a ring plus a planet. Need some information on angular scales and so forth. Need to work with Juergen to complete this description.

ALMA Simulation Tools

ALMA observers have two powerful simulation tools available: the CASA sim_observe/sim_analyze tasks, 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. The ALMA sensitivity calculator is the primary tool used for time estimates during the technical assessment of ALMA proposals. Therefore, if using the results from the simulators in a Technical Justification of an observing proposal, users should discuss or reconcile any significant discrepancies between the RMS in the simulated image and that determined by the sensitivity calculator.

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 here.

The OST determines atmospheric opacity in a slightly different way than the CASA sim 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

Simulating ALMA observations in CASA proceeds in the following steps:

  1. Make a model image. The model image is a representation of the sky brightness distribution that you would like to simulate observing, stored initially as a FITS file. There are several paths to making the FITS file, discussed below.
  2. Generate uv data with the simobserve task.
  3. Image the simulated observation with the simanalyze task.

The simobserve task generates the visibilities that would be measured by observing the specified input model of sky brightness. The user needs to specify a telescope to use for the observation. CASA includes a set of configuration files for ALMA, the VLA, CARMA, the SMA, ATCA, and the Plateau de Bure interferometers. The specific configuration files shipped with a CASA installation are available here. 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 simanalyze task is used to 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.

Generating a Model Image

A "model image" is a FITS file that contains a representation of the sky brightness distribution, and it represents the object to be "observed" in the simulation. The FITS file must contain a realistic set of spatial axes, and a realistic flux density. There are several ways to generate a model image.

  • Starting from an existing FITS image

The simplest option is to begin with an existing FITS image of a millimeter or submillimeter observation. The FITS image can be either a single plane (i.e. one observed frequency channel) or a cube. A common simulation exercise is to begin with a FITS file representing an observation of a target, then scale the spatial axes and the flux to shift the data to what would be observed for a similar target at a different distance. The simobserve task has parameters to set the peak flux density, coordinates on the sky, pixel size, frequency of the center channel, and channel width. See the discussion below.

  • Starting from a component list

It may be useful to simulate observations of an idealized model image consisting, for example, of point sources and Gaussians. The CASA component list tool (cl) allows the user to create a FITS file by specifying a set of point sources, Gaussians, disks, and limb-darkened disks. Each of these is a "component" in the FITS image. This CASA guide provides an example of creating a FITS file from a component list.

  • Starting from a GIF or JPG image

A user may wish to convert a GIF or JPG image to a FITS file for simulation in CASA. The image should be converted to a 16-bit FITS image for use with the CASA sim tools. See this page for an example of using gimp to convert a JPG image to a FITS file. Alternatively, you could use ImageMagik, like so: 

convert myfile.jpg myfile.fits

Then proceed to trim and convert the file in CASA like so: 

importfits(fitsimage='myfile.fits',imagename='testimage',overwrite=T)
default 'immath'
imagename = 'testimage'
expr = 'IM0'
box = '0,0,299,299'
outfile = 'testimage2'
immath()
exportfits(imagename ='testimage2',fitsimage ='myfile_16bit.fits',bitpix=16,overwrite=T)

You could also use the parameters in the simobserve task to modify the peak flux density, coordinates on the sky, pixel size, frequency of the center channel, and channel width. See the discussion below.

Generating visibilities with simobserve

The task simobserve takes several steps to generate observed visibilities. The major steps are:

  • Modify Model. If desired, you can modify the header parameters in your data model to mimic different observing targets. 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. The settable parameters include:
    • inbright: Scaling the model flux densities by setting the peak flux density of the brightest pixel, e.g. "1.2Jy/pixel"
    • indirection: Set the center target coordinates, e.g. "J2000 19h00m00 -40d00m00"
    • incell: Set the angular size of each pixel, e.g. "0.1arcsec"
    • incenter: Set the observed sky frequency. If the model is a spectral cube, this parameter applies to the center channel.
    • inwidth: Set the channel width, e.g. "10MHz"
  • Set Pointings. If the angular size of your model image is comparable or larger than the 12-m primary beam, you can simulate observing the target as a mosaic. In this step, the individual pointings are determined and saved in a text file. You can also generate such a text file yourself.
  • Generate visibilities. The visibilities are determined based on the telescope and configuration specified, and the length in time of the observation.
  • Add noise. Finally, noise is added to the visibilities.

Transforming to images with simanalyze

The task simanalyze then images the visibility data using CASA's clean task. It also calculates and displays the difference between the simulates observation and the original model data, and it generates a "fidelity image". Fidelity is defined as I/| I-T |, where I is the observed image intensity and T is the true image intensity, given in this case by the sky model.

  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.

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. This tutorial demonstrates combining data from each ALMA component "by hand". This guide is of particular interest to those wishing to explore using the 12-m array in combination with the ACA, and those interested in combining data from multiple 12-m array configurations.
Simalma (CASA 4.2)
This tutorial demonstrates how to use simalma, a task that simplifies simulations that include the main 12-m array plus the ACA. Like the previous guide, this one is of particular interest to those wishing to explore multi-component ALMA observations.

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.

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