View source for Simulating Observations in CASA 3.3

[[Category: Simulations]] [[Category: ALMA]]

''This guide is applicable to CASA version 3.3.''

== Introduction ==

Simulation capability in CASA follows the usual two-layered structure: there is a beginner-level Python <tt>task</tt> interface called <tt>sim_observe</tt>, which calls methods in the <tt>sm</tt> C++ <tt>tool</tt>.  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.  <tt>sim_observe</tt> can also add thermal noise (from receiver, atmosphere, and ground) to the visibilities.  The task <tt>sim_analyze</tt> 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 <tt>sm</tt> 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.  <tt>sm</tt> also has more flexibility in adding thermal noise than <tt>sim_observe</tt>.

''Note that <tt>sim_observe</tt> and <tt>sim_analyze</tt> are new tasks in CASA 3.3.  In earlier versions of CASA the functionality of both tasks was contained in a single task called <tt>simdata</tt>.  <tt>simdata</tt> is still available in CASA 3.3 but is deprecated and may be removed in a future CASA release.''  

CASA simulation uses the [http://www.mrao.cam.ac.uk/~bn204/alma/atmomodel.html aatm] atmospheric model, a thin wrapper of Juan Pardo's [http://damir.iem.csic.es/PARDO/class_atm.html 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 <tt>sim_observe</tt> 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: [[Getting Started in CASA#Installing CASA | Install CASA]].

<tt>sim_observe</tt> and <tt>sim_analyze</tt> 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 <tt>sim_observe</tt>, you can modify the sky model, then predict ACA visibilities, then run again and predict ATCA 12m visibilities.  With <tt>sim_analyze</tt> 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 <tt>sim_analyze</tt> 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 <tt>clean</tt>.
# [[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) [http://almaost.jb.man.ac.uk/ ]. This is a web-based interface to an ALMA simulator hosted by the University of Manchester, UK. Like the <tt>sim_observe</tt>, it is based on the CASA <tt>sm</tt> toolkit, but uses different wrapper scripts, and, in particular, has a different treatment of atmospheric effects. Comparisons to the [https://almascience.nrao.edu/call-for-proposals/sensitivity-calculator ALMA sensitivity calculator] made in March 2011 suggest that both <tt>sim_observe</tt> and the OST give similar noises for observations in bands 3-8, but the OST diverges in bands 9 and 10. <font color="red"> In general, however, because the ALMA sensitivity calculator will be used for the technical assessment of ALMA proposals, only values from it, not <tt>sim_observe</tt> or the OST, should be used to estimate exposure times for ALMA Science Goals.</font>

=== ALMA updates ===

No updates currently for CASA 3.3.

== Tutorials, Recipes, and Example images ==

{| style="width: 100%; valign: top; background-color:#E0FFFF; border:1px solid #3366FF; text-align: center; cellpadding=0"

! [[Simulation Guide for New Users (CASA 3.3)]]
| rowspan=2 style="border-bottom:1px solid black;" | [[File:30Dor_ES.png|100px]]
|-
| style="border-bottom:1px solid black;" | 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)]]
| rowspan=2; style="border-bottom:1px solid black;" | [[File:M51thumb.png|100px]]
|-
| style="border-bottom:1px solid black;" | 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)]]
| rowspan=2; style="border-bottom:1px solid black;" | [[File:Analyze_fits_list.jpg|100px]]
|-
| style="border-bottom:1px solid black;" | Tutorial for simulating data based on multiple sources (using both a FITS image and a component list).
|-

! [[PPdisk simdata (CASA 3.3)]]
| rowspan=2; stype="border-bottom:1px solid black;" | [[File:Psimthumb.png|100px]]
|-
| style="border-bottom:1px solid black;" | 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)]]
| rowspan=2; style="border-bottom:1px solid black;" | [[File:N891thumb.png|100px]]
|-
| style="border-bottom:1px solid black;" | 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)]]
| rowspan=2; style="border-bottom:1px solid black;" | [[File:einstein_fs_cfg8_1hr.gif|100px]]
|-
| style="border-bottom:1px solid black;" | 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. 
|-

| colspan=2; style="border-bottom:1px solid black;" | [[Sim Inputs | Other example input images]]
|-
| colspan=2; | [[Sim Outputs | 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.
{{Checked 3.3.0}}