Difference between revisions of "Simulating Observations in CASA 3.1"

From CASA Guides
Line 3: Line 3:
 
== Introduction ==  
 
== Introduction ==  
  
Simulation capability in CASA follows the usual two-layered structure: there is a beginner-level python <tt>task</tt> interface called [[simdata]], 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.  The task also can produce a cleaned image of the model visibilities, compare that image with your input convolved with the synthesized beam, and calculate a fidelity image.  <tt>simdata</tt> can add thermal noise (from receiver, atmosphere, and ground) to the visibilities.
+
Simulation capability in CASA follows the usual two-layered structure: there is a beginner-level python <tt>task</tt> interface called [[simdata]], 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.  The task also can produce a cleaned image of the model visibilities, compare that image with your input convolved with the synthesized beam, and calculate a fidelity image.  <tt>simdata</tt> can add thermal noise (from receiver, atmosphere, and ground) to the visibilities. ''Note that small differences (~10%) may exist between the noise predicted by the [http://www.eso.org/sci/facilities/alma/observing/tools/etc/ ALMA sensitivity calculator] and the measured RMS on the simdata output. These differences are believed to be due to the different implementations of the atmospheric models in the two pieces of code. The ALMA sensitivity calculator will be used for the technical assessment of ALMA proposals, and thus values from it should be used to scale exposure times.''
  
 
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>simdata</tt>, for example for new observatories that are unknown to <tt>simdata</tt>.
 
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>simdata</tt>, for example for new observatories that are unknown to <tt>simdata</tt>.
Line 12: Line 12:
  
 
Part of CASA's simulation routines are generic ephemeris and geodesy calculations available in python - see [[simutil.py]].
 
Part of CASA's simulation routines are generic ephemeris and geodesy calculations available in python - see [[simutil.py]].
 +
 +
'''Note on cleaning:''' cleaning images produced by simdata can lead to a spurious decrease in noise on the image. Users should clean simulated
 +
images with care, using a small number of iterations and boxing bright sources.
  
 
<font color="green"> Because <tt>simdata</tt> is still actively being developed, documentation may lag reality, please email rindebet at nrao.edu with any questions - It's my job to help you use this software.  In particular, you may find that some of the presentations and graphics below show parameter inputs that are slightly different from the latest version of CASA.</font>
 
<font color="green"> Because <tt>simdata</tt> is still actively being developed, documentation may lag reality, please email rindebet at nrao.edu with any questions - It's my job to help you use this software.  In particular, you may find that some of the presentations and graphics below show parameter inputs that are slightly different from the latest version of CASA.</font>
Line 24: Line 27:
  
 
| style="width: 98%; valign: top; background-color:#E0FFFF; border:1px solid #3366FF;" |
 
| style="width: 98%; valign: top; background-color:#E0FFFF; border:1px solid #3366FF;" |
<big>'''simdata2'''</big> <font color="red">Only works in CASA 3.0.2</font> [https://safe.nrao.edu/wiki/bin/view/Software/ObtainingCASA Obtaining CASA]<br />  
+
<big>'''simdata'''</big> <font color="red">(simata2 only works in CASA 3.0.2</font> [https://safe.nrao.edu/wiki/bin/view/Software/ObtainingCASA Obtaining CASA]<br />  
  
 
1. [[Getting Started in CASA#Installing CASA | Install CASA]]
 
1. [[Getting Started in CASA#Installing CASA | Install CASA]]
  
[[simdata2]] inputs look like this (v3.0.2; click to enlarge): [[File:Simdata2.png|100px]]
+
[[simdata/simdata2]] inputs look like this (v3.0.2; click to enlarge): [[File:Simdata2.png|100px]]
  
 
The subtasks are modular i.e. as long as you follow a few conventions about filenames, you can run each  
 
The subtasks are modular i.e. as long as you follow a few conventions about filenames, you can run each  
 
bit independently and optionally.  For example, you can modify the sky model, then predict ACA visibilities, then run again and predict
 
bit independently and optionally.  For example, you can modify the sky model, then predict ACA visibilities, then run again and predict
ATCA 12m visibilities and image and analyze both measurement sets together.  You can run once to predict, run interactive clean yourself, and as long as you called your image $project.image, run <tt>simdata2</tt> just to calculate a difference image and analyze the results.
+
ATCA 12m visibilities and image and analyze both measurement sets together.  You can run once to predict, run interactive clean yourself, and as long as you called your image $project.image, run <tt>simdata</tt> just to calculate a difference image and analyze the results.
  
 
2. [[Modify Model]] - relabel (scale) the spectral and spatial coordinates and brightness of the sky model image.
 
2. [[Modify Model]] - relabel (scale) the spectral and spatial coordinates and brightness of the sky model image.
Line 54: Line 57:
 
| rowspan=3; style="border-bottom:1px solid black;" | [[File:M51thumb.png|100px]]
 
| rowspan=3; style="border-bottom:1px solid black;" | [[File:M51thumb.png|100px]]
 
|-
 
|-
!style="solid black;"| [[M51 at z = 0.1|simdata page]] &nbsp;&nbsp; [[M51 at z = 0.1 and z = 0.3|simdata2 page]]
+
!style="solid black;"| &nbsp;&nbsp; [[M51 at z = 0.1 and z = 0.3|simdata page]]
 
|-
 
|-
 
!style="border-bottom:1px solid black;"| NOTE: increasing the [[etime study|exposure time]] to run faster
 
!style="border-bottom:1px solid black;"| NOTE: increasing the [[etime study|exposure time]] to run faster
Line 64: Line 67:
 
| rowspan=2; stype="border-bottom:1px solid black;" | [[File:Psimthumb.png|100px]]
 
| rowspan=2; stype="border-bottom:1px solid black;" | [[File:Psimthumb.png|100px]]
 
|-
 
|-
!style="border-bottom:1px solid black;"| [[PPdisk simdata1| simdata page]] &nbsp;&nbsp; [[PPdisk simdata2| simdata2 page]]
+
!style="border-bottom:1px solid black;" | [[PPdisk simdata2| simdata page]]
 
|-
 
|-
  
Line 72: Line 75:
 
| rowspan=2; style="border-bottom:1px solid black;" | [[File:N891thumb.png|100px]]
 
| rowspan=2; style="border-bottom:1px solid black;" | [[File:N891thumb.png|100px]]
 
|-
 
|-
!style="border-bottom:1px solid black;"| [[http://casaguides.nrao.edu/images/2/29/N891.simdata.py simdata script]] &nbsp;&nbsp; [[N891 simdata2| simdata2 page]]
+
!style="border-bottom:1px solid black;"| [[N891 simdata2| simdata page]]
 
|-
 
|-
  
Line 88: Line 91:
 
* [http://almasimulations.pbworks.com/ Simulation Library] This will become a library of use cases and examples illustrating different science and observation setups.  It is in early stages as of Jan 2010, and we're actively seeking volunteers to turn their simulation projects into use cases.  
 
* [http://almasimulations.pbworks.com/ Simulation Library] This will become a library of use cases and examples illustrating different science and observation setups.  It is in early stages as of Jan 2010, and we're actively seeking volunteers to turn their simulation projects into use cases.  
 
* [https://safe.nrao.edu/wiki/bin/view/ALMA/Jan2010Wkshop Jan 2010 workshop] Including slides and discussion of how simdata and Simulator work "under the hood" and plans for development
 
* [https://safe.nrao.edu/wiki/bin/view/ALMA/Jan2010Wkshop Jan 2010 workshop] Including slides and discussion of how simdata and Simulator work "under the hood" and plans for development
 +
''Italic text''

Revision as of 13:51, 11 October 2010


Introduction

Simulation capability in CASA follows the usual two-layered structure: there is a beginner-level python task interface called simdata, 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. The task also can produce a cleaned image of the model visibilities, compare that image with your input convolved with the synthesized beam, and calculate a fidelity image. simdata can add thermal noise (from receiver, atmosphere, and ground) to the visibilities. Note that small differences (~10%) may exist between the noise predicted by the ALMA sensitivity calculator and the measured RMS on the simdata output. These differences are believed to be due to the different implementations of the atmospheric models in the two pieces of code. The ALMA sensitivity calculator will be used for the technical assessment of ALMA proposals, and thus values from it should be used to scale exposure times.

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 simdata, for example for new observatories that are unknown to simdata.

New for 3.1.0: The simdata task is the task formerly known in 3.0.2 as simdata2. The old version of simdata has been removed.

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: cleaning images produced by simdata can lead to a spurious decrease in noise on the image. Users should clean simulated images with care, using a small number of iterations and boxing bright sources.

Because simdata is still actively being developed, documentation may lag reality, please email rindebet at nrao.edu with any questions - It's my job to help you use this software. In particular, you may find that some of the presentations and graphics below show parameter inputs that are slightly different from the latest version of CASA.

Steps to simulation

Users of the most recent version of CASA, 3.1.0 and later should use simdata. If you are using CASA 3.0.2 and simdata2, you should use simdata2.

simdata (simata2 only works in CASA 3.0.2 Obtaining CASA

1. Install CASA

simdata/simdata2 inputs look like this (v3.0.2; click to enlarge): Simdata2.png

The subtasks are modular i.e. as long as you follow a few conventions about filenames, you can run each bit independently and optionally. For example, you can modify the sky model, then predict ACA visibilities, then run again and predict ATCA 12m visibilities and image and analyze both measurement sets together. You can run once to predict, run interactive clean yourself, and as long as you called your image $project.image, run simdata just to calculate a difference image and analyze the results.

2. Modify Model - relabel (scale) the spectral and spatial coordinates and brightness of the sky model image.

3. Set Pointings - calculate a mosaic of pointings and save in a text file. You could also make the text file yourself.

4. Predict - Calculate visibilities for a specified array on a specified day

5. Corrupt - Corrupt the measurement set with thermal noise, phase noise, cross-polarization, etc.

6. Image A subset of clean to re-image the visibilities

7. Analyze Calculate and display the difference between output and input, and fidelity image.

Tutorials, Recipes, and Example images

Simulated ALMA Observation of M51 at z = 0.1 and z = 0.3: fully annotated tutorial

This uses a BIMA-SONG cube of a nearby galaxy and scales it to greater distance.

M51thumb.png
   simdata page
NOTE: increasing the exposure time to run faster
Protoplanetary Disk: sky model and lightly annotated script

This uses a theoretical model of dust continuum from Sebastian Wolff, scaled to the distance of a nearby star.

Psimthumb.png
simdata page
Nearby edge-on spiral galaxy: sky model, script, and discussion

This uses a Galactic CO cube from the Galactic Ring Survey and places it at 10Mpc, similar to what NGC891 would look like if it were observable from the southern hemisphere.

N891thumb.png
simdata page
Other example input images
Other example output simulations (scripts to reproduce these are coming)



Technical and Planning

I always welcome input on developing the CASA simulator, and these links are meetings, technical documents, and planning discussions. Much of it won't make sense to a new user of CASA::simdata, but may be of interest to those wanting to delve deeper:

  • Simulation Library This will become a library of use cases and examples illustrating different science and observation setups. It is in early stages as of Jan 2010, and we're actively seeking volunteers to turn their simulation projects into use cases.
  • Jan 2010 workshop Including slides and discussion of how simdata and Simulator work "under the hood" and plans for development

Italic text