An Introduction to CASA and Simulations with Simdata May 25, 2011.
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Transcript of An Introduction to CASA and Simulations with Simdata May 25, 2011.
![Page 1: An Introduction to CASA and Simulations with Simdata May 25, 2011.](https://reader035.fdocuments.us/reader035/viewer/2022062714/56649d2c5503460f94a023e7/html5/thumbnails/1.jpg)
An Introduction to CASA and Simulations with
Simdata
May 25, 2011
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General Description• CASA is the post-processing package for ALMA (and EVLA)
both interferometric and single dish• The ALMA pipeline is is being built from CASA toolkit• Toolkit packaged into most commonly used tasks for users• Designed with parallelization in mind• CASA in active community use since October 2007• Release 3.2.0 available since May 18 2011
– Most recent linux flavors, Mac OSX for Leopard & Snow Leopard
– Available to anyone from CASA homepage:
http://casa.nrao.edu
May 25, 2011
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CASA Documentation and Web Resources
http://casaguides.nrao.edu
• Casaguides are fully annotated scripts including screen shots
• There is also a complete CASA user manual
• Currently there are millimeter guides for EVLA, CARMA, and SMA that serve as important learning tools for future ALMA data
• CASAguides for ALMA Science Verification data on near horizon
May 25, 2011
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Inside CASA
> TasklistWill show an organized list of all currently available tasks
Opportunities for full ALMA related CASA tutorials will be available after the proposal call deadline (June 30)
May 25, 2011
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What is Simdata Good For?
• Number of antennas• Antenna configuration• Length of observation• Noise*
Take a model image and find out how it would look if observed with ALMA
• Thermal Noise
• Phase Noise
May 25, 2011
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Model Image30Dor Spitzer IRAC 8um image from SAGE
May 25, 2011
Field of view
at 230 GHz
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Model ImageResized and now at 230 GHz
Field of view
at 230 GHz
Now ~15 timesmore distant!
May 25, 2011
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Fourier Transforms of Images
From http://carmilumban-ap186.blogspot.com
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Scales Measured in Compact Early ScienceAntenna Placement uv-coverage synthesized beam
2 hour observation in Compact Configuration
Point Spread Function
bmaj = 2.3
bmin = 2.0
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Full Science 12m Array - Compact
2 hour observation
Antenna Placement uv-coverage synthesized beam
bmaj = 1.8
bmin = 1.7
Note lower sidelobes
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Full Science 12m Array - Extended
2 hour observation
Antenna Placement uv-coverage synthesized beam
bmaj = 0.38
bmin = 0.19
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Model: Early Science Compact Configuration
Model Image Convolved Model
“Observed” Image
2 hour observation
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Model: Full Science Main Array - Compact
Model Image Convolved Model
“Observed” Image
2 hour observationLarge scale emission: Observe with ACA and possibly TPA
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Model: Full Science Main Array - Extended
Model Image Convolved Model
“Observed” Image
2 hour observation
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Model: Early Science Compact & Extended
Model Image
Compact Array
Extended Array
2 hour observation
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Basic Simdata Workflow
• Start CASA • Input image file into Simdata• Predict what ALMA would see using
Simdata• Add noise (optional)• Compare ALMA image with input
image
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CASA BasicsTo start > casapy
To look at task inputs > inp clean
Grey: expandable parameters
Red: invalid value
Blue: accepted value but not the default
To see full help > help clean
Reset defaults > default clean
Or use script mode - only need to set non-default values: > clean(vis=‘visname’,field=‘2’,mode=‘channel’)
To run > go
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Basic Simdata Inputs
• Image of target• Observing time• Antenna configuration• Optional noise parameters
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Basic Simdata Inputs
Parameters that can be changed
Current values of parameters
Explanation of what the parameters are
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Basic Simdata Inputs
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Model Input FITS FileHeader must include:• Coordinates• Brightness units• Observing frequency• Pixel Scale (angular and spectral)• Polarization (if needed)• OR: Modify FITS image within
Simdata
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Change your input file: modifymodel
• modifymodel = True• skymodel = “30dor.fits”• inbright = ”0.06mJy/pixel”• indirection = "J2000 10h00m00 -40d00m00”• incell = "0.15arcsec”• incenter = "230GHz”• inwidth = '2GHz'
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setpointings
• integration = '600s’ Time average for each data point
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predict
Total time for the observation
(must be ≥ integration times the number of
pointings)
May 25, 2011
To use the Early Science Configurations
• repodir = os.getenv("CASAPATH").split(' ')[0]• antennalist =
repodir+"/data/alma/simmos/alma.cycle0.compact.cfg”• antennalist =
repodir+"/data/alma/simmos/alma.cycle0.extended.cfg”
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predict
To use the most compact Full Science Configuration
• repodir = os.getenv("CASAPATH").split(' ')[0]• antennalist = repodir+"/data/alma/simmos/alma.out01.cfg”
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Simdata Output
• analyze = True• showarray = True• showconvolved = True• showdifference = False• showfidelity = False
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Simdata Output – ES Compact Array
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Simdata OutputNote that there are six pointings, and we previously
set integration time to 600 seconds, so this product is
less than the 7200 seconds set in totaltime
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Simdata Output – Viewerhttp://casa.nrao.edu/CasaViewerDemo/casaViewerDemo.html
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Other Simdata OptionsThermal Noise
• thermalnoise = ‘tsys-atm’• vis=‘$project.noisy.ms’
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Other Simdata OptionsThermal Noise
Sky Model Thermal NoiseNo Thermal Noise
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Atmospheric phase fluctuations• Variations in the amount of precipitable water
vapor (PWV) cause phase fluctuations and result in– Low coherence (loss of sensitivity)– Radio “seeing”, typically 1² at 1 mm– Anomalous pointing offsets– Anomalous delay offsets
Patches of air with different water vapor content (and hence index of refraction) affect the incoming wave front differently.
May 25, 2011
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Other Simdata OptionsPhase Noise
Sky Model No Phase Noise Phase Noise
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Other Simdata OptionsPhase Noise• Start with the visibilities (measurement set) created
by simdata• Use the “toolkit” to add phase noise
– http://casa.nrao.edu/docs/CasaRef/CasaRef.html– http://casaguides.nrao.edu/index.php?title=Corru
pt– sm.openfromms("simPN.ms")– sm.settrop(mode='screen',pwv=1.0,deltapwv=0.
15)– sm.corrupt()– sm.done()
• Use clean (in CASA) to make the image
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Your Turn
• Find a fits file of an object you want to image
• Use simdata to see how it would look if observed with ALMA–Rescale image if necessary–Try different antenna
configurations–Add noise if interested
• Don’t be afraid to ask for help!May 25, 2011
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Your Turn
• Sample images can be found at:– http://casaguides.nrao.edu/index.php?title=
Sim_Inputs
• Simdata walk-throughs available at:– http://casaguides.nrao.edu
• Simulating observations in CASA 3.2
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Extra Slides
May 25, 2011
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Simdata output images
• Most important images are:– $project.image – output simulated cube/plane– $project.image.flat – moment 0 of simulated
cube/plane– $project.skymodel – input cube/plane– $project.psf – synthesized beam (point spread
function)– $project.residual – residuals after cleaning
May 25, 2011
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Simdata hands-on quickstart • Start CASA and look at simdata
– casapy, inp simdata, help simdata• Go through simdata tutorial
– http://casaguides.nrao.edu• Get image for your own simulation
– http://casaguides.nrao.edu/index.php?title=Sim_Inputs
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Simdata FAQ • Find the antenna configuration files
– repodir = os.getenv("CASAPATH").split(' ')[0]– antennalist =
repodir+"/data/alma/simmos/alma.cycle0.compact.cfg”• Problems with FITS file?
– modifymodel = True– Set all parameters manually
• “image detached” error message?– image = True– analyze = True
May 25, 2011
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Simdata FAQ • Simulation running too long?
– Increase value of “integration” – Decrease number of required pointings– Clean for fewer iterations
• Can’t see effect of added thermal noise?– thermalnoise = ‘tsys-atm’– vis = ‘$project.noisy.ms’– Check brightness of input image vs expected noise
May 25, 2011