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Unlocking the Potential of ALMA: Observing Simulations and Imaging Techniques

Dive into the world of ALMA observations through simulations and imaging techniques. Learn about simulating observed images and physical parameters, understanding radiative transfer codes, and evaluating imaging quantitatively. Discover the power of ALMA in observing your favorite astronomical objects and the process of obtaining "Visibilities V(u,v)" to create detailed images. Practice hands-on exercises using the ALMA Observing Simulator task "simdata" in CASA software to create model visibilities and synthetic images. Explore the effects of different antenna configurations in simulating proto-planetary disk formations.

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Unlocking the Potential of ALMA: Observing Simulations and Imaging Techniques

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  1. ALMAUsers Meeting 2011 SIMDATA Simulation Shigehisa Takakuwa (ASIAA)

  2. Preparing for the ALMA Proposal What can you really get with ALMA observations ? Can ALMA provide you with what you want to see ? --> ALMA Observing Simulations

  3. ``Observed’’ Images ``Observed’’ Physical Parameters ALMA observing Simulation Theoretical model Images@ ALMA observing band Theoretical Physical Parameters of your favorite astronomical objects Radiative Transfer Code Compare !! ALMA Imaging Simulation • Science Power of ALMA 2 SED fitting

  4. ALMA Observation • Data we obtain are ``Visibilities V (u,v)’’, Fourier-Transform of Source Images I (x,y) V (u,v) = ∬ I (x,y) exp {-2πi (ux+vy)} dxdy x, y: Sky Spatial Coordinate u, v: Antenna Tracks projected on the sky V (u,v) consists of amplitude and phase

  5. ALMA Imaging --> To obtain I (x,y) Ideally: V (u,v) = ∬ I (x,y) exp {-2πi (ux+vy)} dxdy • In reality: Vobs (u,v) = S (u,v) ∬ I (x,y) • exp {-2πi (ux+vy)} dxdy • S(u,v): Sampling Function • Fourier-Transform of Vobs (u,v) is NOT Real I(x, y), but we call it Dirty-Image ID(x,y) Methods to guess real I(x,y) from ID (x,y) ---> Clean, MEM, etc…

  6. How to evaluate the imaging quantitatively ?---> Introducing ``Fidelity’’ At each image pixel (i, j); abs[ Model (i, j) ] Fidelity (i, j) = -------------------------------------------- abs[Model (i, j) - Simulated (i, j)] So Fidelity is an image. Pety et al. 2001

  7. Model | Model - Simulation | Example of the ALMA Imaging Simulation:Debris Disk Fidelity = ACA is crucial to recover the flux from debris disks. (Takakuwa et al. 2008)

  8. ALMA Observing Simulator Task ``simdata’’: One of the tasks in CASA, the ALMA data reduction software Create model visibilities from model images, with observing parameters such as the antenna location, specified by the user. Fourier-Transform the Visibility, and ``CLEAN’’ the image with the user-specified imaging parameters, such as weighting on the visibility sampling points.

  9. Products from simdata • Synthetic visibilities • A synthesized CLEANed image • Image of the Difference between the model and simulated image, and Fidelity ALMA. (E)VLA, SMA simulations possible ALL you need is a model of the sky

  10. Simdata Example: Forming Planets in Protoplanetary Disk

  11. Effects of the Different antenna configurations Proto-Planetary Disk Simulation

  12. demodemo

  13. Hands-on Exercise:Read input fits image into CASA Input Image File (in FITS） PC with CASA installed ＋ • casapy • default(‘importfits’) • fitsimage=‘input50pc_672GHz.fits’ • imagename=‘input50pc_672GHz’ • go • viewer();

  14. casapy • default(‘importfits’) • fitsimage=‘input50pc_672GHz.fits’ • imagename=‘input50pc_672GHz’ • go • viewer(); • default(‘simdata’) • skymodel = ‘input50pc_672GHz’ • project = ‘psim2’ (rm -rf psim2* for the 2nd time) • direction=‘center’ • mapsize=‘0.76arcsec’ • antennalist=‘/Applications/CASA.app/Contents/data/alma/simmos/alma.out20.cfg’ (Try different antenna configurations !) • imsize=[192,192] • threshold=‘1e-7Jy’ • niter=10000 • totaltime=‘1200s’ • graphics=‘both’ • image=True • analyze=True • go

  15. Hands-on Exercise: simdata simulation, changing input model parameters • modifymodel=True • incenter=‘230GHz’ • inwidth=‘2GHz’ • incell=‘0.0155arcsec’ • go

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