Description

task description

Summary

This task simulates ALMA observation including 12-m, ACA 7-m and total power arrays, and images and analyzes simulated data.

This task makes multiple calls to simobserve (to calculate visibilities and total power spectra), followed by gridding of total power spectra (if total power is requested), concatenation of the simulated visibilities, calls to the simanalyze task for visibility inversion, deconvolution, and calculation of difference, and fidelity images, and feathering of single dish and interferometric data.

These steps may not all be familiar to new users, so the simalma task runs by default in a "dryrun" mode, in which it assesses the user's input parameters and sky model, and prints an informational report including the required calls to other CASA tasks, both to the screen and to a text file in the project directory (defined below).

The user can modify their parameters based on the information, then either run with dryrun=False to actually call the other tasks to create the simulated data, or run the other tasks individually one at a time to better understand and control the process. More information on running simalma can be found here.

NOTE: The ALMA project is refining the optimal method of combining the three types of data. If that best practice is changed after this release of CASA, the user can control the process by modifying the calls to the other CASA tasks.

 

Parameter descriptions

project

The root filename for all output files. A subdirectory will be created, and all created files will be placed in that subdirectory including the informational report.

skymodel

An input image used as a model of the sky. simalma requires a CASA or FITS image. If you merely have a grid of numbers, you will need to write them out as FITS or write a CASA script to read them in and use the image (ia) tool to create an image and insert the data. simalma does NOT require a coordinate system in the header. If the coordinate information is incomplete, missing, or you would like to override it, set the appropriate "in" parameters.

NOTE: Setting those parameters simply changes the header values, ignoring any values already in the image. No regridding is performed.

If you have a proper Coordinate System, simalma will do its best to generate visibilities from that, and then create a synthesis image
according to the specified user parameters. You can manipulate an image header manually with the imhead task.  

inbright

The peak brightness to scale the image to in Jy/pixel, or " " to keep it unchanged.

NOTE: "unchanged" will take the numerical values in your image and assume they are in Jy/pixel, even if it says some other unit in the header.   

indirection

The central direction to place the sky model image, or " " to use whatever is in the image already.

incell

The spatial pixel size to scale the skymodel image, or " " to use whatever is in the image already.

incenter

The frequency to use for the center channel (or only channel, if the skymodel is 2D). E.g., "89GHz", or " " to use what is in the header. Required even for a 2D model.

inwidth

The width of channels to use, or " " to use what is in the image. Should be a string representing a quantity with units e.g., "10MHz".

NOTE: Only works reliably with frequencies, not velocities. It is not possible to change the number of spectral planes of the sky model, only to relabel them with different frequencies. That kind of regridding can be accomplished with the CASA toolkit.

complist

A component list model of the sky, added to or instead of skymodel. Click here for more information.

compwidth

The bandwidth of components. If simulating from components only, this defines the bandwidth of the MS and output images.

setpointings

If True, calculate a map of pointings and write ptgfile. If graphics are on, display the pointings shown on the model image. Observations with the ALMA 12m and ACA 7m arrays will observe a region of size "mapsize" using the same hexagonal algorithm as the ALMA OT, with Nyquist sampling. The total power array maps a slightly (+1 primary beam) larger area than the 12m array does, to improve later image combination. It samples the region with lattice grids of spacing 0.33 lambda/D. If setpointings=False, read pointings from ptgfile.  

ptgfile

A text file specifying directions in the same format as the example, and optional integration times, e.g.,

#Epoch     RA          DEC      TIME(optional)
J2000 23h59m28.10 -019d52m12.35 10.0

If the time column is not present in the file, it will use "integration" for all pointings.

NOTE: At this time the file should contain only science pointings. simalma will observe these until totaltime is used up. 

integration

Time interval for each integration e.g., '10s'.

NOTE: To simulate a "scan" longer than one integration, use setpointings to generate a pointing file, and then edit the file to increase the time at each point to be larger than the parameter integration time. 

direction

Mosaic center direction. e.g., 'J2000 19h00m00 -40d00m00'. If unset, will use the center of the skymodel image. Can optionally be a list of pointings, otherwise simobserve will cover a region of size mapsize according to maptype.

mapsize

Angular size of mosaic map to simulate. Set to " " to cover the model image.

antennalist

A vector of ASCII files containing antenna positions, one for each configuration of 7m or 12m dishes.In this task, it should be an ALMA configuration. Standard arrays are found in your CASA data repository, os.getenv("CASAPATH").split()[0]+"/data/alma/simmos/". A string of the form "alma;0.5arcsec" will be parsed into a 12m ALMA configuration. Examples: ['alma.cycle2.5.cfg','aca.cycle2.i.cfg'], ['alma.cycle1;0.3arcsec','alma.cycle1.1.cfg','aca.i.cfg']  

hourangle

Hour angle of observation e.g., '-3h'.

totaltime

The total time of observations. This should either be a scalar time quantity expressed as a string e.g., '1h', '3600sec', '10min', or a vector of such quantities, corresponding to the elements of the antennalist vector, e.g., ['5min','20min','3h']. If you specify a scalar, that will be used for the highest resolution 12m configuration in antennalist, and any lower resolution 12m configurations, any 7m configurations, and any TP configurations will have observing times relative to totaltime of 0.5, 2,and 4, respectively.  

tpnant

The number of total power antennas to use in simulation.  

tptime

If tpnant>0, the user must specify the observing time for total power as a CASA quantity e.g., '4h'.

NOTE: This is not broken up among multiple days - a 20h track will include observations below the horizon,  which is probably not what is desired.  

pwv

Precipitable water vapor. If constructing an atmospheric model, set 0 for noise-free simulation. When pwv>0, thermal noise is applied to the simulated data. J. Pardo's ATM library will be used to construct anatmospheric profile for the ALMA site: altitude 5000m, ground pressure 650mbar, relhum=20%, a water layer of pwv at altitude of 2km, the sky brightness temperature returned by ATM, and internally tabulated receiver temperatures. See the documentation of simobserve for more details.  

image

An option to invert and deconvolve the simulated MeasurementSet(s)

NOTE: Interactive clean or more parameters than the subset visible here are available by simply running either clean or tclean tasks directly.

If graphics turned on, display the clean image and residual image uses Cotton-Schwab clean for single fields and Mosaic gridding for multiple fields (with Clark PSF calculation in minor cycles).  

imsize

The image size in spatial pixels (x,y). 0 or -1 will use the model image size. Examples: imsize=[500,500]

imdirection

The phase center for synthesized image. Default is to center on the sky model.

cell

Cell size e.g., "10arcsec". cell = " " defaults to the skymodel cell.

niter

The number of clean/deconvolution iterations, 0 for no cleaning.

threshold

The flux level at which to stop cleaning.

graphics

View plots on the screen, saved to file, both, or neither.

verbose

Print extra information to the logger and terminal.

overwrite

Overwrite existing files in the project subdirectory. Please see the documents of simobserve and simanalyze for the list of outputs produced.