simobserve – visibility simulation task – simulation task

Description

This task simulates interferometric or total power measurment sets. It is currently optimized for JVLA and ALMA, although many observatories are included, and adding your own is simply a matter of providing an antenna location file (see below).

simobserve is meant to work in conjunction with the simanalyze task. Calling simobserve one more times will produce simulated measurement set(s), which are then gridded, inverted and deconvolved into output simulated images using simanalyze.

ALMA users are encouraged to use the simalma task, which provides additional information on the multiple simobserve and simanalyze calls required to simulate an ALMA observation which may consist of 12m interferometric, 7m interferometric, and 12m total power data.

More information and examples are availible at http://casaguides.nrao.edu/index.php?title=Simulating_Observations_in_CASA Please contact the Helpdesk with any questions (see https://casa.nrao.edu/help_desk_all.shtml)

Parameters

Title
Parameter	Default	Description
project	`'sim'`	Root prefix for output file names
skymodel	`''`	model image to observe
inbright	`''`	Peak brightness to scale the image to in Jy/pixel
indirection	`''`	Set new direction, e.g. J2000 19h00m00 -40d00m00
incell	`''`	Set new cell/pixel size, e.g. 0.1arcsec
incenter	`''`	Set new frequency of center channel e.g. 89GHz (required even for 2D model)
inwidth	`''`	Set new channel width, e.g. “10MHz” (required even for 2D model)
complist	`''`	Componentlist to observe
compwidth	`'"8GHz"'`	Bandwidth of components
comp_nchan	`int(1)`	Channelization of components
setpointings	`True`	Calculate a map of pointings?
ptgfile	`'$project.ptg.txt'`	List of pointing positions
integration	`'10s'`	Integration (sampling) time
direction	`numpy.array( [ ] )`	Mosaic center direction, e.g J2000 19h00m00 -40d00m00
mapsize	`numpy.array( [ '','' ] )`	Angular size of mosaic map to simulate.
maptype	`'hexagonal'`	how to calculate the pointings for the mosaic observation: hexagonal, square (raster), ALMA, etc.
pointingspacing	`''`	Spacing in between pointings e.g. 0.25PB. ALMA default: INT=lambda/D/sqrt(3), SD=lambda/D/3
caldirection	`''`	pt source calibrator [experimental]
calflux	`'1Jy'`	pt source calibrator flux [experimental]
obsmode	`'int'`	Observation mode to simulate [int(interferometer)\|sd(singledish)\|(none)]
refdate	`'2014/01/01'`	Date of observation. Not critical unless concatting simulations
hourangle	`'transit'`	Hour angle of observation center, e.g. -3:00:00, 5h
totaltime	`'7200s'`	Total time of observation or number of repetitions
antennalist	`''`	Interferometer antenna position file
sdantlist	`'aca.tp.cfg'`	Single dish antenna position file
sdant	`int(0)`	Single dish antenna index in file
outframe	`'LSRK'`	Spectral frame of MS to create
thermalnoise	`'tsys-atm'`	add thermal noise: [tsys-atm\|tsys-manual\|(none)]
user_pwv	`float(0.5)`	Precipitable Water Vapor in mm
t_ground	`float(270.)`	Ground/spillover ambient temperature in K
t_sky	`float(260.)`	Atmospheric temperatur in K
tau0	`float(0.1)`	Zenith opacity
seed	`int(11111)`	Random number seed
leakage	`float(0.0)`	Cross polarization (interferometer only)
graphics	`'both'`	Display graphics at each stage to [screen\|file\|both\|none]
verbose	`False`	Print extra information to the logger and terminal
overwrite	`True`	Overwrite existing files in the project subdirectory

Parameter Explanations

project

'sim'

root prefix for output file names

skymodel

''

Model image to observe

simobserve uses 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 ia tool to create an image and insert the data.

simobserve 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 that setting those parameters simply changes the header values, ignoring any values already in the image. No regridding is performed.

You can also manipulate an image header manually with the “imhead” task.

If you have a proper Coordinate System, simobserve will do its best to generate visibilities from that.

inbright

''

Peak brightness to scale the image to, in Jy/pixel

Subparameter of skymodel Default: ‘’ (i.e., unchanged)

Example: inbright=’1.2Jy/pixel’

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

''

Central direction to place the sky model image: Subparameter of skymodel Default: ‘’ (use whatever is in the image already)

Example: indirection=’J2000 19h00m00 -40d00m00’

incell

''

set new cell/pixel size: Subparameter of skymodel Default: ‘’ (use whatever is in the image already)

Example: incell=’0.1arcsec’

incenter

''

Frequency to use for the center channel (or only channel, if the skymodel is 2D)

Subparameter of skymodel Default: ‘’ (use whatever is in the image already)

Example: incenter=’89GHz’

inwidth

''

Set new channel width

Subparameter of skymodel Default: ‘’ (use whatever is in the image already)

Should be a string representing a quantity with units e.g. inwidth=’10MHz’

NOTES:

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

''

Component list model of the sky, added to or instead of skymodel. See https://casaguides.nrao.edu/index.php/Simulation_Guide_Component_Lists_(CASA_5.4)

compwidth

'"8GHz"'

Bandwidth of components

Subparameter of complist

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

Example: compwidth=’8GHz’

comp_nchan

int(1)

Channelization of components

Subparameter of complist

If simulating from components only, this defines the number of channels of the MeasurementSet

Example: comp_nchan=256

setpointings

True

If true, calculate a map of pointings and write ptgfile. If false, read pointings from ptgfile.

Default: True

If graphics are on, display the pointings shown on the model image

ptgfile

'$project.ptg.txt'

A text file specifying directions

Subparameter of setpointings=False

The text file should have the following format, with optional integration times: 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: simobserve will observe these, then optionally the calibrator, then the list of science pointings again, etc, until totaltime is used up.

integration

'10s'

Time interval for each integration

Subparameter of setpointings=False

Example: integration=’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

numpy.array( [ ] )

Mosaic center direction.

Subparameter of setpointings=True

Example: “J2000 19h00m00 -40d00m00” or “” to center on model

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

numpy.array( [ '','' ] )

Angular size of of mosaic map to simulate.

Subparameter of setpointings=True

Set to “” to cover model

maptype

'hexagonal'

How to calculate the pointings for the mosaic observation?

Subparameter of setpointings=True Options: hexagonal, square (raster), ALMA, etc

“ALMA” for the same hex algorithm as the ALMA Cycle 1 OT or “ALMA2012” for the algorithm used in the Cycle 0 OT

pointingspacing

''

Spacing in between pointings.: Subparameter of setpointings=True

Examples: pointingspacing=”0.25PB” pointingspacing=”” for ALMA default INT=lambda/D/sqrt(3), SD=lambda/D/3

caldirection

''

pt source calibrator [experimental]

calflux

'1Jy'

pt source calibrator flux [experimental]

obsmode

'int'

Observation mode to simulate

Options: int(interferometer)|sd(singledish)|””(none)

Observation mode to calculate visibilities from a skymodel image (which may have been modified above), an optional component list, and a pointing file (which also may have been generated above).

This parameter takes two possible values: - interferometer (or int) - singledish (or sd)

If graphics are on, this observe step will display the array (similar to plotants), the uv coverage, the synthesized (dirty) beam, and ephemeris information
If simulating from a component list, you should specify “compwidth”, the desired bandwidth; and specify “comp_nchan”, the desired channelization if more than one output channel is desired

refdate

'2014/01/01'

Date of simulated observation: Subparameter of obsmode=’int|sd’ Not critical unless concatting simulations

Example: refdate=”2014/05/21”

hourangle

'transit'

Hour angle of observation center.: Subparameter of obsmode=’int|sd’

Examples: hourangle=”-3:00:00”, “5h”, or “transit”

totaltime

'7200s'

Total time of observation or number of repetitions: Subparameter of obsmode=’int|sd’

Example: totaltime=’7200s’ If a number without units, interpreted as the number of times to repeat the mosaic.

antennalist

''

Ascii file containing antenna positions.

Subparameter of obsmode=’int|””’

Each row has x y z coordinates and antenna diameter; header lines are required to specify # observatory=ALMA # coordsys=UTM # datum=WGS84 # zone=19

Standard arrays are found in your CASA data repository,
If “”, simobserve will not not produce an interferometric MS
A string of the form “alma;0.5arcsec” will be

parsed into a full 12m ALMA configuration.

sdantlist

'aca.tp.cfg'

single dish antenna position file: Subparameter of obsmode=’sd|””’

sdant

int(0)

Index of the antenna in the list to use for total power.: Subparameter of obsmode=’sd|””’ Default: first antenna on the list.

outframe

'LSRK'

spectral frame of MS to create: Subparameter of obsmode=’sd|””’

thermalnoise

'tsys-atm'

add thermal noise.

Options: tsys-atm, tsys-manual, “”

This parameter accepts two settings: - tsys-atm: J. Pardo’s ATM library will be used to construct an atmospheric profile for the ALMA site: altitude 5000m, ground pressure 650mbar, relhum=20%, a water layer of user_pwv at altitude of 2km, the sky brightness temperature returned by ATM, and internally tabulated receiver temperatures. - tsys-manual: instead of using the ATM model, specify the zenith sky brightness and opacity manually. Noise is added and then the visibility flux scale is referenced above the atmosphere.

If left unset (empty string) no thermalnoise corruption is performed.

In either mode, noise is calculated using an antenna spillover efficiency of 0.96, taper of 0.86, surface accuracy of 25 and 300 microns for ALMA and EVLA respectively (using the Ruze formula for surface efficiency), correlator efficiencies of 0.95 and 0.91 for ALMA and EVLA, receiver temperatures for ALMA of 17, 30, 37, 51, 65, 83,147,196,175,230 K interpolated between 35, 75,110,145,185,230,345,409,675,867 GHz, for EVLA of 500, 70, 60, 55, 100, 130, 350 K interpolated between 0.33,1.47,4.89,8.44,22.5,33.5,43.3 GHz, for SMA of 67, 116, 134, 500 K interpolated between 212.,310.,383.,660. GHz.

Note: These are only approximate numbers and do not take into account performance at edges of receiver bands, neither are they guaranteed to reflect the most recent measurements. Caveat emptor. Use the sm tool to add noise if you want more precise control, and use the ALMA exposure time calculator for sensitivity numbers in proposals.

user_pwv

float(0.5)

Precipitable water vapor if constructing an atmospheric model (in mm)

Subparameter of thermalnoise=’tsys-atm’

t_ground

float(270.)

Ground/spillover temperature in K: Subparameter of thermalnoise=’tsys-atm|tsys-manual’

t_sky

float(260.)

Atmospheric temperature in K: Subparameter of thermalnoise=’tsys-manual’

tau0

float(0.1)

Zenith opacity at observing frequency

Subparameter of thermalnoise=’tsys-manual’

https://casaguides.nrao.edu/index.php/Corrupt for more information on noise, in particular how to add a phase screen using the toolkit

seed

int(11111)

Random number seed: Subparameter of thermalnoise=’tsys-atm|tsys-manual’

leakage

float(0.0)

add cross polarization corruption of this fractional magnitude (interferometer only)

graphics

'both'

View plots on the screen, saved to file, both, or neither: Options: screen|file|both|none

verbose

False

Print extra information to the logger and terminal: Default: False Options: True|False

overwrite

True

Overwrite files starting with $project: Default: False Options: True|False