Content-type: text/html Manpage of uvgen

uvgen

Section: User Commands (1)
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NAME

uvgen - Compute visibilities for a model source.  

PERSON RESPONSIBLE

rjs  

CATEGORIES

uv analysis, map making  

DESCRIPTION

UVGEN is a MIRIAD task which computes visibility data for a model source distribution at u-v data points specified by a set of antenna positions, hour angle range and sample interval. The model is specified by a set of Gaussian sources with given positions and flux densities. Analytic expressions are used to calculate the value of the visibilities. The calculation includes the response to polarized sources with linear and circularly polarized feeds. U-V trajectories for all pairs of antennas are computed.  

PARAMETERS

source
The name of a text file containing the source components, one component per line. There is no default. The source components are elliptical Gaussian components. Each line consists of at least three and up to nine values:
  flux,dra,ddec,bmaj,bmin,bpa,iflux,ipa,vflux
where
  flux:          Total flux in Jy.
  dra,ddec:      Position offset from the phase center in arcsec.
  bmaj,bmin,bpa: The full width to half maximum of the major and
                 minor axes, and the position angle of the major
                 axis measured from north to the east. The default
                 half width is 0."0001.
  iflux,ipa:     The sources can be partially linearly polarized.
                 This information is given as a percentage
                 polarization and position angle. The default is 0.
  vflux:         Percentage circular polarization. The default is 0.
The text file is free-format, with commas or blanks used to separate the values. Comments (starting with #) can be included in the file.
ant
The name of a text file containing the position of the antennas. There is no default. Each line of the text file gives three values, being the x, y and z location of an antenna. The antenna positions can be given in either a right handed equatorial system or as a local ground based coordinates measured to the north, east and in elevation. See the "baseunit" parameter to specify the coordinate system. Some standard antenna configurations can be found in $MIRCAT/*.ant for ATCA, BIMA and VLA telescopes. The BIMA and VLA antenna tables, use with baseunit=1, whereas for the ATCA, use baseunit=-51.0204.

The text file is free-format, with commas or blanks used to separate the values. Comments (starting with #) can be included in the file.

baseunit
This specifies the coordinate system used in the antenna file. A positive value for "baseunit" indicates an equatorial system, whereas a negative value indicates a local system. The magnitude of "baseunit" gives the conversion factor between the baseline units used in the antenna file, and nanoseconds. The default value is +1, which means that the antenna file gives the antenna position in an equatorial system measured in nanoseconds. E.g. baseunit=-1 for topocentric coordinates in nanosecs,
                baseunit=-3.335668 for topocentric coordinates in meters, 
                baseunit=3.335668 for geocentric coordinates in meters.
telescop
This parameter determine the feed angle variation (i.e. the parallactic angle plus the feed offset angle - evector). It is also used to set the name of the telescop variable in the output dataset. If can take two values, the first gives the antenna mount type, and can be "altaz" or "equatorial". The second value gives the feed offset angle ("evector") in degrees. The default is 0.

Alternatively, you can give the name of a known telescope for this parameter. In this case, the mount type and feed offset angle will be that of that particular telescope.

The default value is "hatcreek" (which is equivalent to "altaz,0").

corr
Defines the correlator setup. The values are:
  nchan:       Number of channels in each spectral window. Use 0
               for a wideband only file.
  nspect:      Number of spectral windows. Default 1; maximum 4.
  f1,f2,...:   "nspect" values giving the offset for the center 
               frequency of each window, in MHz. Default 0.
  df1,df2,...: "nspect" values giving the total widths of each
               spectral window, in MHz. Default 1000.
No checking is made for valid combinations. Default is wideband only for each spectral window.
spectra
Model a Gaussian spectral line. The spectral line model line consists of three values:
  famp:        The line to continuum ratio
  fcen:        Line freq (GHz)
  fwid:        Line width (GHz).
Default is no spectral line.
time
The time of the observation (this corresponds to ha=0). This is in the form
  yymmmdd.ddd
or
  yymmmdd:hh:mm:ss.s
The default is 80JAN01.0. A function of this is also used as a seed for the random number generator. With the unix date command you can use
         date +%y%b%d:%H:%M:%S | tr '[A-Z]' '[a-z]'
A note on the definition of time in a miriad dataset (which is integration centered): this program computes them instantaneous, and could therefor be argued to be off by inttime/2 (as old BIMA data was).
freq
Frequency and IF frequency in GHz. Defaults are 100,0.0 GHz.
radec
Source right ascension and declination. These can be given in hh:mm:ss,dd:mm:ss format, or as decimal hours and decimal degrees. The default is 0,30.
harange
Hour Angle range (start,stop,step) in hours. Default is -6 hrs to + 6 hrs, with a sample interval=0.1 (6 minute) There is a time slip option in the code available to make hour angles into true time hours, but by default this keyword honors earth rotation.
ellim
Elevation limit in degrees. The default is not to limit uv coverage by elevation. If set, then hour angles below the limit are not "observed".
stokes
This selects the polarization parameters formed. Up to 4 polarizations can be formed in one run . They can be 'i' (default),
  stokes=xx,yy,xy,yx
will form a file with the 4 polarisations corresponding to an array with linear feeds. For linear feeds the convention is that the X feed has a position angle of 0, and the Y feed is 90 (measured north towards east).
polar
Polarization patterns for generating time shared polarization data. Up to MAXPOLAR=20 strings of the characters R and L, or X and Y, to represent the polarization of each antenna R(right circular polarization), L(left circular polarization) X(linear polarization PA=0), Y(linear polarization PA=90). E.g. for 3 antennas, the polar=LLL,LRR,RRL,RLR cycles through all combinations of LCP and RCP for each baseline every 4 integrations. The default is to use the stokes keyword.
leakage
Polarization leakage errors, given as a percent. This gives the rms value of leakages of one polarisation feed into another. Polarization leakage errors are constant over the observation. To use this, you must set
  stokes=xx,yy,xy,yx
or
  stokes=rr,ll,rl,lr
The default is 0 (i.e. no polarization leakage).
zeeman
Zeeman effect; the keyword gives the product B * Z, where,
   Stokes V = B * Z * dI/dnu + Leakage * I
   B = line of sight field, and Z = Zeeman splitting term.
This generates a circular polarization for a spectral line. Default = 0.
lat
Latitude of observatory. This can be given in m:ss,dd:mm:ss format, e.g lat=40:49:02.50, or as decimal ees. The default is 40 degrees.
cycle
This gives two values, being the time on-source, and the time off-source cycle times, both in hours. This allows simulation of time segments lost while observing calibrators, etc. For example, if simulating an observation which observes the source for 24 minutes and then is off-source (observing a calibrator) for 6 minutes, use:
  cycle=0.4,0.1
Similarly, if simulating this calibrator, use:
  cycle=0.1,0.4
The default is harange(3),0 (i.e. do not interrupt the observations).
pbfwhm
This dictates the primary beam model used in the simulation. It gives the FWHM of a gaussian primary beam, in arcseconds. The default is no primary beam attenuation.
center
Offset observing centers for a mosaiced observation, in arcseconds. Two values (x and y offset) are required per pointing. Several values can be given. Default is 0,0 (i.e. a plain, single pointing observation). The time spent on each pointing is given by the value of ``cycle(1)''. Note that the default value of cycle(1) means that the observing center changes every integration.
gnoise
Antenna based gain noise, given as a percentage. This gives the multiplicative gain variations, specified by the rms amplitude to be added to the gain of each antenna at each sample interval. The gain error stays constant over the period given by the ``cycle(1)'' parameter (see above). Thus ``cycle(1)'' can be varied to give different atmosphere/instrument stabilities. Note that the default of the ``cycle'' parameter means that the gain changes every integration.

A gain error can also be used to mimic random pointing errors provided the source is a point source. The default is 0 (i.e. no gain error).

pnoise
Antenna based phase noise, in degrees. This gives the phase noise, specified by the rms phase noise to be added to each antenna. Up to 4 values can be given to compute the phase noise
  pnoise(1) + pnoise(2)*(baseline)**pnoise(3)*sinel**pnoise(4)
where ``baseline'' is baseline length in 100m units. Typical values for pnoise(2) are 1mm rms pathlength (e.g. 2 radians at 100 GHz), For Kolmogorov turbulence pnoise(3)=5/6 for baseline < 100m and 0.33 for baseline > 100m (outer scale of turbulence). pnoise(4)=-0.5 for a thick turbulent screen, and -1 for a thin layer. See also the ``gnoise'' parameter. Default is 0,0,0,0 (i.e. no phase error).
systemp
System temperature used to compute additive random noise and total power. One or 3 values can be given; either the average single sideband systemp including the atmosphere (TELEPAR gives typical values), or the double sideband receiver temperature, sky temperature, and zenith opacity, when systemp is computed as:
  systemp = 2.*(Trx + Tsky*(1-exp(-tau/sinel)))*exp(tau/sinel)
where systemp, Trx and Tsky are in Kelvin. Typical values for Hat Ck Trx, Tsky, and tau are 75,290,0.15. (OBSTAU gives values for tau). systemp is used to generate random Gaussian noise to add to each data point. Default is 0,0,0 (i.e. no additive noise).
tpower
Two values can be given to represent the total power variations due to receiver instability (Trms), and atmospheric noise (Tatm).
         tpower = Trms * systemp +  Tatm * pnoise
The receiver instablity is modeled as multiplicative Gaussian noise. The atmospheric noise is modeled to be correlated with the antenna phase noise. Typical values at 3 millimeter wavelength are Trms=10-3 and Tatm=0.2 K/radian (280 degrees/K). Default is tpower=0,0
jyperk
The system sensitivity, in Jy/K. Its value is given by 2*k/(eta * A) where k is Boltzmans constant (1.38e3 Jy m**2 / K), A is the physical area of each antenna (pi/4 * D**2), and eta is an efficiency. For the ATCA, D is 22 meters, and eta is composed of a correlator efficiency (0.88) and an antenna efficiency (0.65 at 6 cm). The overall result is jyperk=12.7. The default jyperk=150, a typical value for the Hat Creek 6.1 m antennas.
out
This gives the name of the output Miriad data file. There is no default. If the dataset exists, visibilities are appended to the dataset, with an appropriate informational message.
options
slip slip time, such that hour angles become clock hours, ignoring
        earths rotation. For short observations this is ok especially
        if you want "nicer" times for your timestamps.
real store correlations as reals, instead of scaled integers
        by default the threshold is (or was) 4 channels, above which
        correlations are stored as scaled integers
complex store correlations as complex. Although allowed as an option
        here, most miriad programs do not support this mode yet.


 

Index

NAME
PERSON RESPONSIBLE
CATEGORIES
DESCRIPTION
PARAMETERS

This document was created by man2html, using the manual pages.
Time: 18:35:38 GMT, July 05, 2011