Basic Information on uvgen

Task: uvgen
Purpose: Compute visibilities for a model source.
Categories: uv analysis, map making

        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.

Key: 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 ten 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.
          alpha:         Spectral index. 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.

Key: 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.

Key: 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 nanosec.
        E.g.    baseunit=-1 for topocentric coordinates in nanosec,
                baseunit=3.33564 for geocentric coordinates in metres.

Key: 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", "xyew", 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").

Key: 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; max 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.

Key: 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.

Key: time
        The time of the observation (corresponding to ha=0) 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.

Key: seed
        Set the seed for the random number generator
        If this is not set or zero, the time is used as the seed

Key: freq
        Frequency and IF frequency in GHz.
        Defaults are 100,0.0 GHz.

Key: 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.

Key: harange
        Hour Angle range (start,stop,step) in hours.  Default is
      -6 hrs to + 6 hrs, with a sample interval of 10 seconds
      (although see caveat in inttime).

Key: inttime
        The cycle integration time to use in seconds. Default is
        10 seconds. If only the third parameter to harange is not
        specified, the sample interval will be this value.

Key: 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".

Key: stokes
        This selects the polarization parameters formed.  Up to 4
        polarizations can be formed in one run.  They can be 'i'
        (default), 'xx', 'yy', 'xy', 'yx, 'lr', 'rl', 'rr' or 'll'.
        For example:
          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).

Key: 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.

Key: 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).

Key: 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.

Key: lat
        Latitude of observatory, in degrees.  Default is 40 degrees.

Key: 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).

Key: pbfwhm
        This dictates the primary beam model used in the simulation.  It
        gives the FWHM of a gaussian primary beam, in arcsec.
        The default is no primary beam attenuation.

Key: center
        Offset observing centers for a mosaiced observation, in arcsec
        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.

Key: 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).

Key: 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 the baseline length in km.  Typical values
        for pnoise(2) are 1mm rms pathlength (e.g. 2 radians at 100GHz),
        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).

Key: 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 Creek 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).

Key: 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 3mm wavelength are
        Trms = 0.001 and Tatm = 0.2 K/radian (280 degrees/K).
        Default is tpower=0,0

Key: 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 metres, 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 6.1m
        antennas.

Key: antfac
        In case not all antennas should have equal sensitivity, specify
        a factor to apply to the noise, followed by a list of antenna
        numbers. Default is to treat all antennas the same.

Key: options
        A number of options can be specified, separated by commas.
          'leakfvar' Add linear variation of leakage parameters across
                     each spectral window
          'delay'    Add delay noise instead of phase noise, i.e., make
                     the phases vary with frequency
          'bandpass' Add a semi random bandpass function to the spectra

Key: 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.