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.

Generated by miriad@atnf.csiro.au on 05 Mar 2024