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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 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.
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 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.
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" 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; 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.
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 (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).
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=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.
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. 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.
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 arcseconds.
The default is no primary beam attenuation.
Key: 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.
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 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).
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 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).
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 3 millimeter wavelength
are Trms=10-3 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 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.
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.
Key: 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.