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Basic Information on opcal
Task: opcal
Purpose: Correct data for atmospheric opacity and flux miscalibration.
Categories: uv analysis
opcal corrects a visibility dataset for atmospheric opacity
and can also attempt to correct for errors in the flux calibration
scale. This should be the first step in the calibration of visibility
data (i.e. before any other calibration steps). It should be performed
on both the source of interest as well as all calibrators.
opcal works by computing the brightness and opacity of the atmosphere.
This is estimated from a model of the atmosphere, given the observing
frequency and elevation, as well as meteorological data.
Opacity correction is probably not warranted if the
fluctuations in opacity are small and the calibrator is quite close
to the program source. However opacity correction will not be
damaging in these instances (it would be just an unnecessary extra
step). At frequencies above 15 GHz, opacity correction is generally
recommended.
To correct for atmospheric opacity, opcal scales up the measured
visibility data to account for this attenuation. It also scales the
system temperature data to an ``above atmosphere'' value.
opcal can also estimate and correct for a miscalibration in the flux
calibration scale. The miscalibration might arise from incorrect
value assumed for the on-line calibrator source (noise diode), or
an incorrect conversion to system temperature. opcal works by
comparing its computed estimates of the sky brightness with the
measured values of system temperature. In estimating a miscalibration
factor, it assumes that the measured system temperature is a result of
the the atmosphere and CMB, plus a constant (i.e. contributions to
system temperature from the receivers, spillover, astronomical source,
etc, are assumed constant). A scale factor to correct the measured
system temperature is then computed. Using this procedure should only
be attempted when the observation samples an appreciable range of
elevations.
NOTE: The model sky brightnesses are just that - models, not reality.
They will probably be quite inaccurate in cloudy weather, and
very inaccurate in rainy weather. Do not use this except in clear
weather.
NOTE: This procedure to estimate flux calibration factor is not
necessarily correct. The system temperature scale may differ from
with the model data because the system efficiency used in the
conversion process (the telescope Jy/Kelvin) was wrong, and the
system temperature scale may have been adjusted to account for this.
Key: vis
The name of the input uv data set. No default.
Key: select
Normal Miriad uv data selection. See the help on select for
more information. This selection criteria is used in checking
which data are to be used in determining the flux scale calibration
(i.e. mode=flux or mode=both). All data (regardless of the select
keyword) is copied to the output file.
Key: out
The name of the output uv data set. No default.
The output dataset contains extra uv variables, which might be
instructive to examine. These are
tsky Expected sky brightness, in Kelvins
trans Expected atmospheric transmissivity (i.e. a fraction,
with 1 indicating a transparent atmosphere).
airtemp Measured air temperature, in celsius.
pressmb Measured air pressure, in millibars.
relhumid Measured relative humidity, as a percent.
antel Antenna elevation, in degrees.
airmass Airmass - cosec(antel)
Key: mdata
Input text file giving the ATCA meteorology data. No default.
For more help on getting the data appropriate to your
observation, see the help on weatherdata.
Key: mode
This determines the calibration operations to perform. Possible
values are ``opacity'', ``flux'' or ``both'' or "neither". The
default is to perform opacity correciton only.