Content-type: text/html Manpage of opcal

opcal

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NAME

opcal - Correct data for atmospheric opacity and flux miscalibration.  

PERSON RESPONSIBLE

rjs  

CATEGORIES

uv analysis  

DESCRIPTION

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.  

PARAMETERS

vis
The name of the input uv data set. No default.
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.
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)
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".
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.


 

Index

NAME
PERSON RESPONSIBLE
CATEGORIES
DESCRIPTION
PARAMETERS

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Time: 18:35:38 GMT, July 05, 2011