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Initial ``fixes'' to millimetre data

The next step after loading is to apply a number of ``fixes'' that are important for 3-mm data. Note that there is no record in the data that these steps have been performed. In particular the software will allow you to apply the fixes twice, which would be just as bad as not applying them at all. The task to do these, atfix, performs the following functions:

• By default atfix applies the ``standard'' gain/elevation model for the antennas. You can disable this by using options=nogainel.

• By default atfix corrects for a ``standard'' instrumental phase model. You can disable this with options=noinst.

• As mentioned above, the ATCA uses the chopper wheel method to measure an above atmosphere system temperature. The nature of the hardware means the measurement is done only typically every 5 to 30 minutes. When observing, the ATCA on-line system can either apply a nominal system temperature value to the visibility data, or it can apply the actual measured system temperature values. In applying these measured values, on-line system will apply the most recent measurement.

  Task atfix includes an algorithm to interpolate measured system temperature values, between measurements, and to correct the visibility data accordingly. In doing this it correctly handles whether a nominal or the measured system temperature value applied to the data on-line. The interpolation algorithm takes into account changes in elevation and meteorological conditions.

  The parameter tsyscal can be used to change this default behaviour. This parameter can take on values to revert the data back to the nominal system temperature value, to make no changes to the system temperature scaling, to use the most recent system temperature measurement, or to use an extrapolation (as distinct from interpolation) algorithm in estimating system temperatures.

• Often you will want to apply a correction to your data to better account for the exact locations of the antennas.

  On the night of an ATCA reconfiguration, a solution for the antenna positions is derived. The practise is to install this solution as the ``standard solution''. The on-line system will then use this solution in subsequent observations in that array configuration. The quality of this solution is important for the ultimate phase calibration of 3mm data.

  Experience has shown that often the position solution derived at the time of the reconfiguration is not necessarily the best possible. For example the solution will depend critically on the atmospheric phase stability at the time of the solution, and this might be less than ideal on the night of the reconfiguration. Consequently improved solutions will often be derived at a later time. If improved solutions become available, then 3-mm observers should correct their data accordingly.

  Updated solutions are stored in parameter files in the Miriad system directory $MIRCAT. These solutions are distributed via the Miriad update mechanism, and have file names of the form ``dantpos.yymmdd''. Here where ``yymmdd'' is the date of the relevant array reconfiguration.

  Task atfix corrects the data for an improved antenna location solution via the dantpos parameter. If an improved solution file is present, you can instruct atfix to read this directly using the indirect parameter input mechanism (see Section 2.5). For example, to use the antenna location solution appropriate for a hypothetical array reconfiguration that happened on 16 October 2002, use

       dantpos=@$MIRCAT/dantpos.021016
  

• One of the flaws of the current ATCA RPFITS files is that antenna locations are not saved when antennas are not in use. While this might not seem a serious flaw (who cares about the antennas if they are not in use?), the unused antennas can still cause shadowing - true antenna locations are needed for shadowing calculation. This was a particular issue with the interim 3-antenna 3-mm system when observing in compact arrays.

  By giving a value to the array parameter, atfix will fill in any antenna locations that are missing in the input visibility file. NOTE: This just fills in missing antenna locations, it does not perform any flagging of shadowed data.

  The value given to this parameter is either an array configuration name (e.g. EW352 or 750A) or a list of six station names (``W106'' and ``N3'' are examples of station names). When giving the station names, these must be in the order of the antennas (i.e. CA01, CA02, CA03 etc), regardless of any possible array shuffles.

  NOTE: When antennas are in a shuffled order, or for arrays using the north spur, you should generally give the list of station names, as the standard array configuration names assume an antenna orderwhich is probably not correct.

  If in doubt the on-line configuration history should be consulted.

Typical inputs to atfix are:

ATFIX
in=vela.uv	Input visibility dataset.
out=vela.fixed.uv	Output corrected visibility dataset.
dantpos=@$MIRCAT/dantpos.021016	Antenna location correction.