Writing Visibility FITS Files

Task fits can also be used to write a visibility FITS file. In addition to the keywords used when reading visibility datasets, fits uses keywords select, stokes and options to control extra processing performed on the visibilities.

Unfortunately fits is more sophisticated at reading FITS files than writing - note the following caveats carefully.

• fits can cope with only a single pointing of a single source, with a single frequency setup and single antenna array configuration (note that, unlike earlier versions, fits writes an antenna table). fits does quite limited checks to see if these conditions are met, and quite happily will use the relevant parameters of the first record.

• fits assumes that the data are a single IF band, with a constant frequency increment between channels. If presented with multiple spectral windows (multiple IFs), an average frequency and increment will be used. This is very undesirable. If you do this, fits will issue warnings about frequencies deviating from linearity. These warnings should be taken seriously. To avoid this, use the select or line parameters to force fits to write out a single spectral window.

• fits does not save Miriad calibration tables. However fits does give you the option of applying the calibration, as it writes out the data (see the options keyword).

• Miriad allows you flexibility in the choice of polarization correlations that you can write out. On-the-fly polarization conversion can be performed. However AIPS will refuse to read in a number of the possibilities that Miriad allows. In particular, AIPS UVLOD refuses to load visibility data where there are three different sorts of polarization correlations (rather than the more usual 1, 2 or 4 sorts).

Typical inputs follow:

FITS
in=0823.uv	Name of input Miriad dataset.
op=uvout	Write out a visibility FITS file.
out=0823.FITS	Output FITS file - probably upper case for AIPS.
select	Select appropriate data.
stokes=i,q,u,v	Select appropriate polarizations.
option	Leave unset to apply calibration,
options=nopol,nocal,nopass	or turn off all calibration.