Description

The exportfits task exports CASA-produced images as FITS files for transporting to other software packages or publication. 

With the default settings, this task will produce FITS image files with 4 dimensions:  RA, Dec, radio frequency, and polarization (Stokes parameters).  Several options influence the shape of the output data array.  Setting dropstokes=True will cause the polarization axis to be omitted.  Setting velocity=True will cause the spectral axis to have units of velocity rather than frequency.  Setting stokeslast=False will interchange the order of the polarization and spectral axes (i.e, polarization will be the third dimension, and frequency the fourth).  Finally, setting dropdeg=True will cause any "degenerate" axes, those with length 1, to be omitted.  For example a total-intensity continuum image would result in a 2-dimensional output array (RA, Dec only) when using this option.

The bitpix parameter specifies the number of bits used to represent a data value in the FITS image array. The allowed values for bitpix are 16 or -32 meaning the data values are represented as 16-bit twos-complement binary integer or IEEE single precision floating point, respectively. We recommend using the default value -32 in most cases. Using 16 bits results in smaller FITS files, at the expense of dynamic range in the image.  It may also be necessary to use specific bitpix values depending on the requirements of any external programs that will be reading the FITS files.

The minpix and maxpix parameters are used to specifiy the minimum and maximum data values when re-scaling to integers in the 16-bit case; this affects the dynamic range of the exported image.  Data values outside this range will be clipped.  If unspecified, these will be determined automatically from the minimum and maximum data values present.

Generally, the conversion follows the official FITS standard 3.0.