FITStoImage

Converting a FITS cube to a CASA Image

If you want to perform analysis on a FITS file in CASA, you first need to import it into `.image` format.

importfits('file.fits','file.image')

If this works, great! You can move on. CASA will treat NaN values in an image as 'masked'.

Dealing with Errors

There may be issues with FITS headers. CASA respects a large number of header keys (reference).

There are many ways to edit a FITS header. One of the most straightforward to use is edhead. Otherwise, one can use CASA's imhead

CASA Keyword	FITS keyword	Description
beammaj	BMAJ	Major axis of the clean beam
beammin	BMIN	Minor axis of the clean beam
beampa	BPA	Position angle of the clean beam
bunit	BUNIT	Brightness unit (K, Jy/beam, etc)
cdeltn	CDELTn	Pixel size, nth axis  (max n is 4)
crpixn	CRPIXn	Pixel coordinate of reference point, nth axis
crvaln	CRVALn	Pixel location of reference point, nth axis
ctypen	CTYPEn	Axis name, nth axis. For FITS, this includes the projection
cunitn	CUNITn	Pixel units, nth axis
datamax	DATAMAX	Maximum pixel value in image
datamin	DATAMIN	Minimum pixel value in image
date-obs	DATE-OBS	Date of the observation
equinox	EQUINOX	Reference frame for directional coordinates
imtype		Image type: intensity,
minpos		Position of the minimum value (world unit)
minpixpos		Same in pixel (array)
maxpos		Position of the maximum value (world unit)
maxpixpos		Same in pixel (array)
object	OBJECT	Source name
observer	OBSERVER	Observer name
projection	CTYPEn	Image projection ('SIN','TAN', or 'ZEA')
reffreqtype		Reference frame for the spectral coordinates
restfreq	RESTFREQ	Rest Frequency
shape	NAXISn	Number of pixels along each axis
telescope	TELESCOP	Telescope name

What does it mean if you get this sort of error?

 CASA <1>: importfits('file.fits','file.image') 
 2013-12-18 13:06:18     WARN    importfits::FITSCoordinateUtil::fromFITSHeader  The wcs function failures are too severe to continue ... 
 2013-12-18 13:06:18     WARN    importfits::ImageFITSConverterImpl::FITSToImage (file /var/rpmbuild/BUILD/casapy-stable/casapy-stable-42.0.26945/casacore/images/Images/ImageFITSConverter.tcc, line 71)       No proper coordinate system defined in FITS file. Using dummy linear system instead.

First, go to the `casapy-yyyymmdd-hhmmss.log` file and look at the errors.

 2013-12-18 13:06:18     INFO    importfits::FITSCoordinateUtil::fromFITSHeader  celfix incurred the error Inconsistent or unrecognized coordinate axis types 
 2013-12-18 13:06:18     INFO    importfits::FITSCoordinateUtil::fromFITSHeader  spcfix incurred the error Inconsistent or unrecognized coordinate axis types 
 2013-12-18 13:06:18     INFO    importfits::FITSCoordinateUtil::fromFITSHeader  cylfix incurred the error Inconsistent or unrecognized coordinate axis types

In my case, the error turned out to be that `CTYPE3` was set to `RADI-LSR`, while it should be `VELO-LSR` to be recognized by the CASA system. The velocity convention is, unfortunately, lost.