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.