CASA Images

Structure of CASA images

CASA images are stored as tables and can be accessed with CASA tasks and tools. E.g. metadata can be listed and edited with imhead, and the content is best visualized with the CASA image viewer (see 'Image Cube Visualization').

 

Image Headers

Image Headers can be accessed and edited via the imhead task and the msmd tool.  Header data can also be inspected with the casabrowser.  See 'Dealing with Image Headers' for details. 

 

Image Axes / Velocity Systems

CASA images typically have the following axis order (0-based python numbers) 0: RA, 1: DEC,  2: Stokes, 3: Frequency.  Spatial axes can also be GLON/GLAT or other systems.  The spectral axis of images created in CASA is always in frequency units.  In addition, (multiple) velocity systems can be added to relabel the spectral axis. The task imreframe offers an easy way to change the velocity labelling. Tools to modify spatial and spectral properties of images are coordsys and measures.  

Note that when data is imported into CASA from FITS, the above conventions may not apply. Also, when exporting to FITS, and the desired exported spectral axis is velocity, the parameter velocity=True must be set in exportfits.  See 'Image Import and Export" for details. 

  

Image Masks

Internal Image Masks are stored as Boolean True/False cubes within the images themselves. There can be multiple masks stored in each data cube and one of them is defined to be the 'default' mask. The default mask is the one visible when the image is displayed, e.g. in the viewer, and that is applied for operations on images. All masks have labels, such as mask0 etc. and they can be selected by specifying the image name followed by the mask name, separated by a colon. E.g. 'mask1' in 'image.im' is used when specifying the image as 'image.im:mask1'. Available masks can be listed with the task makemask which can also assign any mask as the default. The same task can also be used to export masks into separate CASA zero/non-zero cubes and to import such cubes as Boolean masks inside images. In addition, makemask allows to create masks from image regions.  More information on masks is provided on the 'Image Masks' and 'LEL Masks' CASAdocs pages. 

 

CASA Regions

They are region text files outside images that can be edited and modified by the user. More information is available on the 'Region Files' page.