GILDAS Headers

The DEFINE IMAGE command allows SIC to access not only to the content of an image (the data value), but also to all its associated parameters; DEFINE HEADER allows access only to these associated parameters. The header variables have names derived from the generic name by adding the special character % and an extension (such as e.g. NDIM for the number of dimensions) to the generic header name. For example, command DEFINE HEADER VAR file.gdf READ also creates the following variables:

       VAR%GENE       Integer        Length of general section
       VAR%NDIM       Integer        Number of dimensions (ReadOnly)
       VAR%DIM        Integer[7]     Dimensions           (ReadOnly)
       VAR%CONVERT    Double[3,7]    Conversion formulae for the 7 axes:
                                     Reference pixel,
                                     Value at reference pixel,
                                     Increment
       VAR%BLAN       Integer        Length of blanking section
       VAR%BLANK      Real[2]        Blanking and tolerance
       VAR%EXTREMA    Integer        Length of extrema section
       VAR%MAX        Real           Maximum
       VAR%MIN        Real           Minimum
       VAR%MINLOC     Integer[7]     Position of min value
       VAR%MAXLOC     Integer[7]     Position of max value
       VAR%DESC       Integer        Length of units and system section
       VAR%UNIT       Char*12        Image unit
       VAR%UNIT1      Char*12        First axis type
       VAR%UNIT2      Char*12        Second axis type
       VAR%UNIT3      Char*12        Third axis type
       VAR%UNIT4      Char*12        Fourth axis type
       VAR%SYSTEM     Char*12        Coordinate system
       VAR%POSI       Integer        Length of position section
       VAR%SOURCE     Char*12        Source name
       VAR%RA         Double         Right Ascension
       VAR%DEC        Double         Declination
       VAR%LII        Double         Galactic longitude
       VAR%BII        Double         Galactic latitude
       VAR%EQUINOX    Real           Equinox of coordinates
       VAR%PROJ       Integer        Length of projection section
       VAR%PTYPE      Integer        Projection type (code)
       VAR%A0         Double         first coordinate of projection center
       VAR%D0         Double         second coordinate of projection center
       VAR%ANGLE      Double         position angle of projection
       VAR%X_AXIS     Integer        First projected axis
       VAR%Y_AXIS     Integer        Second projected axis
       VAR%SPEC       Integer        Length of spectroscopy section
       VAR%LINE       Char*12        Line name
       VAR%FREQRES    Double         Frequency resolution
       VAR%IMAGFRE    Double         Image Frequency
       VAR%RESTFRE    Double         Rest Frequency
       VAR%VELRES     Real           Velocity resolution
       VAR%VELOFF     Real           Velocity offset
       VAR%F_AXIS     Integer        Frequency/Velocity axis
       VAR%BEAM       Integer        Length of beam section
       VAR%MAJOR      Real           Major axis of beam
       VAR%MINOR      Real           Minor axis of beam
       VAR%PA         Real           Position angle of beam
       VAR%SIGMA      Integer        Noise section length
       VAR%NOISE      Real           Theoretical noise
       VAR%RMS        Real           Actual noise
       VAR%PROPER     Integer        Proper motion section length
       VAR%MU         Real[2]        along RA and DEC, in mas/yr
       VAR%PARALLAX   Real           Parallax in mas

In addition, the following variables are also created when accessing GILDAS UV Tables through the DEFINE UVTABLE command:

       VAR%NCHAN      Integer        Number of channels
       VAR%NVISI      Integer        Number of visibilities
       VAR%NSTOKES    Integer        Number of Stokes parameters
       VAR%NATOM      Integer        Complex visibility size
       VAR%BASEMIN    Real           Minimum baseline
       VAR%BASEMAX    Real           Maximum baseline

VAR becomes a dummy variable of type header, which can only be referenced in a further DELETE /VARIABLE command. The VAR%item variables are ReadOnly or ReadWrite according to the keyword following the filename, except for the dimension variables (VAR%DIM and VAR%NDIM), which cannot be modified. VAR%READONLY indicates whether the variable can be modified or not.

Full headers can be copied to one another, using the command

    LET A% = B%

which copies the header of image B into that of image A (dimensions are not modified, however). Thus a full copy of a 4 dimensions GILDAS data file can be obtained within SIC as follows:

    DEFINE IMAGE A Oldfile.gdf READ
    DEFINE INTEGER N1 N2 N3 N4
    LET N1 A%DIM[1]
    LET N2 A%DIM[2]
    LET N3 A%DIM[3]
    LET N4 A%DIM[4]
    DEFINE IMAGE B[N1,N2,N3,N4] Newfile.gdf REAL
    LET B A      !  Copy A data into B
    LET B% A%    ! Copy A header into B header
    DELETE /VARIABLE B ! Deletes the SIC variables, but not the file...

A simpler (and more generic) way to declare new images is to use the size casting provided by the /LIKE option:

    DEFINE IMAGE A Oldfile.gdf READ
    DEFINE IMAGE B Newfile.gdf REAL /LIKE A  ! Define B like A...
    LET B A      !  Copy A data into B
    LET B% A%    !  Copy A header into B header
    DELETE /VARIABLE B ! Deletes the SIC variables, but not the file...