Reading FITS Files

For many years there was officially no such thing as disc FITS. However, ad hoc implementations have existed for a long time. Of these, FITSDIN will handle files adhering to the FITS rules for blocking (and more), but it doesn't process byte-swapped `FITS' files. Thus it can process files with fixed-length records of semi-arbitrary length; so, for example, files mangled during network transfer, which have 512-byte records rather than the customary 2880, may be read. However, it will not handle, VAX FITS files as may be produced with FIGARO's WDFITS. FITSDIN will accept a list of files with wildcards. However, a comma-separated list must be enclosed in quotation marks. Also wildcards must be protected. Here are some examples so you get the idea.

     % fitsdin '*.fit'
     % fitsdin \*.fit
     ICL> fitsdin *.fit
     % fitsdin '"i*.fit,abc123.fts"'
     ICL> fitsdin "i*.fit,abc123.fts"

In the following example a floating-point file is read (BITPIX=$-$32) and so FMTCNV is not required.

     % fitsdin '*.fits'

        2 files to be processed...

     Processing file number 1: /home/scratch/dro/gr.fits.
     File /scratch/dro/gr.fits(1)  Descriptors follow:
     SIMPLE  =                    T / Standard FITS format
     BITPIX  =                  -32 / No. of bits per pixel
     NAXIS   =                    2 / No. of axes in image
     NAXIS1  =                  512 / No. of pixels
     NAXIS2  =                  256 / No. of pixels
     EXTEND  =                    T / FITS extension may be present
     BLOCKED =                    T / FITS file may be blocked
     
     BUNIT   = 'none given      '   / Units of data values
     
     CRPIX1  =   1.000000000000E+00 / Reference pixel
     CRVAL1  =   0.000000000000E+00 / Coordinate at reference pixel
     CDELT1  =   1.000000000000E+00 / Coordinate increment per pixel
     CTYPE1  = '                '   / Units of coordinate
     CRPIX2  =   1.000000000000E+00 / Reference pixel
     CRVAL2  =   0.000000000000E+00 / Coordinate at reference pixel
     CDELT2  =   1.000000000000E+00 / Coordinate increment per pixel
     CTYPE2  = '                '   / Units of coordinate
 
     ORIGIN  = 'ESO-MIDAS'          / Written by MIDAS
     OBJECT  = 'artificial image'   / MIDAS desc.: IDENT(1)
             :                :                :
             :                :                :
             :                :                :
     HISTORY  ESO-DESCRIPTORS END     ................
     
     END
     OUT - Output image > gr
     Completed processing of disc file /home/scratch/dro/gr.fits to gr.
     File has illegal-length blocks (512).  Blocks should be a multiple (1--10) of the
     FITS record length of 2880 bytes.
     Processing file number 2: /home/scratch/dro/indef.fits.
     File /home/scratch/dro/indef.fits(1)  Descriptors follow:
     SIMPLE  =                    T  /  FITS STANDARD
     BITPIX  =                   32  /  FITS BITS/PIXEL
     NAXIS   =                    2  /  NUMBER OF AXES
     NAXIS1  =                  256  /
     NAXIS2  =                   20  /
     BSCALE  =      3.7252940008E28  /  REAL = TAPE*BSCALE + BZERO
     BZERO   =      7.9999999471E37  /
     OBJECT  = 'JUNK[1/1]'  /
     ORIGIN  = 'KPNO-IRAF'  /
             :                :                :
             :                :                :
             :                :                :
     END
     OUT - Output image > iraf
     Completed processing of disc file /home/scratch/dro/indef.fits to iraf.

NDFTRACE shows that the object name is written to the NDF's title, that axes derived from the FITS headers are present, and that gr is a _REAL NDF.

     % ndftrace gr
 
        NDF structure /home/scratch/dro/iraf:
           Title:  artificial image
           Units:  none given
 
        Shape:
           No. of dimensions:  2
           Dimension size(s):  512 x 256
           Pixel bounds     :  1:512, 1:256
           Total pixels     :  131072
 
        Axes:
           Axis 1:
              Label : Axis 1
              Units : pixel
              Extent: -0.5 to 511.5
 
           Axis 2:
              Label : Axis 2
              Units : pixel
              Extent: -0.5 to 255.5
 
        Data Component:
           Type        :  _REAL
           Storage form:  PRIMITIVE
           Bad pixels may be present
 
        Extensions:
              FITS             <_CHAR*80>

Both FITSIN and FITSDIN write the FITS headers into an NDF extension called FITS within your NDF. The extension is a literal copy of all the 80-character `card images' in order. These can be inspected or written to a file via the command FITSLIST. There is more on this NDF extension and its purpose in the FITS Airlock.