2008-05-26

Astronomical image processing guide (How to open a FITS image?)

A structure of a FITS file is a little bit complicated, but not too much. The FITS itself includes a header and a data part. The header of an image consists from meta-information about the image. The most important are the size (width and height) and data representation of the image.

The header is set of 80-byte (character) length records. Every record is represented by text line with structure:
KEYWORD = VALUE
The KEYWORD must be no longer than 8 characters. The '=' must be in 9 column. The width and height of an image is coded by the way:
NAXIS1 = 1628
NAXIS2 = 1236

How to get an image size?

To get this basic information by use of the cfitsio library, we can use of the piece of the code (source code):

! to compile: gfortran -Wall -o FITSsize o.f90 -L/usr/local/lib -lcfitsio

program FITSsize

implicit none

integer :: status, bitpix, naxis, naxes(2)
! status ... FITS status (0=no error)
! naxis .. number of axes in image (we require =2)
! naxes .. dimension of the image (2-element array)

integer :: blocksize,pcount,gcount
logical :: extend, simple
! required by cFITSIO

character(len=666) :: name = 'image.fits'
! name .. fill with name of the image to open
status = 0
call ftopen(25,name,0,blocksize,status)
call ftghpr(25,2,simple,bitpix,naxis,naxes,pcount,gcount,extend,status)
call ftclos(25,status)

if( status == 0 ) then
write(*,*) 'The image ',trim(name),' has the size:',naxes
else
write(*,*) 'The image "',trim(name),'" not found or not accessible.'
end if

end program FITSsize

The code may by compiled by command:

host$ gfortran -Wall -o FITSsize FITSsize.f90 -L/usr/local/lib -lcfitsio

where switch -Wall prints some warnings, -o specify name of the generated binary file (name of the routine), -L points path to cfitsio library (may be omitted, usually any system directory) and -lcfitsio links cFITSIO library (libcfitsio.a).

Type:

host$ ./FITSsize

to run. The utility will try to open file named as 'image.fits' (can be changed in declaration name = 'image.fits'). If this file is accessible it will print the size of the image. If the name can't be open, an error will appeared.

This code demonstrates basic idioms of FITS-specific ones:
  • its look horribly
  • there is a lot of declarations which meaning is too hard to remember
  • the variable status must be set to zero before calling of any of cFITSIO routines
  • the name of the image to open is a second argument to ftopen
  • the routine ftghpr reads important parameters (coded by KEYWORDS as above) of the image
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