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

Astronomical image processing guide (Intro)

From time to time, I'm thinking about the most useful way to process of an astronomical image data. An use of wide-known software package utilities like Gaia, ds9, IRAF may be better or worse in comparison of a direct coding of a self-made routine? Both approaches has its own advantages and disadvantages so, I think, there is no an universal way to process its. For example, a lot of work can be done inside IRAF's (http://iraf.noao.edu/) environment, but sometimes may be faster and more suitable of coding of an own utility. Both approaches may be possible useful and important especially for a particular processing. Also, I thinks the develop of any own routine may be much, much better and simpler then use of prepared ones. While there is a lot of documents describing of various software packages, the processing in a computer language is described poorly. That's why I'm starting write this guide.

The basic operation to work with data is data file handling. A FITS format (http://heasarc.gsfc.nasa.gov/docs/heasarc/fits.html) is a wide-used format to storing of an astronomical data including both an images and a data tables. A general structure of FITS files may be really very complicated. Fortunately, W. Pence and etc. made the cFITSIO library to provide of a standard way to create, open and modify of any FITS files. The library offers interfaces for C and Fortran languages. There is also a lot of wrappers to others (Perl, Python, C++, ..). I'll use of Fortran (fortran 90/95/2003 dialect) in this guide to create a simple code but the change to C (for example) is straightforward.

A computer library (like cfitsio) is a file with a special structure which includes a set of (useful) functions. The file is usually created by compiling of a computer language (cfitsio is in C) to a machine-specific code.

How to install an environment to open FITS files?

1. Use of your package system and install gfortran (Fortran compiler).
2. Use of your package system and install cfitsio.

ad 1). The gfortran is included in most modern Linuxes. Its is possible to use also Intel's ifort (apparently faster). Solaris offers f90/f95. The GNU g95 provides binaries for other systems (BSD's, Mac OSX, ..).

ad 2) Many modern distributions (Debian, Ubuntu, Fedora..) offers cfitsio as a package. If your doesn't, download directly tarball. Move it to /tmp and execute:
host:/tmp$ tar zxf cfitsio-x.y.z.tar.gz
host:/tmp$ cd cfistio-x.y.z
host:/tmp$ ./configure # check output
host:/tmp$ make # compile
host:/tmp$ su # switch to root account
host:/tmp$ make install # install to /usr/local

To install it in an another place use --prefix parameter for configure (./configure --help). To uninstall it, type (as root) make uninstall. A successful installation is indicated by presence of drvrsmem.h,fitsio.h, fitsio2.h and longnam.h in /usr/local/include and libcfitsio.a in /usr/local/lib.