.. note::
    :class: sphx-glr-download-link-note

    Click :ref:`here <sphx_glr_download_generated_examples_io_modify-fits-header.py>` to download the full example code
.. rst-class:: sphx-glr-example-title

.. _sphx_glr_generated_examples_io_modify-fits-header.py:


==================
Edit a FITS header
==================

This example describes how to edit a value in a FITS header
using `astropy.io.fits`.

-------------------

*By: Adrian Price-Whelan*

*License: BSD*

-------------------




.. code-block:: python


    from astropy.io import fits







Download a FITS file:



.. code-block:: python


    from astropy.utils.data import get_pkg_data_filename

    fits_file = get_pkg_data_filename('tutorials/FITS-Header/input_file.fits')







Look at contents of the FITS file



.. code-block:: python


    fits.info(fits_file)





.. rst-class:: sphx-glr-script-out

 Out:

 .. code-block:: none

    Filename: /home/docs/.astropy/cache/download/py3/ba96b4b76d8eff8a716f3a8aeb814ace
    No.    Name      Ver    Type      Cards   Dimensions   Format
      0  PRIMARY       1 PrimaryHDU       7   (100, 100)   float64   
      1                1 ImageHDU         7   (128, 128)   float64


Look at the headers of the two extensions:



.. code-block:: python


    print("Before modifications:")
    print()
    print("Extension 0:")
    print(repr(fits.getheader(fits_file, 0)))
    print()
    print("Extension 1:")
    print(repr(fits.getheader(fits_file, 1)))





.. rst-class:: sphx-glr-script-out

 Out:

 .. code-block:: none

    Before modifications:

    Extension 0:
    SIMPLE  =                    T / conforms to FITS standard                      
    BITPIX  =                  -64 / array data type                                
    NAXIS   =                    2 / number of array dimensions                     
    NAXIS1  =                  100                                                  
    NAXIS2  =                  100                                                  
    EXTEND  =                    T                                                  
    OBJECT  = 'KITTEN  '                                                            

    Extension 1:
    XTENSION= 'IMAGE   '           / Image extension                                
    BITPIX  =                  -64 / array data type                                
    NAXIS   =                    2 / number of array dimensions                     
    NAXIS1  =                  128                                                  
    NAXIS2  =                  128                                                  
    PCOUNT  =                    0 / number of parameters                           
    GCOUNT  =                    1 / number of groups


`astropy.io.fits` provides an object-oriented interface for reading and
interacting with FITS files, but for small operations (like this example) it
is often easier to use the
`convenience functions <http://docs.astropy.org/en/latest/io/fits/index.html#convenience-functions>`_.

To edit a single header value in the header for extension 0, use the
`~astropy.io.fits.setval()` function. For example, set the OBJECT keyword
to 'M31':



.. code-block:: python


    fits.setval(fits_file, 'OBJECT', value='M31')







With no extra arguments, this will modify the header for extension 0, but
this can be changed using the ``ext`` keyword argument. For example, we can
specify extension 1 instead:



.. code-block:: python


    fits.setval(fits_file, 'OBJECT', value='M31', ext=1)







This can also be used to create a new keyword-value pair ("card" in FITS
lingo):



.. code-block:: python


    fits.setval(fits_file, 'ANEWKEY', value='some value')







Again, this is useful for one-off modifications, but can be inefficient
for operations like editing multiple headers in the same file
because `~astropy.io.fits.setval()` loads the whole file each time it
is called. To make several modifications, it's better to load the file once:



.. code-block:: python


    with fits.open(fits_file, 'update') as f:
        for hdu in f:
            hdu.header['OBJECT'] = 'CAT'

    print("After modifications:")
    print()
    print("Extension 0:")
    print(repr(fits.getheader(fits_file, 0)))
    print()
    print("Extension 1:")
    print(repr(fits.getheader(fits_file, 1)))




.. rst-class:: sphx-glr-script-out

 Out:

 .. code-block:: none

    After modifications:

    Extension 0:
    SIMPLE  =                    T / conforms to FITS standard                      
    BITPIX  =                  -64 / array data type                                
    NAXIS   =                    2 / number of array dimensions                     
    NAXIS1  =                  100                                                  
    NAXIS2  =                  100                                                  
    EXTEND  =                    T                                                  
    OBJECT  = 'CAT     '                                                            
    ANEWKEY = 'some value'                                                          

    Extension 1:
    XTENSION= 'IMAGE   '           / Image extension                                
    BITPIX  =                  -64 / array data type                                
    NAXIS   =                    2 / number of array dimensions                     
    NAXIS1  =                  128                                                  
    NAXIS2  =                  128                                                  
    PCOUNT  =                    0 / number of parameters                           
    GCOUNT  =                    1 / number of groups                               
    OBJECT  = 'CAT     '


**Total running time of the script:** ( 0 minutes  0.504 seconds)


.. _sphx_glr_download_generated_examples_io_modify-fits-header.py:


.. only :: html

 .. container:: sphx-glr-footer
    :class: sphx-glr-footer-example



  .. container:: sphx-glr-download

     :download:`Download Python source code: modify-fits-header.py <modify-fits-header.py>`



  .. container:: sphx-glr-download

     :download:`Download Jupyter notebook: modify-fits-header.ipynb <modify-fits-header.ipynb>`


.. only:: html

 .. rst-class:: sphx-glr-signature

    `Gallery generated by Sphinx-Gallery <https://sphinx-gallery.readthedocs.io>`_