.. DO NOT EDIT.
.. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY.
.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "generated/examples/io/modify-fits-header.py"
.. LINE NUMBERS ARE GIVEN BELOW.

.. only:: html

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

        :ref:`Go to the end <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*

.. GENERATED FROM PYTHON SOURCE LINES 16-20

.. code-block:: Python


    from astropy.io import fits
    from astropy.utils.data import get_pkg_data_filename








.. GENERATED FROM PYTHON SOURCE LINES 21-22

Download a FITS file:

.. GENERATED FROM PYTHON SOURCE LINES 22-25

.. code-block:: Python


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








.. GENERATED FROM PYTHON SOURCE LINES 26-27

Look at contents of the FITS file

.. GENERATED FROM PYTHON SOURCE LINES 27-30

.. code-block:: Python


    fits.info(fits_file)





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

 .. code-block:: none

    Filename: /home/user/.astropy/cache/download/url/519010d87325a22575dc1d16f3a05d26/contents
    No.    Name      Ver    Type      Cards   Dimensions   Format
      0  PRIMARY       1 PrimaryHDU       8   (100, 100)   float64   
      1                1 ImageHDU         8   (128, 128)   float64   




.. GENERATED FROM PYTHON SOURCE LINES 31-32

Look at the headers of the two extensions:

.. GENERATED FROM PYTHON SOURCE LINES 32-41

.. 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

 .. 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  = '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     '                                                            




.. GENERATED FROM PYTHON SOURCE LINES 42-50

`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 <https://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':

.. GENERATED FROM PYTHON SOURCE LINES 50-53

.. code-block:: Python


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








.. GENERATED FROM PYTHON SOURCE LINES 54-57

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:

.. GENERATED FROM PYTHON SOURCE LINES 57-60

.. code-block:: Python


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








.. GENERATED FROM PYTHON SOURCE LINES 61-63

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

.. GENERATED FROM PYTHON SOURCE LINES 63-66

.. code-block:: Python


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








.. GENERATED FROM PYTHON SOURCE LINES 67-71

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:

.. GENERATED FROM PYTHON SOURCE LINES 71-83

.. 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

 .. 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     '                                                            





.. rst-class:: sphx-glr-timing

   **Total running time of the script:** (0 minutes 0.036 seconds)


.. _sphx_glr_download_generated_examples_io_modify-fits-header.py:

.. only:: html

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

    .. container:: sphx-glr-download sphx-glr-download-jupyter

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

    .. container:: sphx-glr-download sphx-glr-download-python

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


.. only:: html

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

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