+
+
+
+Edit a FITS header#
+ +Learning Goals#
+-
+
Read a FITS file
+Retrieve FITS header metadata
+Edit the FITS header
+Write the modified file as a FITS file
+
Keywords#
+FITS, file input/output
+Summary#
+This tutorial describes how to read in and edit a FITS header, and then write
+it back out to disk. For this example we’re going to change the OBJECT
+keyword.
from astropy.io import fits
+astropy.io.fits provides a lot of flexibility for reading FITS
+files and headers, but most of the time the convenience functions are
+the easiest way to access the data. fits.getdata() reads only the
+data from a FITS file, but with the header=True keyword argument will
+also read the header.
data, header = fits.getdata("input_file.fits", header=True)
+There is also a dedicated function for reading only the +header:
+hdu_number = 0 # HDU means header data unit
+fits.getheader("input_file.fits", hdu_number)
+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 '
+MYHDRKW = 'My Header Keyword'
+But getdata() can get both the data and the header, so it’s a useful
+command to remember. Since the primary HDU of a FITS file must contain image data,
+the data is now stored in a numpy array. The header is stored in an
+object that acts like a standard Python dictionary.
# But hdu_number = 0 is the PRIMARY HDU.How many HDUs are in this file?
+fits_inf = fits.open("input_file.fits")
+fits_inf.info()
+fits_inf[0].header
+Filename: input_file.fits
+No. Name Ver Type Cards Dimensions Format
+ 0 PRIMARY 1 PrimaryHDU 8 (100, 100) float64
+ 1 1 ImageHDU 7 (128, 128) float64
+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 '
+MYHDRKW = 'My Header Keyword'
+Using fits.open allows us to look more generally at our data. fits_inf[0].header gives us the same output as fits.getheader. What will you learn if you type fits_inf[1].header? Based on fits_inf.info() can you guess what will happen if you type fits_inf[2].header?
Now let’s change the header to give it the correct object:
+header["OBJECT"] = "M31"
+Finally, we have to write out the FITS file. Again, the convenience +function for this is the most useful command to remember:
+fits.writeto("output_file.fits", data, header, overwrite=True)
+That’s it; you’re done!
+Two common and more complicated cases are worth mentioning (but if your needs +are much more complex, you should consult the full documentation http://docs.astropy.org/en/stable/io/fits/).
+The first complication is that the FITS file you’re examining and +editing might have multiple HDU’s (extensions), in which case you can +specify the extension like this:
+data1, header1 = fits.getdata("input_file.fits", ext=1, header=True)
+This will get you the data and header associated with the index=1 extension
+in the FITS file. Without specifying a number, getdata() will get the
+0th extension (equivalent to saying ext=0).
Another useful tip is if you want to overwrite an existing FITS
+file. By default, writeto() won’t let you do this, so you need to
+explicitly give it permission using the clobber keyword argument:
fits.writeto("output_file.fits", data, header, overwrite=True)
+A final example is if you want to make a small change to a FITS file, like updating a header keyword, but you don’t want to read in and write out the whole file, which can take a while. Instead you can use the mode='update' read mode to do this:
with fits.open("input_file.fits", mode="update") as filehandle:
+ filehandle[0].header["MYHDRKW"] = "My Header Keyword"
+Exercise#
+Read in the file you just wrote and add three header keywords:
+-
+
‘RA’ for the Right Ascension of M31
+‘DEC’ for the Declination of M31
+‘RADECSRC’ with text indicating where you found the RA/Dec (web URL, textbook name, your photographic memory, etc.)
+
Then write the updated header back out to a new file:
+