Add a column in fits file

import pyfits

data = pyfits.open('old_file')[1].data
head = pyfits.open('old_file')[1].header
cols = []
cols.append(
    pyfits.Column(name='new_column_name', format='D', array= new_data_array)
    )
orig_cols = data.columns
new_cols = pyfits.ColDefs(cols)
hdu = pyfits.BinTableHDU.from_columns(orig_cols + new_cols)
hdu.writeto('new_table.fits')  # just rewrite the file.




Also using astropy: