使用 astropy 打印 FITS 文件的标题：NameError: name 'header' is not defined

Question

I am trying to learn to use FITS operations through astropy via http://docs.astropy.org/en/stable/io/fits/

I am following the directions on this website. They are:

"To see the entire header as it appears in the FITS file (with the END card and padding stripped), simply enter the header object by itself, or print(repr(header)) "

But when I type header , I get the following error :

NameError: name 'header' is not defined

I get the same error when I put print(header) or print(repr(header)) commands.

My question is why is it that the "header" command did not work?

Am I supposed to define it somehow?

My code:

from astropy.io import fits
hdulist = fits.open('test1.fits')
hdulist.info()
header

I am using jupyter notebook via Canopy.

Answer 1

My question is why is it that the "header" command did not work?

Am I supposed to define it somehow?

In short: It's not a command and you don't need to define it. It's actually an attribute, so you have to look it up on the hdulist .

The hdulist contains hdu s and each hdu contains a data and a header , so to access the header of the first hdu you use:

print(repr(hdulist[0].header))

The [0] is because I wanted the first HDU (python uses zero-based indexing) and the .header accesses the header attribute of this HDU.

Even though I said you don't need to define it, but you can define a variable called header:

header = hdulist[0].header   # define a variable named "header" storing the header of the first HDU
print(repr(header))  # now it's defined and you can print it.

How many HDUs are present should be shown by the hdulist.info() , so you can decide which one you want to print or store.

Note that you should always use open as a context manager so it closes the file automatically (even in case of Errors):

from astropy.io import fits

with fits.open('test1.fits') as hdulist:  # this is like the "hdulist = fits.open('test1.fits')"
     hdulist.info()
     for hdu in hdulist:
         print(repr(hdu.header))
# After leaving the "with" the file closes.

This example also shows how you can use a for loop to go over all HDUs in a HDUList .