Accessing data stored as a table in a multi-extension FITS (MEF) file

FITS files can often contain large amount of multi-dimensional data and tables. This example opens a FITS file with information from Chandra’s HETG-S instrument.

The example uses astropy.utils.data to download multi-extension FITS (MEF) file, astropy.io.fits to investigate the header, and astropy.table.Table to explore the data.

By: Lia Corrales, Adrian Price-Whelan, and Kelle Cruz

License: BSD

Use astropy.utils.data subpackage to download the FITS file used in this example. Also import Table from the astropy.table subpackage and astropy.io.fits

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

Download a FITS file

event_filename = get_pkg_data_filename('tutorials/FITS-tables/chandra_events.fits')

Display information about the contents of the FITS file.

fits.info(event_filename)

Filename: /home/user/.astropy/cache/download/url/333246bccb141ea3b4e86c49e45bf8d6/contents
No.    Name      Ver    Type      Cards   Dimensions   Format
  0  PRIMARY       1 PrimaryHDU      30   ()
  1  EVENTS        1 BinTableHDU    890   483964R x 19C   [1D, 1I, 1I, 1J, 1I, 1I, 1I, 1I, 1E, 1E, 1E, 1E, 1J, 1J, 1E, 1J, 1I, 1I, 32X]
  2  GTI           3 BinTableHDU     28   1R x 2C   [1D, 1D]
  3  GTI           2 BinTableHDU     28   1R x 2C   [1D, 1D]
  4  GTI           1 BinTableHDU     28   1R x 2C   [1D, 1D]
  5  GTI           0 BinTableHDU     28   1R x 2C   [1D, 1D]
  6  GTI           6 BinTableHDU     28   1R x 2C   [1D, 1D]

Extension 1, EVENTS, is a Table that contains information about each X-ray photon that hit Chandra’s HETG-S detector.

Use Table to read the table

events = Table.read(event_filename, hdu=1)

Print the column names of the Events Table.

print(events.columns)

<TableColumns names=('time','ccd_id','node_id','expno','chipx','chipy','tdetx','tdety','detx','dety','x','y','pha','pha_ro','energy','pi','fltgrade','grade','status')>

If a column contains unit information, it will have an associated astropy.units object.

print(events['energy'].unit)

eV

Print the data stored in the Energy column.

print(events['energy'])

  energy
    eV
---------
13874.715
2621.1938
12119.018
3253.0364
14214.382
1952.7239
3267.5334
3817.0366
2252.7295
6154.1094
      ...
4819.8286
12536.866
2599.5652
15535.768
6653.0815
14362.482
14653.954
 6652.827
 9672.882
1875.9359
Length = 483964 rows