13.3 Reading Fits Files

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13.3 Reading Fits Files#

Flexible Image Transport System (FITS) files are used to store astronomical images. As seen in the figure below, a FITS file is made up of two parts: an ASCII header and a binary image. The header usually contains important metadata that describes photometric and spatial calibration information.

Some of the more important header information includes:

  • NAXIS1, NAXIS2 : the number of pixels along the axes.

  • CTYPE1, CTYPE2: coordinates used for the axes (often celestial coordinates).

  • In order to have a unique (RA, DEC) coordinate for each pixel in the image, it needs to be astrometrically calibrated. The three most important astrometry keywords (for each axis) are:

    • CDELT - the size of a pixel, usually given in degrees. For the header shown on the previous slide, CDELT1 = -2.777e-4 degrees = -2.777e-4 x 3600 = -1 arc second.

    • CRPIX - the pixel number of a reference pixel whose RA/DEC is given by the corresponding CRVAL keyword.

    • CRVAL - the RA/Dec of the reference pixel (as specified by CRPIX).

WCS and fits#

The astropy.wcs.WCS sub-package is used to create an object to represent the world coordinate system (WCS) transformation of an image.

from astropy.wcs import WCS

The astropy.io.fits sub-package is used to read-in FITS images.

from astropy.io import fits
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