def wcs(self):
        """World coordinate system (`~astropy.wcs.WCS`)."""
        wcs = WCS(naxis=2)
        wcs.wcs.crval = self.config['crval']
        wcs.wcs.crpix = self.config['crpix']
        wcs.wcs.cdelt = self.config['cdelt']
        wcs.wcs.ctype = self.config['ctype']

        return wcs
World coordinate system (`~astropy.wcs.WCS`).