Counts image (`~astropy.io.fits.ImageHDU`).

def image(self):
        """Counts image (`~astropy.io.fits.ImageHDU`)."""
        events = self.event_table
        skycoord = SkyCoord(events['GLON'], events['GLAT'], unit='deg', frame='galactic')
        pixcoord = PixCoord.from_sky(skycoord=skycoord, wcs=self.wcs)

        shape = self.config['shape']
        bins = [np.arange(shape[0] + 1), np.arange(shape[1] + 1)]
        sample = np.vstack((pixcoord.y, pixcoord.x)).T
        data, _ = np.histogramdd(sample=sample, bins=bins)
        data = data.astype('float32')

        header = self.wcs.to_header()
        return fits.ImageHDU(data=data, header=header, name='image')