Animate a finder chart.

def animate(self, filename='constellation.mp4', epochs=[1900,2100], dt=5, dpi=300, fps=10, **kw):
        '''
        Animate a finder chart.
        '''

        scatter = self.finder(**kw)
        plt.tight_layout()
        figure = plt.gcf()

        if '.gif' in filename:
            try:
                writer = ani.writers['pillow'](fps=fps)
            except (RuntimeError, KeyError):
                writer = ani.writers['imagemagick'](fps=fps)
            except:
                raise RuntimeError('This python seems unable to make an animated gif.')
        else:
            try:
                writer = ani.writers['ffmpeg'](fps=fps)
            except (RuntimeError,KeyError):
                raise RuntimeError('This computer seems unable to ffmpeg.')


        with writer.saving(figure, filename, dpi or figure.get_dpi()):
            for epoch in tqdm(np.arange(epochs[0], epochs[1]+dt, dt)):

                # update the illustration to a new time
                coords = self.atEpoch(epoch)
                scatter.set_offsets(list(zip(coords.ra.value, coords.dec.value)))
                plt.title('{} in {:.1f}'.format(self.name, epoch))

                writer.grab_frame()