Download star index files.
    The default range was useful for my cameras.

def download(odir: Path, source_url: str, irng: Sequence[int]):
    """Download star index files.
    The default range was useful for my cameras.
    """
    assert len(irng) == 2, 'specify start, stop indices'

    odir = Path(odir).expanduser()
    odir.mkdir(parents=True, exist_ok=True)

    ri = int(source_url.split('/')[-2][:2])

    for i in range(*irng):
        fn = f'index-{ri:2d}{i:02d}.fits'
        url = f'{source_url}{fn}'
        ofn = odir / fn
        if ofn.is_file():  # no clobber
            print('skipping', ofn)
            continue
        print(f'{url} => {ofn}', end='\r')

        urlretrieve(url, ofn)