def simple_nbyn_hdu(n):
    return fits.PrimaryHDU(data=empty_nbyn(n),
                           header=headers.generate_nxn_header(n))
def simple_3by3_hdu():
    return fits.PrimaryHDU(data=simple_3by3(),
                           header=headers.generate_nxn_header(3))
def centered_gaussian_hdu():
    return fits.PrimaryHDU(data=centered_gaussian(),
                           header=headers.generate_nxn_header(9))