phew, it worked: 
```python
In [3]: tmp = read_fits.read_fits_spectrum1d('sample_13CO.fits')
In [4]: tmp.dispersion
Out[4]: 
<Quantity [  40000.10101113,  40212.63686928,  40425.17272744,
             40637.70858559,  40850.24444375,  41062.7803019 ,
             41275.31616006,  41487.85201821,  41700.38787636,...., 179636.15981851, 179848.69567666] m / s>
```