@cdeil  Sure, I can define it as a module-level constant, but it requires `scipy`.  `AiryDisk2D` also later tries to import `scipy`, raising `ImportError` if it's not available, so is this OK when the module is input?:

```
try:
    from scipy.special import jn_zeros
    AIRYDISK2D_RZ = jn_zeros(1, 1)[0] / np.pi
except:
    pass
```