I think what is there now is OK. It will work for users that have `scipy` and will fail with an `ImportError` in `AiryDisk2D.__init__` for users that don't have `scipy`.
But @astrofrog, if you prefer putting `1.2196698912665045` instead of `jn_zeros(1, 1)[0] / np.pi` or moving the computation to the `AiryDisk2D` `__init__` or `eval`, that's fine with me, I think it doesn't matter much how this is implemented.

I made the initial comment to take the computation of this constant out of `eval` for better efficiency, but now I realise that making it a module-level constant actually means that many users that import `astropy.modeling.models` but are not using `AiryDisk2D` will execute this line, so it might actually be less efficient for most users (so sorry for making this somewhat pointless suggestion in the first place).