> My thought was that outside the individual wavelength ranges, the SED is just undefined, as opposed to zero.

OK.  That's a fair behavior, and I think it does match what the `__call__` function currently does.  The LookupTable would raise an exception if the wavelength was outside the allowed range, so that does make sense.

My only request then is that this behavior be documented in either the `__call__` docstring or the main docstring for SED (or both would probably be even better).  

If you want to be more fancy, it might even be worth adding an explicit check in the `__call__` function to make sure the wavelength is within the allowed range and raise an exception explaining what happened.  The exception from the LookupTable (which actually comes up from the C++ layer I believe) might not be the easiest thing for the user to parse if they accidentally use a bandpass outside of the range where the SED is defined.  Saying explicitly that the SED isn't defined a whatever wavelength might be more helpful.