Well, the idea of inheriting from ```NDData``` was so that we could pass around the objects from one tool to the next, so I would hope that the ```CCDData``` object would be usable by this class.   As @mwcraig mentions, there shouldn't be anything in ```CCDData``` that should prevent it from being used vs. any other ```NDData``` object.  So I might change the above in the API to something like 'if an object is inherited from an NDData, it can be passed to the function. ' rather than just an ```NDData``` object.