For the most part, I don't think you need to bring up a disability at all. In general, it is none of their business if it doesn't impact your ability to deliver excellence in the job. However, let's say your disability will affect your ability to take on certain tasks of the position. For example, say you have to be able to lift 25 pounds in order to operate a telescope or some other instrument, and your disability precludes you from doing that. In this instance, you can indicate the issue, and how you would work around it so as not to impact your ultimate productivity.