The only difference seems to be that observation discusses a singular object, and observational-astronomy discusses multiple.
Looks to me that these tags could be merged since basically observational-astronomy encompasses observation and questions could be altered to define if the question is about a singular object or multiple.
This is a simple enough request, so I've gone ahead and performed the merge; I can't think of a reason not to. In the process, I also created a synonym mapping hst to hubble-telescope. If we see something like hubble-space-telescope arise, I can merge that, if need be. But for now it looks like we're all good.