What Right Ascension and Declinations are observable by a telescope from a particular location? I'm new to Observational Astronomy, and I just started observations with a telescope and read about RA and Declination(celestial co-ordinates). Although I think I understood the concept, I'm confused about what RA/Dec is visible from the location at present(Summer Sky).

From what I have understood, since I'm in Sendai, Japan(~38.26N, 140.86E), the +38 degree declination is at my zenith at midnight. So almost -30 Declination will be at my horizion. That should mean that I can see stars from -30 Declination to 90 Declination at my location. Is that correct?

What about RA though, what range of RAs will it be possible to view while its dark(2000 hours to 0300 hours)
The declination limit, looking south, is about -52 degrees.  This is basically your latitude minus 90.

At 8pm, due south will be about RA 12hrs, and at 3 am it will be about 19hrs.  The eastern horizon will be about 6 hrs ahead, and that increases as you look further north, until you hit the circumpolar region, when all RAs are visible.