Cheap hotels in Madrid near to the Madrid Planetarium are great if you are in to astronomy. The planetarium is equipped with fantastic projectors which give slide shows of the solar system, exploding stars, galaxies and the like. But perhaps the most impressive thing of all is the optical projector, because it allows the night sky to be projected on the inside of the planetarium. There are also a series of exhibition halls, some with permanent and some with temporary exhibitions, as well as a telescope of course and a gift shop.
The planetarium is positioned 800 meters above sea level, is open for limited times between April and June and admission is free. The position is not ideal, as it is so close to the artificial light such as city lights, street lights etc of Madrid, but it is still possible to see quite a bit. So if you want to look at the stars, then just book your discount hotel near to the Madrid Planetarium in Madrid.