A small astronomical observatory above Hospedaje Central takes advantage of Tarma's high-elevation location, where the clear nights of June, July and August provide ideal opportunities for stargazing (though the surrounding mountains do limit the amount of observable heavens). Admission includes a talk (in Spanish) on constellations and a peek at some stars. The bad news: it only opens sporadically in the aforementioned months.
Try the Facebook page for details of openings, which are often during festival days or holidays.