Astrophysics is the branch of Astronomy (from the Greek "the laws of the stars") that studies the physics of the Universe. Nearly all our information for the study of distant cosmic objects in our Universe comes from the observation of electromagnetic waves and particles that reach the Earth. In the history of Astronomy, all the discoveries have been accompanied hand-in-hand by technological developments sometimes to improve our perception of the Universe and at other times to open new as yet unexplored windows. This Thesis intends to introduce the reader to one of the windows, opened in the recent past, since 1989, to the most violent phenomena of the Universe that take place at high energies and which has given rise to a new branch in Astronomy: High Energy Astronomy. The new window of observation has improved thanks to the development of new instrumentation, which needs calibration and monitoring tasks and which sometimes has to face up to new intrinsic difficulties. This work is based on the ground-based telescopes used to observe the Universe at high energies, in particular the MAGIC Telescope. Specifically, this Thesis covers the MAGIC telescope monitoring, its detection capabilities, the new challenges opened up by the effect of the Earth's magnetic field at low energies and the first results for the physics of some members of a particular kind of star, called pulsars. It describes in detail the efforts to monitor the behaviour of the telescope, which has to be stable enough to guarantee a good understanding of the results. Finally, this work presents the upper limits of two of the most promising pulsar gamma-ray candidates: PSR J0205+6449 and PSR J0218+4232.