The search for cosmic particle accelerators capable of reaching the PeV (10^15 eV) range, PeVatrons, is a key science objective for the very-high-energy (VHE) community (gammaray domain around TeV = 10^12 eV). As the PeV particles are accelerated, gamma rays in the ∼100 TeV range should be produced. Thus, investigations into this energy domain are necessary in the search for PeVatrons. For the purpose of identifying PeVatrons with the MAGIC telescopes a novel observational method of performing observations at very large zenith angles (≳ 70°) is introduced in the first part of the thesis. The goals and technical characteristics are explained, together with the benefits and drawbacks of the method. The performance of this observational method has been performed on the "standard candle" of the VHE gamma-ray astronomy, the Crab Nebula. After the Very Large Zenith Angle (VLZA) method was validated it was applied on two extended sources of VHE gamma-rays, MGRO J2019+37 and HESS J1809-193 which covers the second part of the thesis. Since the sources are extended a spatial likelihood analysis was performed on these sources. In the last part of the thesis a technical contribution to the (Large Size Telescope) LST-1 is presented called Camera Displacement Monitor (CDM). It is a system for monitoring the displacement of the LST-1 camera which I was in charge of implementing and testing. The software implementation is explained together with the tests performed in the laboratory and on site. From the technical observations the CDM performance was evaluated and the results show that the requirements are fullfilled and the CDM is capable of measuring the oscillations of LST-1 photomultiplier tube camera.