Optimization And Verification Of The Optical System Of Wide Field Of View Cherenkov Telescope

Large high altitude air shower observatory(LHAASO)is a large comprehensive cosmic ray observation station under construction in China,which is also the most sensitive ground cosmic ray detection station in the world.Wide FOV Cherenkov Telescope Array(WFCTA)is one of the important components in LHAASO whcih consists of 18 Cherenkov telescopes.The WFCTA aims to measure the Cherenkov light and fluorescence caused by the cosmic rays,whose energy is respectively 1014eV-1017 eV and 1015eV-1018 eV via a joint detection combined with Square Kilometer Detector Array and Water Cherenkov Detector Array.The optical system of WFCTA is assembled by 25 regular hexagon spherical mirrors in which the length of each mirror is 297.1mm and the spherical radius is 5800 m.The optical system of Cerenkov telescope has a great influence on the image qualtiy,efficiency inconsistency,and angle resolution and so on.This paper focuses on analyzing various kinds of errors of the optical mechanical system,including deformation error,alignment error and thermal error,by simulations and experiments.It is found that the collection efficiency of the photomultiplier tube(PMT)with diameter of 25.4 mm and quality of imaging on the best focal plane has be much reduced by the deformation error when we simulate optimizing each error by using the parallel light beam.It is also shown that collection efficiency of photons in the best focal plane is 63.5% and the inconsistency of efficiency is 16% after considering all kinds of errors.The above optimization can be achieved by the multiple inspections of the sub mirror,the rubber block,the adjusting mechanism and moving the best focal plane.The WFCTA is mainly used to detect Cherenkov photons and optimization of the optical system has a very important significance on Cherenkov photon collection,event selection,and energy reconstruction.After we analyze the effect of all kinds of errors on the performance of the telescope,we find that the Cherenkov image distribution is not much affected by the errors.The efficiency of component distinguishing decreases by 3.34% while the energy resolution increases by 0.12%.In conclusion,based on the above parameters obtained,the telescope optical system satisfies the experimental requirements of the cosmic ray observatory at 4400 m a.s.l.and this work plays a guiding role in the installation and production of telescope.