  The absolute reflectance of a reflector and its point spread function are the
key parameters of a telescope for measuring light flux. Typically, one is using
low-cost technologies for producing mirrors for the needs of astro-particle
physics experiments. As a rule, these are operating telescopes in open air
conditions at desert or mountainous locations, for cost reasons without
protecting domes. The mirrors on such telescopes are exposed to sand in strong
winds, precipitation and large temperature variations. Due to weathering, their
reflectance is declining within few years. In this report we describe in a
great detail the application of an in-situ method to the MAGIC imaging air
Cherenkov telescopes for regularly monitoring their absolute reflectance and
the point spread function. Compared to similar work that was previously
performed, in this report we focus on important details of light losses due to
scattering. These allowed us to further refine the method and significantly
improve its precision. Also, we report on an in-situ comparison of two mirror
types produced with different technologies.
