Comments on the Use of Masking
All observers must choose a spatial region over which the light from a source will be integrated. Traditionally this region has been shaped like a circle and the radius is a compromise between wanting to include as much signal as possible and wanting to include as little background as possible. The technique discussed here is simply an extension of this idea to an annular shaped region. The choice of the inner radius is again a compromise between desirable signal (the unpiled events) and undesirable background (the piled-up events).