... These Facts and Questions pages mainly tell you how to use the Aberrator-interface and also explain the nature of the aberrations. ... Airy disc: the brightest spot of light when you look at a star in focus. This disc can contain a maximum of 84% of all the light that arrives from a star (perfect refractor telescope). MCT : Maksutov-Cassegrain telescope. A closed-tube mirror-telescope with a secondary obstruction. Less common than the SCT, this telescope uses a corrector (real glass). ...