These proprietary multi-coated doublet compensators were specially designed for astronomical binoviewers by Carl Zeiss and AstroPhysics to actually compensate for the chromatic aberration, or color fringing, that exists in all binocular viewers, as well as the spherical aberration caused by binoviewers (from the long lightpath through the glass in the prisms).  In addition to correcting these color and spherical errors, the Baader Glasspath Compensators also reduce the amount of telescope back-focus (in-travel) needed.  This is especially critical in reflectors and many refractors.  For help in determining the appropriate compensator, please click on the More Info link.