The principal applications of off-axis optical systems are to avoid obstruction of the primary aperture by secondary optical elements, instrument packages, or sensors, and to provide ready access to instrument packages or sensors at the focus. The engineering tradeoff of an off-axis optical system is an increase in image aberrations.
Source: From Federal Standard 1037C
FRACKING FLUIDS POISON A NATIONAL FOREST -- NEW STUDY DETAILS CHANGES IN SOIL CHEMISTRY AND DEVASTATION OF TREES AND PLANTS.
Jul 06, 2011; WASHINGTON, DC -- The following information was released by Public Employees for Environmental Responsibility (PEER): A new study...