Astromania 2" 90-degree 99% Enhanced Dielectric Diagonal Mirror uses multi-layered dielectric coatings in order to guarantee a maximum reflection of 99% with the highest contrast and optimal sharpness.The residual stress in the plane mirror which can occur with the dielectric vapour deposition process is ruled out here. This slip fit 2-inch diagonal with 1.25-inch adapter, plus the Schmidt Cassegrain thread to 2-inch accessory adapter and other Schmidt Cassegrain rear thread screw-on systems, is compatible with both slip-fit and screw-on mounting systems. The optics are made with Pyrex glass and comes with dielectric coatings for the most light transmission possible. Beside eyepieces, a star diagonal is one of the most important accessories. What is the use of a good objective or the best eyepiece if the star diagonal is inferior? We recommend the Astromania 2" 90-degree 99% Enhanced Dielectric Diagonal Mirror to everyone, including those working with high end instruments. The star diagonal permits full illumination of 2” wide field eyepieces.