The Baader Planetarium energy rejection filters include new all-dielectric IR rejection coatings to eliminate IR energy heat to produce cold focus. This ensures proper cooling between the telescope and etalon prologing filter life and avoiding tube heat currents. Cassegrain users will find excessive use of these filters because it reduces heat effect on the mirrors.
Energy rejection ultra-hard dielectric coatings are on both sides of the glass substrate in addition to the high grade optical glass used, making it excellent quality and highly resistant to scratching. The thick substrates are polished plane-parallel, smooth, and flat to an amazing 1/10 wave p-v over the entire surface. Only by using thick substrates can a high surface accuracy be achieved and held in use (for example, the 90mm ERF is 5.5mm thick. Thinner substrates are not capable of holding such high surface accuracy). The special dielectric filter stacks are precisely balanced on opposite sides to maintain the superb optical flatness after coating.