Baader Planetarium D-ERF Energy Rejection Filter, 135mm with IR-Cut Dielectric Coating
Baader planetarium has introduced a new evolution in their premium energy rejection filters, from 70mm up to 180mm! These filters incorporate a new all-dielectric IR rejection coating that eliminates the heating from IR energy to produce a truly cold focus (hence, "Cool ERF"). As a result, the telescope and etalon remain cool and safe from excessive heating. The result is longer filter life, and freedom from tube currents caused by heating. Users with compound telescopes (SCTs, Cassegrains, Maks) will really appreciate the D-ERF, due to its reduced heat load on the mirrors and the reduction in tube currents caused by internal heating.
Unlike every other ERF produced up until now, the Baader D-ERF are produced from a high quality optical clear glass, ensuring freedom from the internal impurities that caused so much headache in producing the older red-glass ERFs. The red glass used in producing the older generation of ERFs is of very poor and variable internal quality. In order to produce ERFs from red glass, Baader had to complete and polish each ERF, then interferometrically inspect every one for internal defects, resulting in having to dispose of far more ERFs than they were able to keep. Its no wonder why other red glass ERFs have such a poor reputation, particularly in sizes over 3" in aperture.
The energy rejection is now accomplished using ultra-hard dielectric coatings on both sides of the glass substrate, and also serve to protect the glass, making it highly resistant to scratches. D-ERF filters are made from high grade optical glass which is free of any internal defects. 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.