The Baader Energy Rejection Filters (D-ERF) allows H-alpha solar observation with the full-aperture resolving power of the telescope. They also dramatically reduce the thermal energy that goes through the telescope to the Solar Spectrum filter and also greatly reduces the thermally-induced seeing effects as well.
The Baader D-ERF filter-layers are designed with the utmost care. The coatings are applied with the world‘s most advanced optical coating machines, with the resultant final filters all being inspected and passed in an autocollimator on a Carl Zeiss optical bench in Germany.
To create the D-ERF filters, Baader use a 43-layer dielectric DWDM-coating which delivers a "cool"-beam of 45nm wide pure red light around the H-alpha wavelength, towards a solar H-alpha filter system. The filter glass substrate is clear BK7 glass that is of the highest homogeneity. It is fine-optically polished to the same degree of objective-lens surfaces, and the filter has all the reflection performance coatings on one side. This reflects all the unwanted wavelengths from 280 nm up to 1500 nm, but only leaving that 45nm-wide spectral window around the H-alpha wavelength. Furthermore, these Baader Planetarium D-ERF filters are coated in a way to keep the heat load on the glass as low as possible, but still ensures that it has an extremely hard, scratch-resistant coating.