BAADER D-ERF ENERGY REJECTION FILTER

The Baader D-ERF Energy Rejection Filters are dielectrically coated to reject the majority of light at both ends of the spectrum, with a narrow transmission window of only 80nm half-bandwidth around the H-alpha line. This enables safe usage of H-alpha solar filters with telescopes larger than 80mm in aperture by preventing as much extra energy as possible from entering the telescope.

Note: an additional filter cell will need to be custom 3D-printed for your exact telescope. Contact us for more information.

Dielectrically-coated energy rejection filters block almost all extraneous light, ensuring that the etalon in your h-alpha filter does not degrade over time and that the central wavelength passed by the etalon stays on-band. By being placed at the front of the optical path, this provides the additional benefits of reducing turbulence inside the telescope and protect against off-axis rays hitting the walls of the tube, which could damage the telescope.

Baader's 43-layer dielectric DWDM-coating delivers a cool beam of pure red light. The glass substrate is clear BK7 of the highest homogeneity, fine-optically polished to the quality grade of objective-lens surfaces, and with all the reflection performance coatings on one side. This reflects all the unwanted spectrum from 280 nm up to 1500 nm, while leaving open just that 80nm-wide spectral window around H-alpha.

It is notoriously difficult to design such a coating, as well as the counterside anti-reflection multicoating, in such a way that no coating-induced stress builds up inside the glass substrate. If these coatings on either side of the plate are not done exactly right to cancel out the stresses induced by the coating layers, the plane-parallel surfaces may easily be distorted. This means the 1/10 wave flat surface deforms into an irregular curved substrate that acts as a (bad) lens.

Baader D-ERF filter-layers are designed with the utmost care, applied with the world‘s most advanced optical coating machines, and the final filters are all inspected and passed in autocollimation on their Carl Zeiss optical bench.