Baaders D-ERF filters are exceptional solar energy rejection filters that are made in Germany
Made from plane parallel circular disks, both surfaces are precision optically polished to 1/10 Lambda. Typically, common absorbative glass type ERF filters are only 1/4~1/5 Lambda which can have a detrimental effect on the optical quality of the system, not so the Baader D-ERF which does not compromise the optics of your system.
An infrared reflection compensation (dielectric interference layer system with many individual layers), prevents the penetration of heat into the telescope tube. The blocking of IR (thermal radiation) holds the D-ERF filters coplanar even after prolonged observation period. Thermally induced air turbulence in the tube is prevented. Above all, the D-ERF filter protects the expensive H-alpha etalon filter against focus shift and against premature aging caused by heat stress. In addition, the centre wavelength, which otherwise with the change of temperature drifts constantly, is stabilized.
D-ERF filters are non-reflective and multilayer. They use the best possible layer system with 0.2% residual reflection to prevent reflections on the plane surfaces, that and would reduce the contrast of the solar image on the surface and the protuberances. To such a create exact plane surface and to obtain the planarity despite the applied forces through the different layers of coating, a high technical effort is necessary. From a large amount of raw glass only the best pieces that have tension-free annealed quality are chosen. If the greatest of care is not taken at every stage of production, the glass deforms with improper preparation of the coating on older evaporation systems. The final product is therefore almost useless. Baader D-ERF filters are coated on modern ion beam-supported systems in order to keep the thermal load on the glass as low as possible and still provide an extremely hard, scratch-resistant coating.
