The D-ERF filters are plane-parallel circular disks, both surfaces are precision optically polished, an infrared-reflective coating (layered system with many individual layers) prevents the penetration of heat into the telescope tube. The blocking of IR (heat radiation) holds the D-ERF filter plane-parallel even with longer observation times.
Only the best quality raw glass is used in the production of these filters, this ensures that the final product is able to cope with the stresses of solar use, whilst ensuring fantastic images and crystal clear observing. Above all, the D-ERF filter protects the expensive H-alpha filter, whilst in focus, against premature aging caused by heat stress.
Baader Planetarium filters are coated using ion-beam systems to keep the heat load on the glass as low as possible and still ensure an extremely hard, scratch-resistant coating.
Never use any eyepiece filter for solar observation without objective front filter.