Plano-parallel round discs, both surfaces have been plano-parallel precision-optically polished to lambda/10. An infrared antireflection coating (dielectric interference coating system with multiple individual layers) prevents heat penetrating into the telescope OTA. The plano-parallel D-ERF filter blocks IR (heat radiation) even over longer observing sessions.
Thermally induced air currents in the OTA (and thus image degradation) are thereby prevented. But, most importantly, the D-ERF protects the expensive H-alpha filter that is close to the focal point from premature aging due to heat stress! In addition, the set wavelength, which would otherwise constantly drift due to changes in temperature, remains stable. D-ERF filters are multi-layer coated! We use the best known coating system, with 0.2% residual reflection, for preventing reflections from surfaces that would otherwise reflect back and forth several times inside the system and reduce the image contrast of the Sun's surface and of any prominences.
To be able to produce a precisely flat surface and achieve this flatness in spite of the forces generated due to the deposition of the various coating layers requires substantial technical expertise. From a large amount of raw glass, one must select only the best stress-free tempered pieces in order only to hope for a high-quality end product! All too easily, the glass deforms when inappropriate manufacturing methods are used or when it is coated using older deposition machines. The final product is almost useless unless the greatest care is taken in every step of production.
Our filters are processed in modern ion beam coating machines in order to keep heat loading of the glass as low as possible and to ensure an extremely hard, scratch-resistant coating.
NOTE: D-ERF-energy rejection filters are PRE-FILTERS and are ONLY SUITABLE for use with SolarSpectrum H-alpha surface filters (and prominence set-ups). This filter must ALWAYS BE MOUNTED IN FRONT OF where the light enters. Orientation towards the Sun (marked on the edge of the filter) must be complied with. White light observing of the Sun's surface WITHOUT AN ADDITIONAL light attenuation filter will irreparably damage your eyes!
Please note: These are unmounted filters!