Solar Spectrum Research Grade RG18 - 0.30Å

We must point out that purchasing the ERF from another source can lead to a repair becoming chargeable in the event of a defect during the warranty and guarantee period. A normal ERF is cheaper to buy. But it does not have the same specifications as our DERF: Simple ERF filters are made of solid-colored red glass (Schott RG 610 or RG 630). This glass is a long-pass filter; it allows all energy to pass through from approx. 570 nm, including the full thermal radiation (NIR) above 630 nm. The heat at the focal point is three times higher with this type of "protective glass" than with our dielectrically blocked D-ERF filter. As a result, the immersion oil in the stack of block glasses, polarizers and etalon in the H-alpha filter literally is getting boiled. An H-alpha filter that is permanently overheated in this way ages very quickly. We would not be able to maintain our 5-year warranty under any circumstances. Incidentally, the colored glass from which these simple ERFs are made is very "soft" and chemically unstable (= prone to weathering). The soft molten glass tends to form streaks. When we were still working with this glass, we had to double-pass test each piece on the optical bench and had a high reject rate. This means that simpler ERF filters age due to the oxygen in the air if they are not coated, the surface becomes "rough" after a few years and the image contrast collapses. The problem with the glass quality of the RG 610 remains even if a UV/IR-reducing coating is vapor-deposited. We have already done this many years ago as C-ERF filter (= Cool ERF), but after long-term experience we abandoned it again and the lesson learned was the development of the D-ERF (Dielectric ERF). Above all, the soft glass often warped during coating, which led to even more rejects. There are manufacturers who offer such ERF or C-ERF filters but do not test every lens as thoroughly as we did on the optical bench. This explains the cheap offers of C-ERF and ERF filters on the market - for which the customer ultimately pays much more than if he had purchased a high-quality DERF, with significantly reduced image quality and reduced service life of the Halpha filter. Our D-ERF filters are made of highly homogeneous, streak-free, finely polished BK-7 glass. They are painstakingly coated with a dielectric coating system that is resistant to ageing and precisely matched to the Solar Spectrum filters. The Solar Spectrum filter only shows its most important advantages (ageing resistance, sharpness and contrast) with our dielectric „D“-ERF coating system were we have full control over the coating process and can verify it´s effectivity. Only then can we guarantee for the 5 years of ageing resistance for the Solar Spectrum H-alpha etalon-stack. Especially with a research grade filter, usage of a "normal" ERF is tantamount to "degrading" the filter. Every chain is only as strong as its weakest link.