We absolutely must point out that an ERF from another dealer may void the warranty of the SolarSpectrum H-alpha Filter, so that repairs may be payable even during the warranty period.
You can buy cheaper standard ERFs, but they don't meet the same specifications as our D-ERF. Simple energy rejection filters are made of red dyed glass (Schott RG 610 or Schott RG 630). These are long pass filters which let all energy beyond ca. 570nm pass - this includes the full near infrared spectrum (heat radiation) above 630nm.
The heat at the focal point behind such an ERF filter is three times higher than behind one of our D-ERFs. This way, the immersion oil in the stack of block-, polarization and Etalon-filters will boil away. An H-Alpha-filter which is permanently overheated this way will age very fast. We can not cover this with our 5-year-warranty.
By the way: The coloured glass of these simple ERFs is rather "soft" and chemically unstable, so it can degrade over the years. The soft glass melt tends to form streaks. If they are not coated simple ERF-filters will age because of the oxygene in our atmosphere, and after some years the surface will get rough, and contrast is lost. When we were still using this glass, we had to check each and every filter on our optical bench, and many filters failed the double pass test.
The problem with the glass quality of an RG 610 remains even when it is coated with an UV/IR-reducing coating. We tried this, too, with our C-ERF (Cool ERF), but we abandoned it after some years. The soft glass often got warped during the coating procedure, so that even less filters were useful after being coated. Our experiences with this combination lead to the development of the D-ERF (Dielectrical ERF).
There are dealers and manufacturers who still offer such ERF or C-ERF filters but do not test them as thoroughly on an optical bench as we did. This is the reason why there are many cheaper C-ERFs and ERFs on the market - but the customer will pay for the cheaper costs a much higher price: a reduced image quality and a reduced lifespan of the H-Alpha filter, compared to a high-quality D-ERF.
Our D-ERF filters are made of very homogeneous, fine-polished BK-7-glass without streaks. They are coated with great effort with a dielectrical coating system which is age-resistant and fine-tuned for the Solar Spectrum filters.
A Solar Spectrum filter will outplay all its benefits (resistance to aging, sharpness and contrast) only in combination with a D-ERF, and only with this combination can we guarantee its resistance to aging. Especially a Research Grade Filter requires a good D-ERF. Every chain is only as strong as its weakest link.
Answer by: Baader Web Team (Admin) on Jan 23, 2017 10:39:00 AM