A brief introduction to the function of narrowband filters.
Narrowband filters have revolutionized CCD photography and now more and more image acquisition with CMOS cameras for the “amateur astronomer” in the past two decades in incredible ways. It was now possible for small telescopes, even in light polluted city areas, to photograph faint nebula – and generally the universe surrounding us in the incredible variety of coloured “gaseous areas” – without restriction of exposure times, and by combining the exposure of each colour of the various emission lines – even the faintest nebula were suddenly registered despite strong light-polluted skies.
Suddenly, the smallest backyard telescope can collect the real light of gaseous nebula to produce image results there were otherwise only reserved for telescopes with several meters of light-collecting surface without any time limit.
The narrow emission area and spectral half width of these filters ensures that only the light emitted or reflected from the gaseous nebula literally “punches” through, even from light polluted city and bright sky background. The stars, which would otherwise outshine the light of the nebula completely, remain tiny. The final image now succeeds colour composites (consisting indeed only from the many hours of collected photons) and is much better, you can even conclude with an unfiltered image (by use of so-called L-filter) to add the real starry sky back into the image.
Also false colour and artefacts of the stars are greatly reduced by the Baader line-filters, even the brightest of stars do not generate extremely disturbing reflections and halos. However, this freedom from reflections is only possible with a very complex structure of many layers of coating, from which such a line filter must be to reflect back all unwanted light and pass only the very narrow, desired spectral range. Such a filter layer structure is extremely complex and there are very large differences in quality – which depends especially on quality and purity, and the rarity of the coating materials used in so-called “rare earth”.
Earlier filters were called “single-cavity filter” (many cheap filters still are). Such a filter had only a tiny region of maximum spectral transmittance and is characterised only for use with a particular aperture ration (such as F/10). Baader line-filters are all dual-cavitym, Ultra-Narrowbands even triple-cavity and have a wide plateau, making them usable for a much wider range of aperture ratios without transmission loss and with best signal-to-noise ratio in the transmission maximum.
Mechanical Properties
- Parfocal and plane polished substrates. Each individual filter is optically fine-polished to 1/4 wave.
- Baader line filters are hard coated individually.
- Baader filters especially are not cut out of large size plate-glass, which is a typical manufacturing process for economy filters (cut-out filters exhibit micro-cracks around all edges. Capillary action between glass and coating layers will lead to premature ageing due to moisture deposition – read more here).
- All Baader filters have individual coating sealed edges, impermeable against ageing because the penetration of moisture is impossible.
- Baader filters are being tested repeatedly to comply with MIL-specifications. One common process is to boil the test specimen for one hour in salt water. Baader filters remained completely intact as opposed to filters drilled out of large glass plates.
- All Baader filters sport scratch resistant hard coatings which can be cleaned repeatedly throughout their entire lifetime as many times as needed - preferably with Baader Optical Wonder cleaning fluid.
- CMOS-optimized (Ultra-) Narrowband / Highspeed Filters feature a Life-Coat™ Warranty
Optical Properties
- Very low halo and reflective angle - compared to competing line filters.
- Balanced sensitivity and excellent coordination in modern CMOS characteristic allows the H-alpha, O III and S II lines recordings with virtually the same exposure time - very important for automated continuous shooting.
- Maximum colour contrast for each of four colour channels - achieved through steep slopes at all transmission curves combined with science approved placement of spectral windows.
- Dual cavity design at CMOS Narrowband-filters, Triple-Cavity Design at Ultra-Narrowband Filters - for applicability over a wide focal length range of f / 10 to f / 3.5 system.
- The O-III and H-beta emission lines are clearly separated - no colour distortion - results in the highest deep-sky quantum yield.
- Both photographically and visually the stars do not appear by any of the Baader line filters as multi-coloured "dunghill", since each filter passes only the light to pass the respective emission line. In this context, a careful comparison of the various manufacturers published Transmission data sheets is recommended. In all Baader filter curves the entire spectral range is always shown with comparison to what a modern CMOS chip can represent, typically (from 300 to at least 1200 nm). It can be seen, whether the offered filter has undesirable weaknesses in the spectral blocking. Many manufacturers only show the spectral region around the filter and are "silent" about their filter so-called "side lobes" (= holes in light suppression), which decreases the contrast and the cleanliness in the reproduction of the star colours sensitive.
