Astronomik SII filter
The Astronomik SII filter is ideal for imaging nebulae showing sulfur emissions from both light-polluted and dark observation sites.
With a narrow bandwidth of either 12nm, 6nm or 4nm and high transmission rates, the filter effectively blocks all unwanted light from wavelengths other than 671.7nm and 673nm from ultraviolet (UV) to infrared (IR).
Difference between Astronomik SII 4nm, 6nm and 12nm filters
The choice between 4nm, 6nm or 12nm FWHM filters depends on your imaging conditions and camera type. There is no universal "best" option - each FWHM serves a specific purpose.
The 4nm Astronomik Narrowband-Emissionline filters are the best choice for astrophotography under extremely light-polluted skies. Astrophotographers who take pictures in a big city from their home garden or balcony can use the 4nm filters to create great, detailed, and high-contrast images of faint objects even under really bad skies. The 4nm Narrowband-Emissionline filters suppress stars even more than the 6nm filters. Compared to the 12nm filters, the number of stars is only about one third.
The 6nm Astronomik Narrowband-Emissionline filters are ideal for imaging faint objects in star-crowded regions of the milky way, as they typically halve the number of stars in the image compared to the 12nm filter. They are optimal if you are observing from a location with strong light pollution.
The 4nm and 6nm Astronomik Narrowband-Emissionline filters are recommended if you are using a CMOS astro camera or a modern system camera with very low dark current.
The 12nm Astronomik Narrowband-Emissionline filters are better suited for use with typical DSLR cameras with CMOS sensors and CCD cameras with normal or high dark current or if you are observing from suburban or dark-sky locations. They are also the preferred choice for cameras with an integrated or dual guiding sensor, as they show more guiding stars compared to the 4nm and 6nm filter.
In areas with little light pollution, the background signal in images is usually limited by the sensor's dark current rather than light pollution. Therefore, further suppression of the sky background by reduction of the FWHM does not bring out more details in the object.
Difference between Astronomik MFR and MaxFR coating
Both the Astronomik MFR and MaxFR coatings offer extremely high contrast, minimized stray light, no halos and needle-sharp stars. In addition, they provide the best possible blocking across the entire spectral range from UV to IR.
The MFR filters can be used without any significant performance loss on the following instruments:
- 12nm MFR-coated filters: Instruments with aperture ratios from f/3 to f/15
- 6nm MFR-coated filters: Instruments with aperture ratios from f/4 to f/15
- 4nm MFR-coated filters: Instruments with aperture ratios from f/4.5 to f/15
The Astronomik MaxFR coating is specially designed for use with very fast optical systems.
Filters with MaxFR coating can be used without restriction on the following instruments:
- 12nm filters with MaxFR coating: Instruments with aperture ratios from f/1.7 to f/6 and approximately 15% reduced transmission at f/1.4
- 6nm filters with MaxFR coating: Instruments with aperture ratios from f/2.2 to f/6 and approximately 10% reduced transmission at f/2
- 4nm filters with MaxFR coating are currently in development.
- Filters for even faster optics are available on request.
Technical data
- Guaranteed transmission of more than 90% (12nm, 6nm and 4nm) at both SII lines (671.7nm and 673.0nm)
- Typical transmission of 96% (12nm) and 93% (6nm and 4nm) at both SII lines
- Glass thickness: 1mm (except Astronomik OWB Type 3 and XT filters)
- Finely polished optical glass, free of any striae, residual stresses, and bubbles
- Extremely scratch-resistant and durable coating
- Completely resistant against high humidity and aging effects
- Astronomik MFR coating (Astronomik Deep-Sky RGB, UV-IR Block L-1, L-2, L-3, and Narrowband-Emissionline filters)
- Parfocal with other Astronomik filters
- Diffraction-limited
- Optimized spectral windows
- Delivered in a high-quality, long-lasting filter box
Due to the high-precision machining of the carrier substrate, all Astronomik filters are parfocal with wedge errors far below all common tolerances. Astronomik filters are diffraction-limited, ensuring that the optical performance (resolution and contrast) of your telescope is not compromised. Issues concerning focal distances and the exact alignment of images from each color channel are therefore effectively eliminated.
All Astronomik filters are ideal for modern cameras, including both CMOS- and CCD-based models, ensuring you capture the best possible images with your equipment. Sharpness, contrast, and the absence of any reflections and halos are guaranteed. Even with bright stars in the field of view, nebulae and galaxies will be captured with the finest structures.
Every Astronomik filter undergoes a rigorous quality control process before delivery. Additionally, each Astronomik Narrowband-Emissionline filter is measured using a high-end PerkinElmer Spectrophotometer to ensure it meets the specifications published on our website. On the back of each Narrowband-Emissionline filter box, you will find a label detailing the precise spectral data for that specific filter, including the maximum transmission and the recommended range of focal ratios.
