Astronomik Filter S II 4 nm - M48 (2")

Astronomik Filter S-II 4 nm - M48 (2")

The Astronomik SII filter is ideal for imaging nebulae showing sulfur emissions from both light-polluted and dark observation sites. It significantly enhances the contrast between objects emitting light at 671.7 nm and 673 nm ([S II] lines) and the sky background. The Astronomik SII filter blocks all emission lines from high- and low-pressure sodium (Na) and mercury (Hg) vapour lamps, the maxima of typical LED lights, most lines of natural airglow and distracting moonlight. For particularly high-contrast results, the Astronomik SII filter is optimised to prevent H-alpha light from passing through.

The filter effectively blocks all unwanted light from wavelengths other than 671.7 nm and 673 nm from ultraviolet (UV) to infrared (IR).

Why 4 nm?
The choice between 12 nm, 6 nm and 4 nm Full-Width-Half-Maximum (FWHM) filters depends on your imaging conditions and camera type. There is no universal "best" option - each FWHM serves a specific purpose.

The new 4 nm line filters are the first choice for all astrophotographers who take pictures under extremely bright skies, for example from their balcony or terrace in the city. Like all Astronomik filters, the new 4 nm filters are optimized for use with dedicated CMOS astro cameras as well as modern DSLR and mirrorless system cameras.

Due to the low full width at half maximum, the number of stars in the image field is greatly reduced. Compared to images taken with a 12 nm filter, only a third of the stars are visible, and even extremely bright stars are only very small. The 4 nm filters are therefore also the first choice for overview images of the Milky Way, where the large number of stars would otherwise make it very difficult to see the nebulae.

The filters are also the right tool if you have a camera with very low dark current, allowing you to take very long exposures without significant background signal.

MFR coating specially developed by Astronomik
The new 4 nm filters are also produced with the proven MFR coating.

The Astronomik MFR coating ensures images free of any halos and razor-sharp stars. You get images with extremely high contrast and minimized stray light thanks to the best out-of-band blocking across the entire wavelength range from UV to IR.

In addition, the MFR coating allows Astronomik narrowband emission line filters to be used with almost all instruments. This means that customers do not have to purchase different filters for different focal ratios, as is the case with almost all competitors. Unlike filters from other manufacturers, which often require special "high-speed" versions, Astronomik´s MFR filters retain their transmission properties on both fast optics and optics with lower light intensity.

4 nm MFR-coated filters can be used in all instruments with focal ratios from f/4.5 to f/∞ without significant performance loss.

Size of filter
Astronomik filters come in a wide range of standard sizes, both mounted and unmounted, to suit various optical setups and cameras.

This filter is a mounted filter in a threaded 2" cell (M48x0.75 thread).

Technical Data

Guaranteed transmission of more than 90% at both [S II] lines (671.7 nm and 673.0 nm)

Typical transmission of 93% at both [S II] lines

Glass thickness: 1 mm

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

Thanks to the high-precision machining of the carrier substrate, all Astronomik filters are parfocal. The 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. Therefore, 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.

Filter transmission curve:

The horizontal axis is the wavelength in nanometers (nm). 500 nm is blue-green, at 600 nm the human eye senses orange and at 650 nm red.

The transmission in % is plotted on the vertical axis.

The red line shows the transmission of the filter.

The [S II] line of single ionized sulfur in nebulae is shown in orange yellow. The square brackets indicate that these lines are forbidden.

Safety informations: PDF Download