This is an Astronomik 6nm S-II CCD MFR Narrowband Emission-line Filter
Mounted in a 36mm Low Profile Non-Threaded Round Cell.
The Astronomik SII Narrowband Filters are suitable for false color imaging of emission nebulae emanating singly ionized sulfur. The contrast between an object glowing at 672nm and the background is increased significantly due to the high transmission and effective out-of-band blocking!
The SII line (672nm) is in the deep red portion of the spectrum, beyond H-alpha.
This Astronomik SII CCD MFR Narrowband filter has a 6nm FWHM. The extra narrow bandpass of 6nm filters are most appreciated with cameras that offer extremely low dark current and good cooling, or when imaging in areas with severe light pollution. The higher level of out-of-band blocking allows you to take longer exposures and choose objects embedded in normally over-bright, crowded areas of the Milky Way. Extra blocking causes faint stars to disappear from the background, and brighter stars will be pin-pricks of light.
Now Mounted in Low Profile Non-Threaded Cell!
Sometimes unmounted filters can be hard to handle. The smaller diameters, such as this 36mm SII filter, can be especially difficult to pick up and insert or remove from the filter wheel, and that can lead to possible damage as well as unwanted reflections from incorrectly installed filters. Astronomik has solved this problem by mounting their 31mm and 36mm filters in a low profile (3mm) non-threaded cell. O-rings, washers, or spacers are no longer required to hold the filter in place, and because the cell is only 3mm thick, it works well in all appropriately sized filter wheels. Mounted 31mm and 36mm Astronomik filters will also play nice with unmounted versions in the same carousel.
Astronomik CCD MFR Narrowband Emission Line Filter Highlights
Special Coatings Allow Filter Use Across Most Focal Ratios: Astronomik Narrowband Emission-line filters have been designed to be used with a wide range of focal ratios. The special MFR (multi-focal ratio) coatings applied to the 12nm FWHM filters make them compatible with optics as fast as f/3, and the 6nm models perform well with focal ratios as fast as f/4. This means you won't need to buy different filters for your fast refractor and your SCT, and you won't have to pay premium prices for high-speed filters.
Amazingly High Out of Band Blocking: Astronomik Narrowband Emission-line Filters offer superb blocking of unwanted wavelengths, from UV up to IR. Transmission of 90-percent is guaranteed by Astronomik, but the typical transmission can be as high as 96-percent on both 6nm and 12nm Narrowband filters. This feature translates to extremely high contrast and pleasingly dark background skies, even under heavily light-polluted conditions, allowing you to take longer exposures if you so desire without creating an overly bright (or saturated) background. Another benefit of good out-of-band blocking is the reduction of stray light and halos. Stars display as sharp points of light on a velvety black background, providing the perfect backdrop for the deep-sky object you are imaging.
Image During Full Moon or Under Light Polluted Skies: Because Astronomik Narrowband Emission Line Filters have a high transmission rate for the chosen wavelength but blocks pretty much everything else, you can image from your own backyard, even if your neighborhood suffers from light-pollution (which is pretty much everybody). If it's a beautiful night with great seeing but the Moon phase doesn't support visual deep-sky astronomy, you can pull out your telescope, camera, and Astronomik filters and still get great results.
Ten Year Warranty! Astronomik filters will last for many years without degradation of the coatings, due to the quality of the substrate and coatings used. The high-quality glass is optically polished and is free of stress and striae. The coatings used on Astronomik filters are applied in multiple layers, making the resulting filter scratch resistant and unaffected by moisture or aging that some filters of less quality may be prone to. Astronomik filters are also parfocal due to the small thickness tolerance allowed. Astronomik backs up this quality with a ten-year warranty.