{"product_id":"explore-scientific-2-inch-sulphur-ii-12nm-nebula-filter","title":"Explore Scientific 2-inch Sulphur II 12nm Narrowband Nebula Filter","description":"\u003ch2\u003eProduct Overview\u003c\/h2\u003e\u003cp\u003eThe Explore Scientific 2-inch S-II Nebula Filter is a 12 nm narrowband filter tuned to the ionised sulphur emission line at 672 nm, mounted in a 2-inch cell. Explore Scientific explains the principle behind it directly: emission nebulae glow in specific colours tied to specific chemical elements, and this filter blocks all the other colours, and with them almost all artificial light, letting only the sulphur emission through. Each filter ships with an individual test report for that particular piece of glass.\u003c\/p\u003e\u003ch2\u003eWho It's For\u003c\/h2\u003e\u003cp\u003eThe reason to buy this rather than the 1.25-inch version is aperture, and the reason to buy 12 nm rather than 6.5 nm is what a large aperture does to a narrowband filter. On a big sensor the light reaching one corner of the frame arrives at the filter with a different angle and a different range of angles than the light landing at the centre. Because an interference coating shifts its passband toward the blue as the angle of incidence increases, that means the corners of a full-frame chip are effectively looking through a slightly different filter than the middle. With a 12 nm window there is enough room either side of 672 nm to absorb that variation, and your corners and your centre record the sulphur line at comparable efficiency. Tighten the window and the difference starts to appear in the data as a radial falloff that is not a vignette and does not calibrate out cleanly.\u003c\/p\u003e\u003cp\u003eSo this is the sulphur filter for someone running a full-frame or large APS-C monochrome camera in a 2-inch filter wheel or drawer, or for an observer with a large-aperture visual instrument who wants to try sulphur contrast on bright planetary nebulae through a 2-inch diagonal. It also suits fast, wide astrographs where both the focal ratio and the field size are working against a narrow band at once. Given that sulphur is already the faintest of the commonly mapped lines, the extra photons a 12 nm window collects are worth more here than the additional background suppression a 6.5 nm filter would provide, unless your sky is exceptionally bright.\u003c\/p\u003e\u003ch2\u003eKey Features \u0026amp; Design\u003c\/h2\u003e\u003cul\u003e\n\u003cli\u003e2-inch mounted cell, sized for full-frame sensors, 2-inch filter wheels and 2-inch diagonals\u003c\/li\u003e\n\u003cli\u003e12 nm passband centred on the ionised sulphur emission line at 672 nm\u003c\/li\u003e\n\u003cli\u003eRejects the remainder of the visible spectrum, including the artificial light that dominates an urban sky\u003c\/li\u003e\n\u003cli\u003eBandwidth chosen to stay consistent from field centre to corner across a large sensor\u003c\/li\u003e\n\u003cli\u003eIndividual test report for the filter you receive, included in the scope of delivery\u003c\/li\u003e\n\u003cli\u003eExplore Scientific names planetary nebulae, emission nebulae and supernova remnants as its targets\u003c\/li\u003e\n\u003cli\u003eUsable both photographically and, on large apertures, for visual contrast work on the brightest planetaries\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch2\u003eFilter Design\u003c\/h2\u003e\u003cp\u003eThe 672 nm passband is produced by a multilayer dielectric coating, and its behaviour depends on geometry as much as on chemistry. Each layer is a fraction of a wavelength thick, and the interference condition that defines the transmission peak depends on the optical path a ray takes through the stack. Rays arriving perpendicular to the surface see the design condition. Rays arriving at an angle satisfy the same condition at a shorter wavelength, so the peak migrates blueward. In a 2-inch filter placed in a converging beam ahead of a large sensor, the angle of incidence varies systematically across the aperture of the filter, which is why the 12 nm width is the practical choice at this size: it is broad enough that the whole illuminated area of the coating still delivers the sulphur line near peak transmission. A wider window does admit slightly more sky continuum, but with a line as faint as S-II, uniform transmission across the field is usually the more valuable property.\u003c\/p\u003e\u003ch2\u003eRecommended Uses\u003c\/h2\u003e\u003cul\u003e\n\u003cli\u003eSulphur data for mapped-colour narrowband imaging on full-frame and large APS-C monochrome cameras\u003c\/li\u003e\n\u003cli\u003eWide-field emission nebula work with fast astrographs and short focal lengths\u003c\/li\u003e\n\u003cli\u003eSupernova remnants where the sulphur structure diverges from the hydrogen structure\u003c\/li\u003e\n\u003cli\u003eImaging from suburban and urban sites where broadband work is not practical\u003c\/li\u003e\n\u003cli\u003eVisual contrast work on bright planetary nebulae through a 2-inch diagonal with generous aperture\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch2\u003eCompatibility and Accessory Notes\u003c\/h2\u003e\u003cp\u003eThe cell is threaded to the standard 2-inch filter thread, so it screws into the barrel of a 2-inch eyepiece, into a 2-inch nosepiece or star diagonal, and into most 2-inch filter wheels. It does not fit wheels that take unmounted 36 mm or 50.8 mm discs, and its cell thickness is worth checking against tight filter drawers before you commit. If you already own the 1.25-inch Explore Scientific narrowband filters, note that a 2-inch cell will not step down without an adapter, and that the mounted glass adds a small amount of optical path that shifts focus rearward, which matters when you are setting the spacing between a field flattener and your sensor. If you are unsure whether your wheel, drawer or diagonal will take a mounted 2-inch filter, contact our team in Bolton with the model and we will confirm it.\u003c\/p\u003e\u003ch2\u003eGood to Know Before You Order\u003c\/h2\u003e\u003cp\u003eExplore Scientific publishes no shipping weight for this item, so we are confirming it on our own scales before the listing goes live. That is why it is currently unpublished. Contact us and we will give you a shipping figure for your address. Also worth setting expectations on: sulphur is the weakest of the three lines most narrowband imagers work with, so this channel will always need more integration time than hydrogen-alpha to reach the same depth, whatever the bandpass. Explore Scientific does not publish peak transmission, substrate material or a full out-of-band blocking range for this filter, and we will not invent those numbers; the individual test report supplied with the filter carries the measured curve for your copy. The delivery is the filter and that report, with no diagonal, wheel, adapter or camera included.\u003c\/p\u003e\u003ch2\u003eFrequently Asked Questions\u003c\/h2\u003e\u003cp\u003e\u003cstrong\u003eDo I actually need a 2-inch filter?\u003c\/strong\u003e If your sensor is larger than about Four Thirds, or your filter wheel and drawer are built around 2-inch threads, yes. A 1.25-inch filter in front of an APS-C or full-frame chip will vignette the corners noticeably. If you are running a small mono camera, the 1.25-inch version does the same optical job for less.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eIs this usable visually?\u003c\/strong\u003e On a large aperture and a bright planetary nebula it can show real structure, and Explore Scientific does mention urban observers alongside astrophotographers for the S-II line. But 672 nm is deep in the red where the dark-adapted eye is weakest, so treat visual use as a bonus rather than the purpose.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhy not go to 6.5 nm for more contrast?\u003c\/strong\u003e Because on a big sensor, a fast beam, and the faintest of the three lines, you would be paying twice: once in signal and once in field uniformity. The 6.5 nm version rewards slower optics, smaller fields and long integration. This one is the more forgiving instrument.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWill it work with a colour camera?\u003c\/strong\u003e Only inefficiently. Bayer-array cameras dedicate roughly a quarter of their photosites to red, and the colour filters themselves attenuate what reaches them. Narrowband work of this kind belongs to monochrome sensors.\u003c\/p\u003e\u003ch2\u003eBottom Line\u003c\/h2\u003e\u003cp\u003eThis is the sulphur filter for a large sensor and a fast telescope, where uniform transmission across a wide field matters more than the last decibel of background rejection. It fits standard 2-inch threads, ships with its own measured test report, and behaves predictably from centre to corner. Contact us if you would like help deciding between this and the 6.5 nm version for your particular optics.\u003c\/p\u003e","brand":"Explore Scientific","offers":[{"title":"Default Title","offer_id":53958705905775,"sku":"310110","price":249.95,"currency_code":"CAD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0686\/0049\/6239\/files\/310110_2.jpg?v=1785960889","url":"https:\/\/ontariotelescope.com\/products\/explore-scientific-2-inch-sulphur-ii-12nm-nebula-filter","provider":"Ontario Telescope and Accessories","version":"1.0","type":"link"}