{"product_id":"zwo-sii-7nm-narrowband-filter","title":"ZWO S-II 7nm Narrowband Filter","description":"\u003ch3\u003e\u003cstrong\u003eProduct Overview\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003eThe \u003cstrong\u003eZWO S-II 7nm Narrowband Filter\u003c\/strong\u003e passes a \u003cstrong\u003e7 nm\u003c\/strong\u003e band centred on the ionised sulphur line at \u003cstrong\u003e672 nm\u003c\/strong\u003e and rejects effectively everything else. Peak transmission is around \u003cstrong\u003e90%\u003c\/strong\u003e, off-band rejection is \u003cstrong\u003eOD3 (below 0.1%)\u003c\/strong\u003e, and infrared from \u003cstrong\u003e700–1100 nm is cut\u003c\/strong\u003e. Every size is polished to \u003cstrong\u003e1\/4 wavefront on both surfaces\u003c\/strong\u003e.\u003c\/p\u003e\n\u003cp\u003eAvailable in \u003cstrong\u003ethree sizes — 1.25\" mounted, 36 mm unmounted, and 2\" mounted\u003c\/strong\u003e. Each is a \u003cstrong\u003esingle filter\u003c\/strong\u003e, not a set. S-II is the third channel of the SHO trio and the faintest of the three, which matters when you plan integration time.\u003c\/p\u003e\n\n\u003ch3\u003e\u003cstrong\u003eChoosing the Right Size\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003eSize is decided by your sensor, not your telescope. \u003cstrong\u003eA filter too small vignettes the corners, and no amount of processing recovers it\u003c\/strong\u003e — the cell wall is a physical obstruction.\u003c\/p\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003e1.25\" mounted — for sensors up to about 4\/3\":\u003c\/strong\u003e ASI1600, ASI294, ASI533 and similar. Threaded cell, 1.9 mm glass. \u003cstrong\u003eWill vignette APS-C and full-frame\u003c\/strong\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003e36 mm unmounted — for APS-C and smaller:\u003c\/strong\u003e ASI2600MM, ASI294MM and similar. Bare glass, no cell or thread, so the full 36 mm is clear aperture. Fits the ZWO 36 mm EFW (7×36 mm). 2 mm glass. \u003cstrong\u003eWill not cover full-frame\u003c\/strong\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003e2\" mounted — for sensors up to full-frame:\u003c\/strong\u003e ASI6200MM, ASI2400 and similar. Threaded cell, fits 2\" wheels, drawers, and standard M48 threads\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003cp\u003eIf you are between sizes, size up.\u003c\/p\u003e\n\n\u003ch3\u003e\u003cstrong\u003eWho It's For\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003eThis is a good match if you are \u003cstrong\u003ealready imaging in H-alpha with a monochrome camera\u003c\/strong\u003e and want to complete the SHO set for Hubble-palette work. S-II sits close to H-alpha in the deep red, but nebulae emit far less strongly in it — so it is a filter you add once the workflow is established, not a starting point.\u003c\/p\u003e\n\n\u003ch3\u003e\u003cstrong\u003eKey Features \u0026amp; Design\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003e672 nm centre wavelength, 7 nm bandpass:\u003c\/strong\u003e targets the ionised sulphur emission line\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eFWHM 7 ± 0.5 nm:\u003c\/strong\u003e a tight, specified passband\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eApproximately 90% peak transmission\u003c\/strong\u003e at the S-II line\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eOD3 off-band blocking:\u003c\/strong\u003e under 0.1% transmission outside the passband\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eIR cut from 700–1100 nm:\u003c\/strong\u003e matters more here than on the other channels, since 672 nm sits near the infrared shoulder\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003e1\/4 wavefront both surfaces:\u003c\/strong\u003e fine-polished, so star shapes are not degraded\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eThree sizes:\u003c\/strong\u003e 1.25\" mounted (1.9 mm glass), 36 mm unmounted (2 mm), and 2\" mounted\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch3\u003e\u003cstrong\u003eRecommended Uses\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eSHO \/ Hubble-palette imaging\u003c\/strong\u003e alongside the H-alpha and O-III 7 nm filters\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eSupernova remnants\u003c\/strong\u003e — the Veil and similar, where S-II traces shocked gas\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003ePlanetary nebulae\u003c\/strong\u003e, where S-II separates ionisation zones that H-alpha alone blends together\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eEmission nebulae\u003c\/strong\u003e where you want the full three-channel palette rather than HOO bicolour\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eImaging under moonlight or heavy light pollution\u003c\/strong\u003e\n\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch3\u003e\u003cstrong\u003eCompatibility and Accessory Notes\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eMonochrome cameras only.\u003c\/strong\u003e On a one-shot-colour sensor the Bayer matrix wastes most of the signal\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003e36 mm is unmounted:\u003c\/strong\u003e no cell and no thread. It is retained by M2 screws in a filter wheel recess — those screws ship with the ZWO 36 mm EFW\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003e1.25\" and 2\" are mounted:\u003c\/strong\u003e threaded cells that fit filter wheels, drawers, and standard filter threads\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eThird-party wheels:\u003c\/strong\u003e ZWO specifies filter thickness between 1.5 mm and 3.0 mm\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eMatch thickness across the wheel\u003c\/strong\u003e so focus does not shift between channels\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eOrientation:\u003c\/strong\u003e ZWO advises the \u003cstrong\u003ecoated side should face the telescope\u003c\/strong\u003e on Mark II filters\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eBackfocus:\u003c\/strong\u003e glass shifts focus by roughly one third of its thickness — about 0.6–0.7 mm depending on size\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch3\u003e\u003cstrong\u003eImportant Limitations\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eS-II is the faintest of the three SHO channels.\u003c\/strong\u003e Expect substantially longer integration than H-alpha for comparable signal. That is astrophysics, not a filter fault\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eSize must match your sensor.\u003c\/strong\u003e 1.25\" vignettes APS-C and full-frame; 36 mm will not cover full-frame\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eUnmounted 36 mm glass must be handled by the edges\u003c\/strong\u003e — no cell protects the coating\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eNot for one-shot-colour cameras or DSLRs\u003c\/strong\u003e — monochrome imaging only\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eNot useful on its own.\u003c\/strong\u003e S-II without H-alpha and O-III is one channel of a three-channel palette\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eNot for visual observing\u003c\/strong\u003e — a 7 nm band at 672 nm is far too dark for the eye\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eA filter wheel and monochrome camera are required\u003c\/strong\u003e and sold separately\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch3\u003e\u003cstrong\u003eFrequently Asked Questions\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003e\u003cstrong\u003eWhy is my S-II data so much weaker than my H-alpha?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eBecause nebulae emit far less strongly at 672 nm than at 656 nm. That is normal and expected. Plan on longer total integration in S-II than in H-alpha — many imagers run roughly twice as long.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eWhich size do I need?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eMatch it to your sensor, not your telescope. Up to about 4\/3\" (ASI1600, ASI294, ASI533), the 1.25\". APS-C and smaller (ASI2600MM), the 36 mm unmounted. Up to full-frame (ASI6200MM), the 2\".\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eWhat is the difference between mounted and unmounted?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eA mounted filter sits in a threaded cell you can screw into things. An unmounted filter is bare glass held by screws in a filter wheel. The 36 mm is unmounted because a cell has walls, and walls block the corners of a big sensor.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eDo I need this if I already have H-alpha and O-III?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eOnly if you want the full Hubble palette. HOO bicolour from H-alpha and O-III is a perfectly respectable stopping point, and it costs a lot less integration time.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eWill it work with my colour camera or DSLR?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eNot effectively. Only the red-sensitive pixels of a Bayer matrix see 672 nm, so you discard most of your resolution and light.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eDo I need a UV\/IR-cut filter as well?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eNo. This filter cuts 700–1100 nm itself, which matters for S-II because 672 nm sits close to the infrared.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eWhich way round does it go?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eZWO advises that on the Mark II filters the coated side should face the telescope.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eCan I image with the Moon up?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eYes. A 7 nm passband rejects moonlight and light pollution well enough to make those nights productive.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eIs this good for beginners?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eIt is not a first filter. Start with H-alpha, learn the mono narrowband workflow, then add O-III and S-II.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eIs one filter included, or three?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eOne. Each size option is a single S-II filter.\u003c\/p\u003e\n\n\u003ch3\u003e\u003cstrong\u003eBottom Line\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003eIn short: a 7 nm S-II filter at 672 nm with roughly 90% peak transmission, OD3 blocking, and 1\/4 wavefront polish — the third SHO channel, and the one that needs the most integration time. Pick the size from your sensor: 4\/3\" takes 1.25\", APS-C takes 36 mm, full-frame takes 2\".\u003c\/p\u003e","brand":"ZWO Optical","offers":[{"title":"1.25\" mounted","offer_id":53804739952751,"sku":"SII7nm1.25","price":164.0,"currency_code":"CAD","in_stock":true},{"title":"36mm unmounted","offer_id":53804739985519,"sku":"SII7nmD36","price":214.0,"currency_code":"CAD","in_stock":true},{"title":"2\" mounted","offer_id":53804740018287,"sku":"SII7nmD2","price":316.0,"currency_code":"CAD","in_stock":true}],"url":"https:\/\/ontariotelescope.com\/products\/zwo-sii-7nm-narrowband-filter","provider":"Ontario Telescope and Accessories","version":"1.0","type":"link"}