{"product_id":"zwo-oiii-7nm-narrowband-filter","title":"ZWO O-III 7nm Narrowband Filter","description":"\u003ch3\u003e\u003cstrong\u003eProduct Overview\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003eThe \u003cstrong\u003eZWO O-III 7nm Narrowband Filter\u003c\/strong\u003e passes a \u003cstrong\u003e7 nm\u003c\/strong\u003e band centred on the doubly-ionised oxygen line at \u003cstrong\u003e500 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. O-III sits in the blue-green where most CMOS sensors peak, making it the most efficient of the three SHO channels.\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 image \u003cstrong\u003eplanetary nebulae and supernova remnants with a monochrome camera\u003c\/strong\u003e. O-III is where those targets show structure that H-alpha simply does not record — the Veil's filaments and the Helix's inner shell are O-III features.\u003c\/p\u003e\n\u003cp\u003eIt is also the second filter most imagers add after H-alpha, and the partner for HOO bicolour work if you are not ready for the full SHO set.\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\u003e500 nm centre wavelength, 7 nm bandpass:\u003c\/strong\u003e targets the major doubly-ionised oxygen 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 O-III line\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eOD3 off-band blocking:\u003c\/strong\u003e rejects mercury and sodium vapour lighting and atmospheric skyglow\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eSits near peak sensor QE:\u003c\/strong\u003e back-illuminated CMOS sensors peak in the blue-green, so O-III photons are captured efficiently\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\u003ePlanetary nebulae\u003c\/strong\u003e — the Ring, Dumbbell, and Helix, where O-III carries the inner structure\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eSupernova remnants\u003c\/strong\u003e — the Veil, whose filamentary detail is largely an O-III signal\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eHOO bicolour imaging\u003c\/strong\u003e, mapping H-alpha to red and O-III to green and blue\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eSHO \/ Hubble-palette imaging\u003c\/strong\u003e alongside the H-alpha and S-II 7 nm filters\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\u003eFast optics shift narrowband passbands.\u003c\/strong\u003e Below roughly f\/4 the steep cone angle blue-shifts the effective centre wavelength of any narrowband filter — O-III is the channel where that shows first\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\u003c\/ul\u003e\n\n\u003ch3\u003e\u003cstrong\u003eImportant Limitations\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eNot for visual observing.\u003c\/strong\u003e A 7 nm imaging filter is far darker than the visual O-III filters sold for eyepiece use — do not confuse the two\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\u003eHalos around bright stars are the classic O-III complaint.\u003c\/strong\u003e ZWO's Mark II filters improved this, but any narrowband filter can halo on a very bright star in the field. Framing helps more than processing\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\u003eNarrowband raises contrast, not brightness.\u003c\/strong\u003e Expect long sub-exposures\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\u003eIs this the same as a visual O-III filter?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eNo, and this is a common and expensive mix-up. Visual O-III filters for eyepiece use run much wider passbands so there is enough light to see. A 7 nm imaging filter is far too dark for the eye. Buy the visual version if you observe.\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\u003eWhich do I buy after H-alpha?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eO-III, usually. It gives you HOO bicolour immediately, and it records structure in planetary nebulae and supernova remnants that H-alpha misses entirely.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eWhy do I get halos around bright stars?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eIt is the classic narrowband O-III artefact. ZWO's Mark II filters reduced it, but a very bright star in the field can still halo. Reframing to move the star out is more effective than trying to fix it later.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eWill this work at f\/3?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eWith a caveat. Below about f\/4 the steep light cone shifts a narrowband filter's effective centre wavelength blueward, and O-III is where you notice it first — transmission at the true line drops. It still works; it just works less well than at f\/5 or slower.\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 some pixels of a Bayer matrix see 500 nm, so you discard most of your resolution and light.\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 well enough to make those nights productive.\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 O-III filter.\u003c\/p\u003e\n\n\u003ch3\u003e\u003cstrong\u003eBottom Line\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003eIn short: a 7 nm O-III filter at 500 nm with roughly 90% peak transmission, OD3 blocking, and 1\/4 wavefront polish — the second narrowband filter most mono imagers buy, and the channel that carries planetary nebulae and supernova remnants. Not a visual O-III filter. 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":53804748701807,"sku":"OIII7nm1.25","price":164.0,"currency_code":"CAD","in_stock":true},{"title":"36mm unmounted","offer_id":53804748734575,"sku":"OIII7nmD36","price":214.0,"currency_code":"CAD","in_stock":true},{"title":"2\" mounted","offer_id":53804748767343,"sku":"OIII7nmD2","price":316.0,"currency_code":"CAD","in_stock":true}],"url":"https:\/\/ontariotelescope.com\/products\/zwo-oiii-7nm-narrowband-filter","provider":"Ontario Telescope and Accessories","version":"1.0","type":"link"}