{"product_id":"zwo-d21-ar-protective-window","title":"ZWO D21 AR Protective Window","description":"\u003ch3\u003e\u003cstrong\u003eProduct Overview\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003eThe \u003cstrong\u003eZWO D21 AR Protective Window\u003c\/strong\u003e is a \u003cstrong\u003e21 mm\u003c\/strong\u003e replacement window for ZWO ASI cameras. It is \u003cstrong\u003e1.1 mm\u003c\/strong\u003e thick, figured to \u003cstrong\u003e1\/4 λ flatness\u003c\/strong\u003e, and carries an \u003cstrong\u003eanti-reflective coating on both faces\u003c\/strong\u003e. It sits directly in front of the sensor and seals the sensor chamber against dust and humidity.\u003c\/p\u003e\n\u003cp\u003eAn AR window is optically clear. It does \u003cstrong\u003enot\u003c\/strong\u003e block ultraviolet or infrared — it only reduces surface reflections. That distinction matters, and it is covered below.\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 own a ZWO camera that shipped with a \u003cstrong\u003eD21 × 1.1 mm AR window\u003c\/strong\u003e and need to replace one that is scratched, fogged, or contaminated beyond cleaning.\u003c\/p\u003e\n\u003cp\u003eIt is also the right choice if you run your \u003cstrong\u003eUV\/IR-cut filtering in the imaging train\u003c\/strong\u003e — in a filter drawer, wheel, or threaded filter — and want the window at the sensor to stay optically neutral rather than cutting the band a second time.\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\u003e21 mm diameter, 1.1 mm thick:\u003c\/strong\u003e the D21 form factor used across ZWO's smaller-sensor bodies\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eAnti-reflective coating:\u003c\/strong\u003e reduces reflection losses and internal glare at the window surfaces, which is where halos around bright stars can originate\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eOptically clear:\u003c\/strong\u003e passes UV and IR straight through — all wavelength control happens elsewhere in your train\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003e1\/4 λ flatness:\u003c\/strong\u003e the window sits in a converging beam immediately ahead of the sensor, so surface figure directly affects star shapes\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eDust and humidity seal:\u003c\/strong\u003e the window is the sensor chamber's only physical barrier\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\u003eReplacing a scratched or contaminated AR window\u003c\/strong\u003e on a D21-bodied ASI camera\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eMonochrome imaging\u003c\/strong\u003e, where filtering is handled by a filter wheel and a sensor-side IR-cut would be redundant\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eNarrowband and photometric work\u003c\/strong\u003e, where you want no unnecessary coatings shaping the passband\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eInfrared and methane-band planetary imaging\u003c\/strong\u003e, which an IR-cut window would defeat outright\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\u003eMatch models:\u003c\/strong\u003e ASI120 series, ASI178 series, ASI290 series, ASI224MC series, ASI385 series, ASI185MC series, ASI220 series, ASI462 series, ASI662 series, ASI678 series, ASI664 series, and ASI715 series\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eDiameter is not interchangeable:\u003c\/strong\u003e D21 fits 21 mm bodies only. ASI174-series bodies take the D25; ASI1600 \/ ASI294 \/ ASI183 \/ ASI533 take the D32; ASI6200 \/ ASI2400 \/ ASI2600 take the D60\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eWant IR-cut instead?\u003c\/strong\u003e Fit the D21 UV IR-Cut window in the same body — same diameter and thickness, different coating\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eNo installation direction:\u003c\/strong\u003e ZWO confirms the window has no preferred orientation\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eReplacement procedure:\u003c\/strong\u003e see ZWO's official guide, \u003ca href=\"https:\/\/i.zwoastro.com\/wp-content\/uploads\/2025\/12\/c36da643469647ac8334179b49aa8b11.pdf\" target=\"_blank\" rel=\"noopener\"\u003eHow to Replace the Camera Protective Window\u003c\/a\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eMechanical part only\u003c\/strong\u003e — no electronic function, no camera communication\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\u003eThis window does not cut UV or IR.\u003c\/strong\u003e ZWO states that plainly. If you are imaging in colour through a refractor with no filter in the train, infrared will focus at a different point than visible light and soften your image. Use the D21 UV IR-Cut window instead, or add a filter\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eNot a threaded filter.\u003c\/strong\u003e It is a bare optical window that only works installed inside the camera body\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eReplacement means opening the sensor chamber.\u003c\/strong\u003e Dust introduced during the swap lands on or near the sensor. ZWO specifically advises against user replacement on some bodies — including the ASI462MC\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eFits 21 mm bodies only.\u003c\/strong\u003e Check your camera against the list above before ordering\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eSold as a single window.\u003c\/strong\u003e No tools, gaskets, or retaining ring included\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\u003eWhat is the difference between the AR window and the UV IR-cut window?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eThe AR window is anti-reflective coated but optically clear — it passes ultraviolet and infrared through to the sensor. The UV IR-cut window blocks both. ZWO confirms directly that the AR window does not cut UV.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eWhich cameras use the D21 AR window?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eThe ASI120, ASI178, ASI290, ASI224MC, ASI385, ASI185MC, ASI220, ASI462, ASI662, ASI678, ASI664, and ASI715 series.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eShould I choose AR or IR-cut?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eIf you already filter in the imaging train — a filter wheel, drawer, or threaded UV\/IR-cut — keep the AR window and let those do the work. If you shoot one-shot-colour with nothing in the train, the IR-cut window does the job at the sensor and frees your drawer for something else.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eDoes the window have to face a particular direction?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eNo. ZWO confirms there is no specific installation direction, so the anti-reflective coating works either way round.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eCan I fit this to my ASI462MC myself?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eIt is the correct window for that camera, but ZWO does not recommend replacing it yourself. Their guidance, not ours.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eMy protective window is scratched — can I just remove it?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eThe camera will focus and operate without it, but the sensor is then open to dust and humidity, which is a far worse problem than the scratch. Replace it.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eWill this fit my ASI1600 or ASI294?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eNo. Those bodies use the 32 mm D32 AR window. This is 21 mm.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eIs this good for beginners?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eThe part is simple; fitting it is not a beginner task, since it means opening the sensor chamber. If you are not confident working clean, have it done.\u003c\/p\u003e\n\n\u003ch3\u003e\u003cstrong\u003eBottom Line\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003eIn short: a 21 mm × 1.1 mm anti-reflective window at 1\/4 λ flatness — the correct spare for D21-bodied ASI cameras, and the right choice when your UV\/IR filtering already happens in the imaging train. It does not cut UV or IR; if you need that at the sensor, fit the D21 UV IR-Cut instead.\u003c\/p\u003e","brand":"ZWO Optical","offers":[{"title":"Default Title","offer_id":53804443926639,"sku":"D21AR","price":20.0,"currency_code":"CAD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0686\/0049\/6239\/files\/f8fc754efe2799ca917eee21d7350efd.jpg?v=1784083457","url":"https:\/\/ontariotelescope.com\/products\/zwo-d21-ar-protective-window","provider":"Ontario Telescope and Accessories","version":"1.0","type":"link"}