{"product_id":"zwo-d21-uv-ir-cut-protective-window","title":"ZWO D21 UV IR-Cut Protective Window","description":"\u003ch3\u003e\u003cstrong\u003eProduct Overview\u003c\/strong\u003e\u003c\/h3\u003e\n\u003cp\u003eThe \u003cstrong\u003eZWO D21 UV IR-Cut 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 sits directly in front of the sensor, where it does two jobs at once: it seals the sensor chamber against dust and humidity, and it blocks ultraviolet and infrared light outside the visible band.\u003c\/p\u003e\n\u003cp\u003eThis is a consumable spare, not an upgrade. If the window on your camera is scratched, fogged, or contaminated beyond cleaning, this is the part that replaces it.\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 UV IR-cut window\u003c\/strong\u003e — the ASI120MC-S or ASI178MC — and need to replace a damaged one.\u003c\/p\u003e\n\u003cp\u003eIt is also the correct part if you own a camera fitted with a \u003cstrong\u003eD21 AR window\u003c\/strong\u003e and want to convert it to IR-cut, so that you no longer need a separate UV\/IR-cut filter in the imaging train. Confirm your camera's window diameter before ordering: a 21 mm window will not fit a body machined for 25 mm or 32 mm.\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\u003eUV IR-cut coating:\u003c\/strong\u003e passes visible light and rejects ultraviolet and infrared, unlike an AR window, which passes both\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003e1\/4 λ flatness:\u003c\/strong\u003e the window sits in a converging beam directly ahead of the sensor, so surface figure matters — a poorly figured window degrades 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\u003cli\u003e\u003cstrong\u003eOptical glass with coatings on both faces\u003c\/strong\u003e\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 window\u003c\/strong\u003e on an ASI120MC-S or ASI178MC\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eConverting a D21 AR-equipped camera to IR-cut\u003c\/strong\u003e, removing the need for a separate UV\/IR-cut filter\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003ePlanetary and lunar imaging with refractors\u003c\/strong\u003e, where uncorrected infrared focuses at a different point than visible light and softens the image\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eDaytime or terrestrial use\u003c\/strong\u003e of an ASI camera, where infrared leakage shifts colour badly\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\u003eCameras shipped with this window:\u003c\/strong\u003e ASI120MC-S and ASI178MC\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 take the D32; ASI6200 \/ ASI2400 \/ ASI2600 take the D60\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eAR vs IR-cut:\u003c\/strong\u003e an AR window is anti-reflective only and does not cut UV or IR. If your camera came with a D21 AR window and you want IR-cut behaviour at the sensor, this is the part that changes it\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eNo installation direction:\u003c\/strong\u003e ZWO confirms the window has no preferred orientation — it can be fitted either face forward\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eReplacement procedure:\u003c\/strong\u003e ZWO publishes an 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 is a bare optical window, not a filter you can thread into an imaging train.\u003c\/strong\u003e It 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. Work in still, clean air. ZWO does not recommend that users replace windows on cameras where the window is factory-fitted at close tolerance\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eNot a substitute for a light-pollution or narrowband filter\u003c\/strong\u003e — it cuts UV and IR only\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eFits 21 mm bodies only.\u003c\/strong\u003e Check your camera model 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\u003eWhich cameras use the D21 UV IR-cut window?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eZWO ships this window on the ASI120MC-S and ASI178MC. It also physically fits other D21-bodied cameras that came with an AR window, if you want to convert them to IR-cut.\u003c\/p\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 straight through to the sensor. The UV IR-cut window blocks both. ZWO states plainly that the AR window does not cut UV.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eDo I still need a UV\/IR-cut filter in my imaging train?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eNot for that purpose, no. With this window installed, the IR-cut function happens at the sensor. That frees the filter drawer or wheel 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.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eMy protective window is scratched. Do I need to replace it?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eA scratched window scatters light and can produce halos around bright stars. The camera will still focus and operate without it, but the sensor is then exposed to dust and humidity — which is a far worse problem. 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 window. This is 21 mm.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eIs there a D32 UV IR-cut version?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eNo. ZWO produces the D32 in AR only. If you need IR-cut on an ASI1600 or ASI294, use a filter in the imaging train instead.\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eIs this good for beginners?\u003c\/strong\u003e\u003c\/p\u003e\n\u003cp\u003eThe part is simple, but fitting it is not a beginner task — it means opening the sensor chamber. If you are not confident doing that cleanly, have it done rather than risk dust on the sensor.\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 UV IR-cut window at 1\/4 λ flatness — the correct replacement for an ASI120MC-S or ASI178MC, and the part that converts a D21 AR-equipped camera to IR-cut at the sensor. Confirm your diameter first; D21 does not fit D25, D32, or D60 bodies.\u003c\/p\u003e","brand":"ZWO Optical","offers":[{"title":"Default Title","offer_id":53804401000559,"sku":"D21IR-CUT","price":20.0,"currency_code":"CAD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0686\/0049\/6239\/files\/426474105bc9afcb1261044235a281b3-1.jpg?v=1784083160","url":"https:\/\/ontariotelescope.com\/products\/zwo-d21-uv-ir-cut-protective-window","provider":"Ontario Telescope and Accessories","version":"1.0","type":"link"}