ZWO D21 UV IR-Cut Protective Window

Product Overview

The ZWO D21 UV IR-Cut Protective Window is a 21 mm replacement window for ZWO ASI cameras. It is 1.1 mm thick, figured to 1/4 λ flatness, 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.

This 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.

Who It's For

This is a good match if you own a ZWO camera that shipped with a D21 × 1.1 mm UV IR-cut window — the ASI120MC-S or ASI178MC — and need to replace a damaged one.

It is also the correct part if you own a camera fitted with a D21 AR window 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.

Key Features & Design

• 21 mm diameter, 1.1 mm thick: the D21 form factor used across ZWO's smaller-sensor bodies
• UV IR-cut coating: passes visible light and rejects ultraviolet and infrared, unlike an AR window, which passes both
• 1/4 λ flatness: the window sits in a converging beam directly ahead of the sensor, so surface figure matters — a poorly figured window degrades star shapes
• Dust and humidity seal: the window is the sensor chamber's only physical barrier
• Optical glass with coatings on both faces

Recommended Uses

• Replacing a scratched or contaminated window on an ASI120MC-S or ASI178MC
• Converting a D21 AR-equipped camera to IR-cut, removing the need for a separate UV/IR-cut filter
• Planetary and lunar imaging with refractors, where uncorrected infrared focuses at a different point than visible light and softens the image
• Daytime or terrestrial use of an ASI camera, where infrared leakage shifts colour badly

Compatibility and Accessory Notes

• Cameras shipped with this window: ASI120MC-S and ASI178MC
• Diameter is not interchangeable: D21 fits 21 mm bodies only. ASI174-series bodies take the D25; ASI1600 / ASI294 / ASI183 take the D32; ASI6200 / ASI2400 / ASI2600 take the D60
• AR vs IR-cut: 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
• No installation direction: ZWO confirms the window has no preferred orientation — it can be fitted either face forward
• Replacement procedure: ZWO publishes an official guide, How to Replace the Camera Protective Window
• Mechanical part only — no electronic function, no camera communication

Important Limitations

• This is a bare optical window, not a filter you can thread into an imaging train. It only works installed inside the camera body
• Replacement means opening the sensor chamber. 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
• Not a substitute for a light-pollution or narrowband filter — it cuts UV and IR only
• Fits 21 mm bodies only. Check your camera model against the list above before ordering
• Sold as a single window. No tools, gaskets, or retaining ring included

Frequently Asked Questions

Which cameras use the D21 UV IR-cut window?

ZWO 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.

What is the difference between the AR window and the UV IR-cut window?

The 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.

Do I still need a UV/IR-cut filter in my imaging train?

Not 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.

Does the window have to face a particular direction?

No. ZWO confirms there is no specific installation direction.

My protective window is scratched. Do I need to replace it?

A 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.

Will this fit my ASI1600 or ASI294?

No. Those bodies use the 32 mm D32 window. This is 21 mm.

Is there a D32 UV IR-cut version?

No. 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.

Is this good for beginners?

The 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.

Bottom Line

In 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.