ZWO D21 AR Protective Window

Product Overview

The ZWO D21 AR 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 carries an anti-reflective coating on both faces. It sits directly in front of the sensor and seals the sensor chamber against dust and humidity.

An AR window is optically clear. It does not block ultraviolet or infrared — it only reduces surface reflections. That distinction matters, and it is covered below.

Who It's For

This is a good match if you own a ZWO camera that shipped with a D21 × 1.1 mm AR window and need to replace one that is scratched, fogged, or contaminated beyond cleaning.

It is also the right choice if you run your UV/IR-cut filtering in the imaging train — 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.

Key Features & Design

• 21 mm diameter, 1.1 mm thick: the D21 form factor used across ZWO's smaller-sensor bodies
• Anti-reflective coating: reduces reflection losses and internal glare at the window surfaces, which is where halos around bright stars can originate
• Optically clear: passes UV and IR straight through — all wavelength control happens elsewhere in your train
• 1/4 λ flatness: the window sits in a converging beam immediately ahead of the sensor, so surface figure directly affects star shapes
• Dust and humidity seal: the window is the sensor chamber's only physical barrier

Recommended Uses

• Replacing a scratched or contaminated AR window on a D21-bodied ASI camera
• Monochrome imaging, where filtering is handled by a filter wheel and a sensor-side IR-cut would be redundant
• Narrowband and photometric work, where you want no unnecessary coatings shaping the passband
• Infrared and methane-band planetary imaging, which an IR-cut window would defeat outright

Compatibility and Accessory Notes

• Match models: 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
• Diameter is not interchangeable: 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
• Want IR-cut instead? Fit the D21 UV IR-Cut window in the same body — same diameter and thickness, different coating
• No installation direction: ZWO confirms the window has no preferred orientation
• Replacement procedure: see ZWO's official guide, How to Replace the Camera Protective Window
• Mechanical part only — no electronic function, no camera communication

Important Limitations

• This window does not cut UV or IR. 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
• Not a threaded filter. It is a bare optical window that only works installed inside the camera body
• Replacement means opening the sensor chamber. Dust introduced during the swap lands on or near the sensor. ZWO specifically advises against user replacement on some bodies — including the ASI462MC
• Fits 21 mm bodies only. Check your camera against the list above before ordering
• Sold as a single window. No tools, gaskets, or retaining ring included

Frequently Asked Questions

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 through to the sensor. The UV IR-cut window blocks both. ZWO confirms directly that the AR window does not cut UV.

Which cameras use the D21 AR window?

The ASI120, ASI178, ASI290, ASI224MC, ASI385, ASI185MC, ASI220, ASI462, ASI662, ASI678, ASI664, and ASI715 series.

Should I choose AR or IR-cut?

If 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.

Does the window have to face a particular direction?

No. ZWO confirms there is no specific installation direction, so the anti-reflective coating works either way round.

Can I fit this to my ASI462MC myself?

It is the correct window for that camera, but ZWO does not recommend replacing it yourself. Their guidance, not ours.

My protective window is scratched — can I just remove it?

The 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.

Will this fit my ASI1600 or ASI294?

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

Is this good for beginners?

The 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.

Bottom Line

In 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.