ZWO D25 AR Protective Window

Product Overview

The ZWO D25 AR Protective Window is a 25 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. It sits directly in front of the sensor and seals the sensor chamber against dust and humidity.

An AR window is optically clear. It reduces surface reflections but does not block ultraviolet or infrared — that distinction drives most of the buying decision here, and it is covered below.

Who It's For

This is a good match if you own an ASI174-series camera or an ASI485MC — both ship with the D25 AR window — and need to replace one that is scratched, fogged, or contaminated beyond cleaning.

It is also the right choice if you filter in the imaging train rather than at the sensor, or if you do infrared work that an IR-cut window would defeat outright.

Key Features & Design

• 25 mm diameter, 1.1 mm thick: the D25 form factor used on ASI174-series and ASI485MC bodies
• Anti-reflective coating: reduces reflection losses and internal glare at the window surfaces, a common origin of halos around bright stars
• Optically clear: passes UV and IR straight through — all wavelength control happens elsewhere
• 1/4 λ flatness: the window sits in a converging beam directly 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 an ASI174-series camera or ASI485MC
• Infrared and methane-band planetary imaging, which requires infrared to reach the sensor
• Monochrome imaging where a filter wheel already handles wavelength selection
• Narrowband and photometric work, where you want no unnecessary coating shaping the passband
• High-frame-rate solar and lunar work with the ASI174, where the sensor's speed is the point and filtering is external

Compatibility and Accessory Notes

• Match models: the ASI174 series. The ASI485MC also ships with this window
• Want IR-cut instead? Fit the D25 UV IR-Cut window — same diameter and thickness, different coating. ZWO names it specifically as the ASI485MC's IR-cut conversion
• Diameter is not interchangeable: D25 fits 25 mm bodies only. Smaller planetary bodies take the D21; ASI1600 / ASI294 / ASI183 / ASI533 take the D32; ASI6200 / ASI2400 / ASI2600 take the D60
• No installation direction: ZWO confirms the window has no preferred orientation, so the coating works either way round
• 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. Shooting one-shot-colour through a refractor with nothing in the train means infrared focuses at a different point than visible light and softens the image. Use the D25 UV IR-Cut instead, or filter in the train
• 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 — work in still, clean air
• Fits 25 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

Which cameras use the D25 AR window?

The ASI174 series. The ASI485MC also ships with it from the factory.

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.

Should I choose AR or IR-cut?

If you filter in the imaging train, or you image in infrared, keep the AR window. If you shoot one-shot-colour with nothing in the train, the IR-cut version does that job at the sensor and frees a filter slot.

I have an ASI485MC and want IR-cut — what do I order?

The D25IR-CUT. ZWO names it as the correct IR-cut replacement for that camera.

Does the window have to face a particular direction?

No. ZWO confirms there is no specific installation direction.

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 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 25 mm.

Is this good for beginners?

The part is simple; fitting it is not, since it means opening the sensor chamber. If you are not confident working clean, have it done.

Bottom Line

In short: a 25 mm × 1.1 mm anti-reflective window at 1/4 λ flatness — the factory spare for the ASI174 series and ASI485MC, and the right choice when you filter in the train or image in infrared. It does not cut UV or IR; if you need that at the sensor, fit the D25 UV IR-Cut.