ZWO O-III 7nm Narrowband Filter

Product Overview

The ZWO O-III 7nm Narrowband Filter passes a 7 nm band centred on the doubly-ionised oxygen line at 500 nm and rejects effectively everything else. Peak transmission is around 90%, off-band rejection is OD3 (below 0.1%), and infrared from 700–1100 nm is cut. Every size is polished to 1/4 wavefront on both surfaces.

Available in three sizes — 1.25" mounted, 36 mm unmounted, and 2" mounted. Each is a single filter, not a set. O-III sits in the blue-green where most CMOS sensors peak, making it the most efficient of the three SHO channels.

Choosing the Right Size

Size is decided by your sensor, not your telescope. A filter too small vignettes the corners, and no amount of processing recovers it — the cell wall is a physical obstruction.

• 1.25" mounted — for sensors up to about 4/3": ASI1600, ASI294, ASI533 and similar. Threaded cell, 1.9 mm glass. Will vignette APS-C and full-frame
• 36 mm unmounted — for APS-C and smaller: ASI2600MM, ASI294MM and similar. Bare glass, no cell or thread, so the full 36 mm is clear aperture. Fits the ZWO 36 mm EFW (7×36 mm). 2 mm glass. Will not cover full-frame
• 2" mounted — for sensors up to full-frame: ASI6200MM, ASI2400 and similar. Threaded cell, fits 2" wheels, drawers, and standard M48 threads

If you are between sizes, size up.

Who It's For

This is a good match if you image planetary nebulae and supernova remnants with a monochrome camera. O-III is where those targets show structure that H-alpha simply does not record — the Veil's filaments and the Helix's inner shell are O-III features.

It is also the second filter most imagers add after H-alpha, and the partner for HOO bicolour work if you are not ready for the full SHO set.

Key Features & Design

• 500 nm centre wavelength, 7 nm bandpass: targets the major doubly-ionised oxygen emission line
• FWHM 7 ± 0.5 nm: a tight, specified passband
• Approximately 90% peak transmission at the O-III line
• OD3 off-band blocking: rejects mercury and sodium vapour lighting and atmospheric skyglow
• Sits near peak sensor QE: back-illuminated CMOS sensors peak in the blue-green, so O-III photons are captured efficiently
• 1/4 wavefront both surfaces: fine-polished, so star shapes are not degraded
• Three sizes: 1.25" mounted (1.9 mm glass), 36 mm unmounted (2 mm), and 2" mounted

Recommended Uses

• Planetary nebulae — the Ring, Dumbbell, and Helix, where O-III carries the inner structure
• Supernova remnants — the Veil, whose filamentary detail is largely an O-III signal
• HOO bicolour imaging, mapping H-alpha to red and O-III to green and blue
• SHO / Hubble-palette imaging alongside the H-alpha and S-II 7 nm filters
• Imaging under moonlight or heavy light pollution

Compatibility and Accessory Notes

• Monochrome cameras only. On a one-shot-colour sensor the Bayer matrix wastes most of the signal
• 36 mm is unmounted: no cell and no thread. It is retained by M2 screws in a filter wheel recess — those screws ship with the ZWO 36 mm EFW
• 1.25" and 2" are mounted: threaded cells that fit filter wheels, drawers, and standard filter threads
• Fast optics shift narrowband passbands. Below roughly f/4 the steep cone angle blue-shifts the effective centre wavelength of any narrowband filter — O-III is the channel where that shows first
• Third-party wheels: ZWO specifies filter thickness between 1.5 mm and 3.0 mm
• Match thickness across the wheel so focus does not shift between channels
• Orientation: ZWO advises the coated side should face the telescope on Mark II filters

Important Limitations

• Not for visual observing. A 7 nm imaging filter is far darker than the visual O-III filters sold for eyepiece use — do not confuse the two
• Size must match your sensor. 1.25" vignettes APS-C and full-frame; 36 mm will not cover full-frame
• Halos around bright stars are the classic O-III complaint. ZWO's Mark II filters improved this, but any narrowband filter can halo on a very bright star in the field. Framing helps more than processing
• Unmounted 36 mm glass must be handled by the edges — no cell protects the coating
• Not for one-shot-colour cameras or DSLRs — monochrome imaging only
• Narrowband raises contrast, not brightness. Expect long sub-exposures
• A filter wheel and monochrome camera are required and sold separately

Frequently Asked Questions

Is this the same as a visual O-III filter?

No, and this is a common and expensive mix-up. Visual O-III filters for eyepiece use run much wider passbands so there is enough light to see. A 7 nm imaging filter is far too dark for the eye. Buy the visual version if you observe.

Which size do I need?

Match it to your sensor, not your telescope. Up to about 4/3" (ASI1600, ASI294, ASI533), the 1.25". APS-C and smaller (ASI2600MM), the 36 mm unmounted. Up to full-frame (ASI6200MM), the 2".

What is the difference between mounted and unmounted?

A mounted filter sits in a threaded cell you can screw into things. An unmounted filter is bare glass held by screws in a filter wheel. The 36 mm is unmounted because a cell has walls, and walls block the corners of a big sensor.

Which do I buy after H-alpha?

O-III, usually. It gives you HOO bicolour immediately, and it records structure in planetary nebulae and supernova remnants that H-alpha misses entirely.

Why do I get halos around bright stars?

It is the classic narrowband O-III artefact. ZWO's Mark II filters reduced it, but a very bright star in the field can still halo. Reframing to move the star out is more effective than trying to fix it later.

Will this work at f/3?

With a caveat. Below about f/4 the steep light cone shifts a narrowband filter's effective centre wavelength blueward, and O-III is where you notice it first — transmission at the true line drops. It still works; it just works less well than at f/5 or slower.

Will it work with my colour camera or DSLR?

Not effectively. Only some pixels of a Bayer matrix see 500 nm, so you discard most of your resolution and light.

Which way round does it go?

ZWO advises that on the Mark II filters the coated side should face the telescope.

Can I image with the Moon up?

Yes. A 7 nm passband rejects moonlight well enough to make those nights productive.

Is one filter included, or three?

One. Each size option is a single O-III filter.

Bottom Line

In short: a 7 nm O-III filter at 500 nm with roughly 90% peak transmission, OD3 blocking, and 1/4 wavefront polish — the second narrowband filter most mono imagers buy, and the channel that carries planetary nebulae and supernova remnants. Not a visual O-III filter. Pick the size from your sensor: 4/3" takes 1.25", APS-C takes 36 mm, full-frame takes 2".