ZWO S-II 7nm Narrowband Filter

Product Overview

The ZWO S-II 7nm Narrowband Filter passes a 7 nm band centred on the ionised sulphur line at 672 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. S-II is the third channel of the SHO trio and the faintest of the three, which matters when you plan integration time.

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 are already imaging in H-alpha with a monochrome camera and want to complete the SHO set for Hubble-palette work. S-II sits close to H-alpha in the deep red, but nebulae emit far less strongly in it — so it is a filter you add once the workflow is established, not a starting point.

Key Features & Design

• 672 nm centre wavelength, 7 nm bandpass: targets the ionised sulphur emission line
• FWHM 7 ± 0.5 nm: a tight, specified passband
• Approximately 90% peak transmission at the S-II line
• OD3 off-band blocking: under 0.1% transmission outside the passband
• IR cut from 700–1100 nm: matters more here than on the other channels, since 672 nm sits near the infrared shoulder
• 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

• SHO / Hubble-palette imaging alongside the H-alpha and O-III 7 nm filters
• Supernova remnants — the Veil and similar, where S-II traces shocked gas
• Planetary nebulae, where S-II separates ionisation zones that H-alpha alone blends together
• Emission nebulae where you want the full three-channel palette rather than HOO bicolour
• 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
• 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
• Backfocus: glass shifts focus by roughly one third of its thickness — about 0.6–0.7 mm depending on size

Important Limitations

• S-II is the faintest of the three SHO channels. Expect substantially longer integration than H-alpha for comparable signal. That is astrophysics, not a filter fault
• Size must match your sensor. 1.25" vignettes APS-C and full-frame; 36 mm will not cover full-frame
• 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
• Not useful on its own. S-II without H-alpha and O-III is one channel of a three-channel palette
• Not for visual observing — a 7 nm band at 672 nm is far too dark for the eye
• A filter wheel and monochrome camera are required and sold separately

Frequently Asked Questions

Why is my S-II data so much weaker than my H-alpha?

Because nebulae emit far less strongly at 672 nm than at 656 nm. That is normal and expected. Plan on longer total integration in S-II than in H-alpha — many imagers run roughly twice as long.

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.

Do I need this if I already have H-alpha and O-III?

Only if you want the full Hubble palette. HOO bicolour from H-alpha and O-III is a perfectly respectable stopping point, and it costs a lot less integration time.

Will it work with my colour camera or DSLR?

Not effectively. Only the red-sensitive pixels of a Bayer matrix see 672 nm, so you discard most of your resolution and light.

Do I need a UV/IR-cut filter as well?

No. This filter cuts 700–1100 nm itself, which matters for S-II because 672 nm sits close to the infrared.

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 and light pollution well enough to make those nights productive.

Is this good for beginners?

It is not a first filter. Start with H-alpha, learn the mono narrowband workflow, then add O-III and S-II.

Is one filter included, or three?

One. Each size option is a single S-II filter.

Bottom Line

In short: a 7 nm S-II filter at 672 nm with roughly 90% peak transmission, OD3 blocking, and 1/4 wavefront polish — the third SHO channel, and the one that needs the most integration time. Pick the size from your sensor: 4/3" takes 1.25", APS-C takes 36 mm, full-frame takes 2".