ZWO 7nm SHO Narrowband Filter Set (Ha, OIII, SII)

Hubble-palette nebula images from a light-polluted backyard

The ZWO 7nm SHO Narrowband Filter Set is three filters, hydrogen-alpha (656nm), oxygen-III (500nm) and sulfur-II (672nm), each passing a band about 7nm wide and blocking almost everything else. Shoot each filter in turn with a mono camera, then combine them into a false-color image in the "Hubble palette" or a two-filter HOO blend. We carry the set in 1.25" (M28.5 threads) or 2" (M48 threads) mounted cells to suit your camera's sensor and filter wheel. ZWO also makes 31mm and 36mm unmounted versions, which we don't stock.

Key specs: Ha 656nm, OIII 500nm, SII 672nm · 7nm ±0.5nm bandwidth · about 90% peak transmission · OD3 off-band blocking (under 0.1%) · 700-1100nm infrared blocked · 1/4-wave polished glass, 1.85mm thick · 1.25" or 2" mounted

What you can image with it

• Large emission nebulae: the Heart and Soul, the North America and Pelican, the Veil and the Elephant's Trunk in IC 1396, all rich in hydrogen with sulfur and oxygen structure that SHO color separates.
• Famous Hubble-palette targets: the Pillars of Creation in the Eagle Nebula (M16), the Rosette, the Lagoon (M8) and the Crescent Nebula (NGC 6888).
• Smaller targets for small sensors: the Bubble Nebula (NGC 7635), the Cave Nebula and planetary nebulae like the Dumbbell (M27) and Ring (M57), which suit 1.25" filters on small-pixel cameras at longer focal lengths.
• Moonlit nights: narrowband keeps working when the Moon is up, so you can image on nights that would be lost to broadband.

How it works

Each filter lets through one emission line. Hydrogen-alpha records the hydrogen gas that fills most nebulae, OIII picks out hotter oxygen-rich regions, and SII shows sulfur, usually the faintest of the three. Streetlights, LED glow and moonlight are spread across the spectrum, so a 7nm window throws most of it away while keeping almost all of the nebula's light at that wavelength. ZWO's transmission curves peak around 90% for all three lines.

In processing, the standard Hubble palette maps SII to red, Ha to green and OIII to blue. Many people start with just Ha and OIII (an HOO blend) and add SII later once they have more time on a target. Plan on several hours per target in total; SII typically needs the most.

Which size do you need?

The filter has to be large enough that it doesn't darken the corners of your sensor. Sensor diagonal is the main thing to check, along with your scope's focal ratio and how close the filter sits to the sensor (a filter wheel sits about 10-20mm away). ZWO's own back-focus guide pairs its 1.25", 31mm and 36mm wheels with 4/3-inch, 1-inch and smaller cameras, recommends 2" or 36mm filters for APS-C because smaller filters may vignette, and recommends only 2" for full frame. Our rule of thumb is a little stricter:

• 1.25" (up to about a 1-inch sensor, roughly 18mm diagonal): the ASI533MM Pro (16mm), ASI585MM (12.9mm), ASI678MM, ASI174MM Mini and ASI432MM. Use it in the EFW 5x1.25" or EFW 8x1.25".
• 4/3-inch sensors (about 23mm diagonal), such as the ASI294MM Pro: ZWO lists 1.25" wheels for 4/3-inch cameras, and in our experience they work at f/5 and slower, but the 7x36mm wheel gives more margin at this in-between size. We don't carry this ZWO set in 36mm; the Optolong SHO 7nm set comes in 36mm unmounted.
• 2" (APS-C and full frame): the ASI2600MM Pro, ASI2600MM Air and ASI2600MM DUO (APS-C, 28.3mm diagonal) and the ASI6200MM Pro (full frame, 43.3mm). The ASI585MM Air also takes 2" filters, because its built-in guide sensor sits beside the main sensor and needs a 36mm-wide light path. Use them in the EFW 5x2" or EFW 7x2".

If you might move up to a larger sensor later, 2" filters will carry over; 1.25" filters will not.

Using it in Colorado

From most of the Front Range, the glow from Denver, Boulder or Colorado Springs is the biggest limit on nebula imaging. Narrowband is the most effective answer: with these filters a Lakewood or Aurora backyard can produce clean SHO data, and a trip to darker skies becomes a bonus rather than a requirement.

What to add for the best results

• A mono camera (required): the ZWO ASI533MM Pro pairs with the 1.25" set; the ASI2600MM Pro pairs with the 2" set.
• A filter wheel: the EFW 8x1.25" holds this set plus LRGB with two slots spare; for 2", the EFW 7x2" holds this set plus LRGB.
• ZWO LRGB filters (1.25" or 2") or Optolong LRGB filters (1.25", 2" or 36mm): add broadband color for galaxies and star color in the same wheel.
• ASIAIR Plus: runs filter sequences and autofocus from your phone or tablet.
• ZWO EAFN electronic focuser: refocuses automatically when the filter changes or the temperature drops.
• ZWO 2" filter drawer (M42) or M54 version: a lower-cost way to use a single 2" filter, swapping by hand, if you're not ready for a wheel.

Compare narrowband options

See all astronomy filters and filter wheels and drawers.

What's in the box

• ZWO 7nm hydrogen-alpha filter
• ZWO 7nm OIII filter
• ZWO 7nm SII filter

All three come in the size you choose (1.25" or 2" mounted). The camera and filter wheel are sold separately.

ZWO 7nm SHO Narrowband Filter Set FAQ

Can I use these filters with a color camera?

You can, but it isn't a good match. On a color sensor only one pixel in four is red, so Ha and SII signal is collected slowly, and OIII falls on the green and blue pixels. For a color camera, the ZWO Duo-Band is the better choice.

1.25" or 2"?

Match the filter to your sensor. 1.25" covers sensors up to about 1 inch, like the ASI533MM Pro and ASI585MM. APS-C and full-frame sensors, like the ASI2600MM Pro and ASI6200MM Pro, need 2".

Is 7nm better than 3nm?

Neither is simply better. 3nm filters give more contrast under bright skies and a full Moon but need longer exposures and are more sensitive to fast focal ratios. 7nm is easier to work with, collects signal faster and is the usual starting point.

Do these filters block light pollution?

They don't make a nebula brighter. They block nearly all the light outside each narrow band, including most artificial light and skyglow, which raises the contrast between the nebula and the sky.

Will I need to refocus between filters?

Plan on it. Focus can shift slightly between wavelengths, especially with refractors. An electronic focuser like the ZWO EAFN with ASIAIR or N.I.N.A. handles it automatically.

Which filter wheel holds them?

1.25" filters fit the ZWO EFW 5x1.25" and EFW 8x1.25". 2" filters fit the EFW 5x2" and EFW 7x2".

Specifications