Optolong

Optolong OIII Filter 2″

Turns the sky behind an emission nebula black, so faint targets like the Veil leap out of the background. One job, done better than anything else.

91/ 100

Polaris Score

#1 of 7 nebula filters we recommend

From$119

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What changes

The nebula was always there.
The filter removes everything else.

Drag to see the same patch of sky with and without the filter. Nothing about the nebula changes — the filter simply blocks the skyglow drowning it, so the contrast does the rest.

No filter

OIII filter

◄►

What it blocks

Skyglow, streetlights and the Moon, the broad, unwanted light that greys out the background.

What it passes

The narrow doubly-ionised oxygen (OIII) lines where planetary and supernova nebulae actually glow.

The result

The sky behind the nebula goes black, so faint tendrils lift clear of the background.

01Where it shines

The targets it was made for.

A nebula filter only helps objects that glow at the right emission line — but on those, nothing else comes close. These are the ones it transforms most.

The showpiece

The Veil Nebula

A supernova remnant of impossibly thin filaments. Almost invisible without a filter; with OIII the lace-work leaps out and fills the field.

Huge & faint

North America Nebula

Enormous, low-surface-brightness gas. The filter darkens the sky enough that the continent shape finally separates from the Milky Way behind it.

Contrast-critical

The Crescent Nebula

A faint shell blown by a dying star. The classic “only visible with OIII” target, off without the filter, obvious with it.

Big planetary

The Helix Nebula

A large, dim planetary nebula that skyglow easily swamps. OIII lifts the ring cleanly off a now-black background.

Wide bloom

The Rosette Nebula

A vast circular bloom around a star cluster. The filter reveals the full petalled ring that a bright sky otherwise erases.

02Be honest with me

What this filter does NOT improve.

Half of buying the right filter is knowing when to leave it in the case. On these targets it makes the view worse, not better — put it away and enjoy them unfiltered.

✕Galaxies

Galaxies glow with broad starlight across all colours. A narrowband filter blocks most of it, they get dimmer, not better.

✕Open clusters

Just stars. There is no nebula emission to isolate, so the filter only dims the very thing you came to see.

✕Globular clusters

Dense balls of stars, again, pure starlight. The filter robs them of brightness and resolution.

✕The Moon

Bright, broadband and reflective. A nebula filter throws away most of the light and adds a colour cast for no gain.

✕Planets

Reflected sunlight, not emission. A narrowband filter does nothing useful here, use it only on emission nebulae.

03Where it fits

Where it lives in your kit.

A filter isn't a standalone instrument — it threads into the light path just before your eye. Here's the chain.

01

Telescope

Gathers the light

Fast Dob or refractor

→

02

Diagonal

Optional · SCT/refractor

Bends the light path

→

03

This filter

Threads onto the eyepiece

Blocks the skyglow

→

04

Eyepiece

Frames the field

2″ wide-field

→

05

Your eye

Sees the contrast

The black sky pops

04Which filter

How the filter types compare.

A handful of filters cover almost all visual observing. The narrower the band, the more contrast — and the fewer targets it works on.

This one

OIII

Narrowest

Bandpass

~12–18 nm

Best for

Planetary & supernova nebulae

Maximum contrast on the right targets, but only those.

UHC

Broad narrowband

Bandpass

~24 nm

Best for

Most emission nebulae

The versatile all-rounder. A little less punch on Veil-type targets, but works on more of them.

H-Beta

Specialist

Bandpass

~9 nm

Best for

A handful of dim nebulae

The “Horsehead filter.” Superb on a few faint targets, useless on everything else.

Broadband

Light-pollution

Bandpass

Wide

Best for

General city observing

Gently cuts skyglow across the board. Helps a little everywhere, transforms nothing.

05The numbers

What These Numbers Mean

Filter typeSpecialist

OIII line filter

It isolates the doubly ionised oxygen lines and rejects nearly everything else, including much of the light from stars. The sky behind the target goes close to black, which is why faint nebulae appear to jump forward.

What changesDramatic

Planetary nebulae and supernova remnants

The Veil Nebula is the classic demonstration: from a faint smudge to visible filaments. The Ring, the Dumbbell and the Helix all gain sharply defined structure that no other filter provides.

What it costs youDimming

Most of the light

Because it blocks nearly the whole spectrum, the image is much dimmer overall and star fields largely disappear. In telescopes under about six inches the view can become too dark to be useful.

Barrel sizeWide field

2 inch

Fits 2 inch eyepieces, which is what you want for large targets like the Veil where the whole object needs to fit in the field at low power.

When not to use itSelective

Galaxies and clusters

Galaxies, globular clusters and reflection nebulae emit across the spectrum, so an OIII filter only dims them. This is a second filter to own after a UHC, not a first one.

Optical

TypeNarrowband, OIII

Bandpass≈ 18 nm FWHM

PassesOIII 496 / 501 nm

Peak transmission> 90%

BlockingUV–IR, deep

SubstratePolished optical glass

Fit & build

Cell size2″ (M48 × 0.75)

Also available1.25″ · 31 mm unmounted

ThreadingBoth faces threaded

MountAnodised aluminium cell

ReflectionFully multi-coated

UseVisual & imaging

06Pairs with

Compatible gear.

Eyepieces

Low power, wide field

2″ Nagler 31 mm

Frames the whole Veil

32 mm Plössl

Budget wide field

24 mm 68°

The 1.25″ option

Telescopes

The faster, the better

8″+ Dobsonian

Aperture feeds contrast

Fast refractor · f/5–6

Whole-nebula framing

SCT · 8–11″

Use with a 2″ diagonal

Wheels & drawers

For quick in / out

2″ filter drawer

Swap without unscrewing

Manual filter wheel

Carry OIII + UHC together

2″ threaded cell

Threads onto any 2″ EP

07Learn more

Guides for nebula observing.

Guide · 6 min

OIII vs UHC: which nebula filter should you buy first?

Most observers start with the wrong one. When the extra contrast of OIII is worth the narrower range, and when UHC is the smarter first buy.

Read the guide →

Guide · 8 min

Why filters do nothing for galaxies (and everything for nebulae)

The physics behind emission lines, in plain English, so you always know before you look whether the filter will help.