Optolong OIII Filter 2″ — image 1
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
Where to buy
Affiliate link — we earn a commission, you pay the same. We rank on fit, not payout.
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.

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.

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.

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.

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.
Dual-band
Imaging only
Bandpass
~3–7 nm × 2
Best for
Colour cameras on emission nebulae
Two narrow windows, H-alpha and OIII, on one piece of glass. Transformative for a one-shot-colour camera in a city, far too dark to look through.
05The numbers

What These Numbers Mean

5 numbers in plain English · full specifications

5 numbers in plain English · full specifications

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.

3 matches for this filter

3 matches for this filter

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.

2 guides that cover this ground

2 guides that cover this ground

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.
Read the guide →