A broadband nebula filter first, nearly always. An O-III transforms planetary nebulae and supernova remnants and makes everything else very dark, which is why it is so often judged useless when it is the first one bought.
All three work on the same principle and differ only in how narrow a window they pass, which is exactly what makes the buying order matter.
What all three do
Emission nebulae radiate at a few specific wavelengths. A filter that passes only those wavelengths removes far more sky glow than nebula light, so the background darkens and contrast rises. Nothing gets brighter.
Why broadband first
A UHC-type filter passes a window wide enough to include several lines, so the star field still looks reasonably natural and a good number of objects respond. It is the filter that can be left in the eyepiece for a whole session.
Why O-III second
Its window is narrow, around the doubly-ionised oxygen lines. On planetary nebulae and supernova remnants the effect is dramatic. On a galaxy, a cluster or a reflection nebula it is a very dark piece of glass.
The figure to compare
Not the name. Two filters sold under the same three letters can pass windows of different widths, and what decides behaviour is the transmission at the lines together with the width of the window. A maker who publishes both is worth preferring.
If this page and the manufacturer’s own documentation disagree, follow the documentation. Every aperture, focal length, payload rating, field, eye relief, transmission figure and thread size on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And for any filter: the name is not the bandpass. Two filters sold under the same three letters can pass windows of very different width, and the figure that decides what a filter does is the transmission at the emission lines together with the width of the window. This page reports the band and the percentage where a maker publishes them, and says they are missing when they are not.
The questions that keep arriving
What does an O-III actually change?
On the objects it suits, a great deal: a faint patch becomes a structured object. On everything else it removes so much light that the field goes nearly black, and a beginner looking at the wrong target concludes the filter is broken.
Is H-beta worth owning?
It serves a short list of targets and is the last of the three to buy. Owners who have one usually bought it for a specific object after exhausting what the other two offered.
Last reviewed 17 September 2026