Which astrophotography filter do you need?
Galaxies and star clusters are usually shot with no filter. A dual-band filter is for emission nebulae from the city or when the Moon is up. This page covers each type and when to use it.
| Filter | Passes | Best for | Sky & Moon |
|---|---|---|---|
| No filter (or UV/IR cut) | Everything the sensor sees | Galaxies, star clusters, reflection nebulae, dark nebulae, the Milky Way | Dark sites, moonless nights |
| Broadband light pollution filter | Most of the spectrum, with a few streetlight colors removed | Galaxies and clusters from suburban skies | Bortle 4-7, moonless nights |
| Dual-band (Ha + OIII) filter | Two narrow bands, hydrogen-alpha (656nm) and oxygen-III (501nm) | Emission nebulae, planetary nebulae, supernova remnants | Anywhere, even a city or a bright Moon |
| Mono narrowband filters (Ha, OIII, SII) | One emission line per filter, 3-7nm wide | Emission nebulae with a mono astro camera, Hubble-palette (SHO) images | Anywhere, any Moon phase |
No filter (or UV/IR cut)
This is where most people should start. Galaxies, clusters and dust glow across the whole spectrum, so a filter that cuts light pollution also cuts the target. You only need a UV/IR cut filter with an astro-modified camera or a dedicated astro camera, or with some refractors that give bloated blue stars.
Examples: Built into most cameras. Clip-in or 2" UV/IR cut filters are available for astro cameras and refractors
Broadband light pollution filter
These block the narrow colors given off by older sodium and mercury streetlights and let most starlight through. They help less each year as cities switch to white LEDs, and they do nothing about moonlight. They can help a little in the suburbs.
Examples: Optolong L-Pro, IDAS LPS-D1/D3, Astronomik CLS
Dual-band (Ha + OIII) filter
With a color camera, this is the filter that lets you shoot nebulae from a city. It only lets through the colors that emission nebulae glow in, so it blocks almost all light pollution and moonlight. Narrower bands (7nm or 3nm) block more moonlight and need longer exposures and careful focus. Keep it off for galaxies and star clusters, because it removes most of their light and turns stars green.
Examples: Optolong L-eNhance, L-eXtreme and L-Ultimate, Antlia ALP-T, Askar ColorMagic duo-band
Mono narrowband filters (Ha, OIII, SII)
With a monochrome camera, separate narrowband filters collect the most signal and the cleanest data. You can map sulfur, hydrogen and oxygen to colors in the Hubble palette. It takes a filter wheel and more processing, so most people try it after starting with a color camera.
Examples: Antlia, Chroma, Astronomik and Optolong 3-7nm sets
Great with a dual-band filter
Emission targets you can shoot from the city or with the Moon up.
Shoot these with no filter
Broadband targets that need a dark, moonless sky.
Picking a filter for tonight
- For a galaxy, star cluster or reflection nebula, use no filter. Wait for the new Moon and go to a dark site if you can.
- For an emission nebula under a dark, moonless sky, no filter gives the most natural colors and stars. A dual-band filter will still add contrast.
- For an emission nebula from a city or with the Moon up, use a dual-band filter. Narrower bands block more moonlight.
- With a mono camera, use separate Ha, OIII and SII filters for nebulae and RGB filters for everything else.
Set your filter in the planner and it will rank targets with these rules in mind. It also warns you when your filter is a bad match for a target.
Filter questions
Do I need a filter for astrophotography?
No. Many of the best beginner targets, including the Andromeda Galaxy, the Pleiades, star clusters and the Milky Way, are shot with no filter. A filter is worth buying once you want to shoot emission nebulae from a light-polluted sky or with the Moon up. For that, a dual-band filter is the most useful one to get.
Can I use a dual-band filter with an unmodified camera?
Yes, though you will get much less hydrogen-alpha signal. Stock DSLRs and mirrorless cameras have a built-in filter that blocks roughly 70-80% of hydrogen-alpha light. Oxygen-III light still gets through, so targets like the Veil and Dumbbell work well. Very red targets like the California Nebula need a lot of exposure time. An astro-modified camera picks up the red much better.
Do light pollution filters work with LED streetlights?
Broadband light pollution filters do little against white LEDs, because LEDs give off light across the whole visible spectrum. Dual-band and narrowband filters still work, because they block everything except two or three narrow colors.
Which filter should I use with the Moon up?
Moonlight is reflected sunlight and covers the whole spectrum, so only narrowband filters block much of it. With a bright Moon, shoot emission nebulae with a dual-band filter (narrower bands work better), or shoot bright star clusters with no filter. Save galaxies for the new Moon.