Your deep sky companionYour deep sky imaging companion
Astrophotography filters
A filter lets some colors of light reach your sensor and blocks the rest. Choose well and a nebula jumps out of a bright sky. Choose badly and you lose most of the light from your target. This guide covers each kind, what it is for, and the targets that suit it.
Filters help against some light, not all
A filter is a color gate. It works well when the thing you dislike (a streetlamp's orange glow) is a different color from the thing you want. It works poorly against the Moon and the broad glow of a city, which are every color at once.
Match the filter to the target
Glowing gas gives off narrow colors, so nebulae suit narrowband filters. Stars and galaxies shine in every color, so galaxies and clusters need broadband or no filter. A narrowband filter on a galaxy leaves nearly nothing.
Match the filter to the camera
A mono camera uses many filters, one after another. A color camera uses one filter that passes several colors at once. A stock DSLR has its own filter inside, which blocks part of the red hydrogen glow.
Darker sky means fewer filters
Every filter costs some light. Under a dark sky, you often need none. The Dark Nights calendar shows when the Moon is out of the way.
The same rainbow, with each kind of filter laid over it. The white lines are the colors that glowing gas gives off.
Choose a target and a sky, then try each filter. The bright shapes show what reaches your sensor and the faint shapes show what the filter throws away.
Everything gets in: the target, and all of the sky.
This is the baseline: all of the target and all of the sky.
A teaching model. The spectra and filter bands are simplified, and real filters and skies vary.
Answer three questions for a starting point.
A dual-band filter is the best single upgrade for a color camera in the suburbs or a city. It also works with a bright Moon, which broadband cannot manage.
Avoid: Very fast scopes below about f/3 can shift the passband. A wider (7 nm or more) filter is the safe choice.
The filter has to be larger than your sensor, or the corners go dark. Pick your camera (or a setup saved in the FOV calculator) to see which sizes cover it.
Tip: set up your camera in the FOV calculator and save it as equipment, and it will appear in the first list here. Setups saved before this feature don’t have a sensor size, so save them again.
APS-C: Sony, Nikon, Fuji (23.5 × 15.6 mm): the sensor is 23.5 × 15.6 mm, a diagonal of 28.2 mm. At f/5 with the filter 12 mm away, the filter needs about 30.6 mm of clear opening. The smallest size that covers it is 36 mm (unmounted).
- 1.25 inch (mounted)Too small
Threaded cell. The usable glass is about 26 mm. 4.6 mm short, so the corners would be cut off.
- 31 mm (unmounted)Too small
Loose glass in a wheel or drawer. About 29 mm usable. 1.6 mm short, so the corners would be cut off.
- 36 mm (unmounted)Fits
Loose glass in a wheel or drawer. About 34 mm usable. Spare room: 3.4 mm.
- 2 inch (mounted)Fits
Threaded cell. The usable glass is about 46 mm. Spare room: 15.4 mm.
Estimates from typical clear openings. Makers differ by a millimeter or two, so check the filter’s stated clear aperture before you buy. The filter should also match what your camera or filter wheel holds.
UV/IR cut (luminance)
Blocks the light your sensor sees but you don't
- What it does
- Sensors respond to ultraviolet and infrared light that eyes cannot see. A UV/IR cut filter removes both and passes the whole visible range. Without it, those wavelengths focus at a slightly different point, so stars on a refractor swell into soft halos.
- Best for
- Mono cameras, and any camera that has had its built-in filter removed (a modified DSLR).
- Example targets
- Everything broadband: galaxies, clusters, reflection nebulae and star fields.
- Cameras
- Mono cameras and modified DSLRs. A stock DSLR already has one built in.
- Good to know
- On a mono camera it doubles as the luminance (L) filter in an L, R, G, B set.
LRGB color filters
Red, green and blue, with a luminance layer
- What it does
- A mono camera sees no color, so you shoot through red, green and blue filters, one after another, and combine them. A luminance (L) set adds sharp, bright detail. It is the mono equivalent of a color sensor.
- Best for
- Mono cameras on targets that shine across the spectrum.
- Example targets
- Andromeda Galaxy (M31), Triangulum (M33), Whirlpool (M51), Pinwheel (M101), Pleiades (M45), globular clusters such as M13.
- Cameras
- Mono cameras only.
- Good to know
- More work than a color camera, since each filter needs its own flats and a full set of exposures.
Broadband light-pollution filter
Trims the colors of streetlamps
- What it does
- Cuts the narrow colors that sodium and mercury street lamps give off, and lets the rest through. The sky background gets darker and contrast improves, with a small color shift that you correct while processing.
- Best for
- Suburban skies, shooting galaxies, clusters and reflection nebulae.
- Example targets
- M31, M33, M81 and M82, M101, M13, the Pleiades, and wide Milky Way fields from the suburbs.
- Cameras
- Any. Clip-in versions fit Canon and Nikon bodies, and screw-in versions fit scopes and lenses. Typical examples are Optolong L-Pro and Astronomik CLS.
- Good to know
- It helps with streetlights and does very little against the Moon or against the broad glow of a big city. Under a dark sky it is not needed.
Dual-band (Hα and OIII)
The best single filter for color cameras and nebulae
- What it does
- Two narrow windows, one for red hydrogen light (Hα, 656 nm) and one for blue-green oxygen light (OIII, about 500 nm). Everything else is blocked. Glowing gas stays bright, while skyglow and the Moon are mostly gone.
- Best for
- Color cameras (one-shot color and modified DSLRs) shooting emission nebulae from a town or city, or with the Moon up.
- Example targets
- Rosette (NGC 2237), North America (NGC 7000), Veil (NGC 6960), Eagle (M16), Lagoon (M8), Trifid (M20), Heart (IC 1805), Pelican (IC 5070), Orion (M42, though the core clips quickly).
- Cameras
- Color astro cameras and modified DSLRs. A stock DSLR passes only part of Hα, so results are weaker but still worthwhile. Typical examples are Optolong L-eXtreme, Antlia and Askar dual-band filters.
- Good to know
- Narrower versions (about 7 nm) give the most contrast, but fast optics below about f/3 can shift the passband, so wider versions suit very fast scopes. Never use it for galaxies or clusters.
UHC (Ultra High Contrast)
The classic nebula filter
- What it does
- A UHC passes the light of hydrogen and oxygen (Hβ, OIII and, in many versions, Hα) in fairly wide windows and blocks most of the rest. It was made for looking at nebulae through an eyepiece, and works just as well for photography. It sits between a light-pollution filter and a narrow dual-band filter: stronger contrast than the first, and a more natural look and brighter image than the second.
- Best for
- Emission and planetary nebulae from suburban skies, with any camera, and a good first nebula filter for a stock DSLR.
- Example targets
- Orion (M42), Lagoon (M8), Trifid (M20), Eagle (M16), Omega (M17), Ring (M57), Dumbbell (M27), Veil.
- Cameras
- Any. Clip-in and screw-in versions exist. Typical examples are Baader UHC-S, Astronomik UHC and Orion Ultra Block. The exact bands differ by brand, and some pass only Hβ and OIII.
- Good to know
- The wider windows let in more skyglow than a 7 nm dual-band filter, so city skies and the Moon favor the narrower filter. Not for galaxies or clusters.
Tri-band and quad-band
Dual-band plus Hβ (and sometimes SII)
- What it does
- The same idea, with extra windows: Hβ (486 nm, a second hydrogen line) and, in quad-band versions, SII (a sulfur line near 672 nm). Slightly wider windows keep more natural star color than a strict dual-band filter.
- Best for
- Color cameras and stock DSLRs, when you want nebula contrast with a gentler look.
- Example targets
- The same targets as dual-band. Typical examples are Optolong L-eNhance and similar tri-band filters.
- Cameras
- Color astro cameras, modified DSLRs, and stock DSLRs.
- Good to know
- A little more skyglow gets through than with a strict dual-band, so they work best from suburban or darker skies.
Narrowband Hα, OIII, SII (mono)
One gas at a time
- What it does
- Single very narrow filters (3, 5, 7 or 12 nm wide), each isolating one gas. Shoot each separately with a mono camera, then combine them into a color picture. The best-known recipe is the Hubble palette (SII to red, Hα to green, OIII to blue).
- Best for
- Mono cameras, nebulae, city skies and any night with a Moon.
- Example targets
- Hα: Heart (IC 1805), Soul (IC 1848), North America (NGC 7000), Veil. OIII: Veil, Dumbbell (M27), Ring (M57), Helix (NGC 7293), Owl (M97), Crescent (NGC 6888). SII: Eagle and its Pillars (M16), Elephant Trunk (IC 1396).
- Cameras
- Mono cameras (and sometimes a color camera, using only the red pixels for Hα).
- Good to know
- Needs long total exposure times, since the light passing through is faint. Each filter needs its own flats and focus offset.
Hβ filter
A rare, faint hydrogen line
- What it does
- A second hydrogen line at 486 nm, much weaker than Hα. Visual observers use it for the Horsehead, and it is a niche choice in photography.
- Best for
- Experienced imagers who want an extra layer of detail from one or two specific nebulae.
- Example targets
- Horsehead (B33 in IC 434) and the California Nebula (NGC 1499).
- Cameras
- Mono cameras.
- Good to know
- Usually included in tri-band filters, so you rarely need a separate one.
Planetary and lunar filters
For the Moon and planets, not deep sky
- What it does
- An infrared-pass filter (around 685 nm or longer) cuts the blur from the atmosphere and gives crisp lunar and planetary detail, even in poor seeing or a bright sky. Colored filters bring out cloud bands on Jupiter and dust storms on Mars. A neutral density or polarizing filter dims the Moon.
- Best for
- High-speed video of the Moon and planets with a mono camera.
- Example targets
- The Moon, Jupiter, Saturn and Mars.
- Cameras
- Mono planetary cameras. A color camera can use a UV/IR cut.
- Good to know
- These are not for deep-sky pictures.
Solar filters
Safety equipment first
- What it does
- Sunlight can blind you or burn out a camera in moments. A white-light solar filter (such as a certified film or glass filter) fits over the whole front of the scope or lens, and shows sunspots. A hydrogen-alpha solar scope shows prominences and surface detail.
- Best for
- Imaging the Sun, and only with filters made for it.
- Example targets
- Sunspots, granulation, prominences, and solar eclipses.
- Cameras
- Any, with the filter fitted over the front aperture.
- Good to know
- Never use a filter that fits behind the lens or at the eyepiece. Never use ordinary filters, sunglasses or exposed film. Buy certified solar filters only.
Size
Common sizes are 1.25 inch and 2 inch (threaded), and 31 mm and 36 mm (unmounted). Check which one your camera or filter drawer takes. A filter too small for the sensor darkens the corners. Use the size checker above.
Where it goes
Right in front of the sensor, in a filter wheel or drawer, a threaded ring in the camera nose, or a clip-in slot inside a DSLR or mirrorless body. For a lens, there are filters that screw onto the front or slot in behind it.
Focus
A filter adds glass to the light path, so refocus after you fit one. On a mono camera, filters in a set are usually close, but each can need a small focus change.
Flats
Each filter has its own dust shadows. Take flat frames for every filter you use. See the calibration frames guide.
Exposure
Narrow filters need longer exposures. Broadband filters need about the same as no filter. Use the histogram guide to check, and adjust gain or exposure per filter.
Fast optics
Very fast systems (below about f/3) send light through a filter at a steep angle, which moves the passband toward blue. Wide (7 nm or more) filters tolerate it best.
Using a dual-band filter on a galaxy
You would get a few stars and almost no galaxy. Use a broadband or light-pollution filter, or none.
Expecting a filter to beat the Moon
Narrowband filters handle moonlight on nebulae. Broadband filters hardly do. For galaxies, wait for a dark night.
Stacking two filters
A light-pollution filter in front of a dual-band filter adds nothing and can cause reflections.
Forgetting flats for each filter
Dust shadows differ filter to filter. Shoot each set.
Buying a filter to fix a bad site
Filters are a boost, not a cure. A darker sky always beats a filter.
Using an unsafe solar filter
Only use certified solar filters, fitted over the front of the scope or lens.
- Do I need a filter at all?
- Not under a dark sky with a decent camera. Filters earn their cost when you shoot from a suburb, a town or under a Moon, and when you choose nebulae.
- What is the first filter to buy?
- For nebulae with a color camera: a dual-band or tri-band filter. For galaxies from a bright site: a broadband light-pollution filter. For a mono camera: LRGB, then narrowband.
- Can I use a dual-band filter with a regular DSLR?
- Yes. Results are better with a modified camera, because a stock DSLR blocks part of the Hα light. Many people still get good results with a stock camera and longer exposures.
- What does the number of nm mean?
- It is the width of the window that the filter lets through. A 3 nm filter is very narrow and very selective. A 12 nm filter is wider and brighter. Narrower gives more contrast and needs longer exposures.
- Does a light-pollution filter work on the Moon?
- Not well. Moonlight spreads across all colors, so a filter that blocks a few colors can't remove it.
- Why do my stars look odd with a filter?
- Narrow filters can make star color look unusual, and on bright stars you may see halos, mostly from cheap or dirty filters. Clean the filter, keep it in its case, and look for halos in your flats too.
Plan around the Moon with dark nights, find targets in the target library, and shoot flats for every filter.