Planetary · Nebula · Light pollution · Solar

Telescope filters: what they can and cannot fix

A filter only removes light. It helps when the light it removes is something you do not want, such as streetlight or glare from a bright planet, and the light it keeps is the target. That one test sorts every filter on the market.

400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm White LED street light: light at almost every wavelength Low-pressure sodium: one line, easy to block High-pressure sodium: a broad hump H-beta O III H-alpha S II Mercury Mercury

Swipe sideways to see the full chart.

Old sodium streetlights glowed at a few wavelengths a filter could block. White LEDs fill the whole spectrum, so only filters that pass just the emission lines of nebulae (white marks) still help.
Filter threads
1.25″ and 2″screw into the eyepiece barrel
Nebula lines
H-beta, O III, H-alphawhat line filters pass
LED street lights
broad spectrumdefeat broadband filters
Solar filters
front onlynever at the eyepiece
First filter to buy
UHCfor emission nebulae

Six kinds of filter, each for a different job

Filter types
FilterWhat it passesUse it forMore
Color (Wratten)One color band, broadBelts on Jupiter, dust storms on Mars, clouds on VenusPlanetary filters
Moon (neutral density or polarizing)A fraction of all lightCutting glare from the Moon and VenusMoon filters
Broadband light pollutionAll but sodium and mercury linesSodium-lit skies; less use under LEDsBelow
Narrowband (UHC)H-beta and O III, about 25 nmEmission and planetary nebulaeNebula filters
Line (O III, H-beta)One line, about 10 nmPlanetary nebulae, the Veil, the HorseheadNebula filters
Imaging narrowband and dual-bandH-alpha, O III, S II, 3 to 7 nmAstrophotography from citiesImaging filters
SolarOne part in 100,000, or a single lineThe Sun, safelySolar filters

LED street lighting has made broadband light-pollution filters nearly useless

Broadband light-pollution filters, also called LPR or CLS filters, were designed for the sodium and mercury lamps that lit cities until the 2010s. Those lamps emit at a few wavelengths, which a filter can block while passing most of the rest. White LEDs emit a smooth spectrum from blue to red, so blocking them means blocking nearly everything. Under LED skies, a broadband filter mostly makes the view darker.

Line filters still work, because nebulae shine at a few exact wavelengths: hydrogen-beta at 486 nm, doubly ionized oxygen (O III) at 496 and 501 nm, and hydrogen-alpha at 656 nm. A filter that passes only those lines lets the nebula through and stops most of the sky glow, LED or not. They do nothing for galaxies and star clusters, which shine in starlight at all wavelengths. For those, only a darker site helps.

Solar filters go over the front of the telescope, never at the eyepiece

Old telescopes sometimes came with a dark glass "sun filter" that screwed into the eyepiece. Focused sunlight can crack one in seconds, with no warning, while you are looking. A safe solar filter covers the whole front of the telescope and finder, so the light is cut before it is focused. See solar filters.