Light Pollution & Filters

The "I live in Bortle 7 and want to image emission nebulae anyway" tier — light-pollution maps and dark-sky finders for site planning, plus optical filters that reject sodium / mercury / LED while passing Hα and OIII. The 2024–26 multi-narrowband filter generation (L-eXtreme, L-Ultimate, NBZ) made urban deep-sky imaging genuinely viable.

For the broader processing pipeline that uses filtered data see Deep-Sky Stacking; for filter wheels that hold these see Filters & Wheels; for cameras / sensors see Astro Cameras & Sensors; for cloud / weather see Astro Cloud & Weather; for sky-quality measurement integrated to HA see Meteor & All-Sky Cameras.

Light-pollution maps

• ★ ★ Light Pollution Map (lightpollutionmap.info) — free, web. The canonical map: VIIRS satellite-based zenith brightness, Bortle scale overlay, world-wide. Plan road trips, find your closest dark site. Free.
• ★ Dark Sky Finder — free Mac/iOS web. Cleaner UI, US-focused, some metric that's a bit less granular than VIIRS but more readable.
• Astrospheric layer — Pro tier, see Astro Cloud & Weather.
• DarkSiteFinder.com — free web; light-pollution + observing-site database with user reviews.
• BlueMarbleNight — free; NASA "Earth at Night" imagery; nice for visual context, less useful for site planning.
• CleanOutside — free, web — see Astro Cloud & Weather.

Filter categories

Three broad classes:

1. Broadband light-pollution filters (e.g., L-Pro / CLS) — pass continuum + bright lines, reject sodium / mercury. For galaxies / clusters / OSC under modest LP.
2. Narrowband single-line filters (Hα / OIII / SII / NII) — pass one emission line; rejection >99%. For mono cameras + LRGB filter wheel.
3. Multi-narrowband filters (L-eXtreme / L-Ultimate / NBZ / Quad-band) — pass two or three lines simultaneously through a single filter; usable on OSC cameras under heavy LP. The 2020s breakout category.

Broadband LP filters

• ★ Optolong L-Pro — paid (~$200 2"). Wider band; passes more continuum; for galaxies / clusters / OSC under suburban Bortle 5–7 skies. Default broadband choice.
• IDAS LPS-D2 (Hutech) — paid (~$280); excellent for nebulae + continuum; the IDAS legacy.
• Astronomik CLS / CLS-CCD — paid (~$160); CCD variant has IR-cut.
• Baader Neodymium / Moon & Skyglow — paid; lighter touch; visual + photo.

Narrowband single-line (mono setups)

• ★ Astrodon Series-E / Series-II — paid; the historical pro standard; expensive (~$300+ each).
• ★ Chroma 3-5 nm Hα/OIII/SII — paid; current pro standard; $400+ each.
• ★ Antlia / Optolong / ZWO 3-7 nm narrowband sets — paid; ~$120–250 each; the affordable narrowband generation that made LRGB+SHO accessible.
• Baader 3.5 / 6.5 nm Hα / OIII / SII — paid; respected.
• NII — useful for special targets (Lagoon Nebula NII rich); rare to own.

Multi-narrowband (the 2020s OSC story)

• ★ ★ Optolong L-eXtreme — paid (~$300 2"). 7 nm dual band — passes Hα and OIII through one filter on a one-shot color (OSC) camera. Demolished urban OSC limits; Bortle 8 skies + L-eXtreme + Sii 294 Pro produces viable Hα-rich nebulae. The 2021–24 default.
• ★ ★ Optolong L-Ultimate — paid (~$400). 3 nm dual band; tighter passband than L-eXtreme; better contrast from the worst skies; slightly tighter star colors. Increasingly the 2024–26 OSC pick over L-eXtreme.
• ★ IDAS NBZ / NBZ-II — paid (~$420). 3 nm dual; 2024 release with shifted bands for better OSC output. Some users prefer over L-Ultimate.
• ★ Antlia ALP-T 5 nm — paid (~$280). Cheaper alternative; popular value pick.
• STC Astro Multispectra — paid; OSC tri-band (Hα + OIII + Hβ); niche.
• SVBONY SV220 — cheaper; brand reputation lower than Optolong/IDAS.

Mounting / stacking

Filters typically clip into the camera nosepiece (1.25"/2") or into a filter drawer/wheel — see Filters & Wheels. Multi-narrowband filters are large in 2"/50 mm; consider sensor size when buying.

Visual filters (for eyepiece use)

• OIII / UHC — Astronomik OIII / UHC, Lumicon OIII, TeleVue Bandmate. Boost emission nebula contrast at the eyepiece dramatically.
• Polaroid lunar filter — for full Moon glare.

Honest limitations

• Filters don't help reflection nebulae or galaxies meaningfully — these emit broadband continuum, not lines.
• Filters don't help broadband stars — they just dim them; expect star-color shifts on mono setups.
• L-eXtreme / L-Ultimate cannot replace true mono narrowband for serious SHO work; they help OSC catch up to the OSC-vs-mono gap, not eliminate it.
• Bortle 9 (city core) is still hard — even L-Ultimate can't fully compensate; consider remote-hosting (Telescope Live, iTelescope.net, AstroCamp).

Sky-quality measurement

Pair filter + light-pollution data with measurement: Unihedron SQM-LU, DIY TSL2591 ESPHome, DSLR / camera SQM math — track over months, time-of-night.

What's changing in 2024–2026

• L-Ultimate / NBZ-II are the new defaults; L-eXtreme sliding to "starter."
• Quad-band OSC filters (Hα + OIII + Hβ + SII) — niche, less effective than dual-band L-Ultimate per nebula.
• Anti-reflection coatings — Optolong / IDAS both improved coatings; halos around bright stars reduced sharply.
• Chinese-OEM 3 nm sets at half the Antlia price; quality variable.

License / pricing summary

• Free: light-pollution maps (Light Pollution Map, Dark Sky Finder, DarkSiteFinder).
• Paid: filters from $80 (entry value) to $400 (L-Ultimate) to $400+ each for pro 3 nm narrowband sets.

Pick this if…

• First filter, suburban OSC: Optolong L-eXtreme or L-Ultimate.
• Dual-band, value: Antlia ALP-T 5 nm.
• Dual-band, premium 2024–26: L-Ultimate or IDAS NBZ-II.
• Galaxies under suburban skies, OSC: Optolong L-Pro.
• Mono LRGB + Hα/OIII/SII: Antlia 3 nm (value), Chroma 3 nm (premium).
• Visual emission-nebula observing: Astronomik OIII or UHC at the eyepiece.
• Find a dark site: Light Pollution Map.