Line filters selectively allow only light from a specific spectral line to pass through. OIII, SII and H-alpha filters are especially popular.
The Roman numerals III and II indicate the degree of ionisation of the chemical element that produces the spectral line. So OIII (pronounce oh three) stands for doubly ionised oxygen, and SII for singly ionised sulphur. OI and SI indicate the electrically neutral atoms.
Line filters are ideal for the visual observation and photography of emission nebulae, supernova remnants and planetary nebulae, since these objects produce defined spectral lines. Star clusters and galaxies on the other hand have a continuous spectrum. For these, filters that generally suppress the sky’s brightness (CLS or UHC filters) are more suitable.
Example of an emission line spectrum (source: Wikipedia)
Omegon
Filters Pro 2'' SII CCD filter
( 1 / 5 )
$ 189.00
ready for shipping in 24
h
Optolong
Filters SII Filter 2"
RRP:
$ 283.00
Our price:
$ 271.00
ready for shipping in 24
h
Astronomik
Filters SII 6nm CCD MaxFR 2"
$ 810.00
ready for shipping in 3-7
days
Omegon
Filters Pro SII 7nm Filter 1.25"
RRP:
$ 179.00
Our price:
$ 149.00
ready for shipping in 24
h
Astronomik
Filters SII 6nm Clip Canon EOS APS-C
$ 354.00
Product can be shipped as of 15.07.2025
Baader
Filters SII CMOS Narrowband 2"
$ 270.00
ready for shipping in 1-2
weeks
Optolong
SII Filters 1.25"
$ 171.00
ready for shipping in 24
h
Omegon
Filters Pro 1.25'' SII CCD filter
$ 129.00
ready for shipping in 24
h
Optolong
Filters Pro Veil SII 3nm 2''
$ 1,070.00
ready for shipping in 24
h
Astronomik
Filters SII 6nm CCD MaxFR 50mm
$ 810.00
ready for shipping in 3-7
days
Explore Scientific
Filters SII 12nm 1.25"
RRP:
$ 141.00
Our price:
$ 71.00
ready for shipping in 1-2
weeks
Explore Scientific
Filters SII 12nm 2"
RRP:
$ 224.00
Our price:
$ 113.00
ready for shipping in 1-2
weeks
Astronomik
Filters SII 6nm 2"
$ 580.00
ready for shipping in 3-7
days
Baader
Filters SII CMOS Ultra-Narrowband 36mm
$ 377.00
ready for shipping in 1-2
weeks
Baader
Filters SII CMOS Ultra-Narrowband 31mm
$ 315.00
ready for shipping in 1-2
weeks
Chroma
Filters Filter SII 36mm unmounted, 3nm
$ 1,000.00
ready for shipping in 3-5
weeks
Astronomik
Filters SII 6nm 36mm
$ 354.00
ready for shipping in 3-7
days
Baader
Filters SII CMOS Narrowband 31mm
$ 168.00
ready for shipping in 1-2
weeks
Baader
Filters SII CMOS f/2 Ultra-Highspeed 65x65mm
$ 640.00
ready for shipping in 1-2
weeks
Baader
Filters SII CMOS Ultra-Narrowband 65x65mm
$ 630.00
ready for shipping in 1-2
weeks
Baader
Filters SII CMOS f/3 Ultra-Highspeed 65x65mm
$ 630.00
ready for shipping in 1-2
weeks
Astronomik
Filters SII 6nm CCD 1.25"
$ 236.00
ready for shipping in 24
h
Baader
Filters Narrowband SII CMOS 1.25"
$ 159.00
ready for shipping in 1-2
weeks
Astronomik
Filters SII 6nm CCD MaxFR Clip Nikon XL
$ 830.00
ready for shipping in 3-7
days
Astronomik
Filters SII 6nm CCD MaxFR Clip Sony alpha 7
$ 830.00
ready for shipping in 3-7
days
Astronomik
Filters SII 6nm CCD MaxFR Clip Canon EOS R XL
$ 830.00
ready for shipping in 3-7
days
Astronomik
Filters SII 6nm CCD MaxFR Clip Canon EOS XL
$ 820.00
ready for shipping in 3-7
days
Baader
Filters Ultra-Narrowband SII CMOS 50.4mm
$ 460.00
ready for shipping in 24
h
Chroma
Filters SII 3nm 2"
$ 1,980.00
ready for shipping in 3-5
weeks
Baader
Filters SII CMOS f/2 Highspeed 65x65mm
$ 428.00
ready for shipping in 24
h
Astronomik
Filters SII 12nm CCD MaxFR 1.25"
$ 212.00
ready for shipping in 3-7
days
Baader
Filters SII CMOS Narrowband 65x65mm
$ 417.00
ready for shipping in 24
h
Baader
Filters SII CMOS Ultra-Narrowband 50x50mm
$ 469.00
ready for shipping in 1-2
weeks
Baader
Filters SII CMOS f/2 Ultra-Highspeed 50x50mm
$ 472.00
ready for shipping in 1-2
weeks
Baader
Filters SII CMOS f/3 Ultra-Highspeed 50x50mm
$ 471.00
ready for shipping in 1-2
weeks
Chroma
Filters SII 5nm 2"
$ 1,820.00
ready for shipping in 3-5
weeks