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)
Optolong
Filters L-Ultimate 2"
$ 412.00
ready for shipping in 24
h
Omegon
Filters OIII filter 1.25''
$ 79.00
ready for shipping in 24
h
Omegon
Filters OIII Filter 2"
( 5 / 5 )
$ 99.00
ready for shipping in 24
h
Optolong
Filters SHO Filter Kit 7nm 2"
$ 800.00
ready for shipping in 24
h
Optolong
Filters L-Synergy 2"
$ 389.00
ready for shipping in 24
h
Optolong
Filters L-Ultimate 1,25"
$ 309.00
ready for shipping in 24
h
Astronomik
Filters 1.25" OIII filter
$ 114.00
ready for shipping in 24
h
Omegon
Filters Pro Veil H-Alpha OIII 7nm 2''
$ 239.00
ready for shipping in 24
h
Omegon
Filters Pro 2'' H-Beta filter
$ 119.00
ready for shipping in 24
h
Askar
Filters Color Magic D2 OIII/SII Duo 2"
$ 435.00
ready for shipping in 24
h
Askar
Filters Color Magic D1 OIII/H-Alpha Duo 2"
$ 435.00
ready for shipping in 24
h
ZWO
Filters Filter set H-alpha, SII, OIII 36mm unmounted
RRP:
$ 760.00
Our price:
$ 680.00
ready for shipping in 3-5
weeks
Chroma
Filters OIII 36mm unmounted, 3nm
RRP:
$ 1,370.00
Our price:
$ 1,200.00
ready for shipping in 24
h
Omegon
Filters Pro OIII 7nm Filter 1,25"
$ 189.00
Sale price:
$ 132.00
ready for shipping in 24
h
Baader
Filters H-alpha/OIII/SII CMOS f/3 Ultra-Highspeed 2"
$ 1,300.00
ready for shipping in 1-2
weeks
ZWO
Filters 1.25" filter set: H-alpha, SII, OIII
RRP:
$ 580.00
Our price:
$ 520.00
ready for shipping in 3-5
weeks
Omegon
Filters Pro 1.25'' OIII CCD filter
$ 129.00
Sale price:
$ 90.00
ready for shipping in 24
h
Optolong
Filters L2 Dual-Combo Set 2"
$ 690.00
ready for shipping in 24
h
Omegon
Filters Pro Veil H-Alpha 7nm 2''
$ 239.00
ready for shipping in 24
h
ZWO
Filters Filter set H-alpha, SII, OIII 31mm unmounted
$ 670.00
ready for shipping in 3-5
weeks
Astronomik
Filters H-alpha 12nm MaxFR XT Clip Sony alpha 7
$ 570.00
ready for shipping in 1-2
weeks
Baader
Filters H-alpha/OIII/SII CMOS Narrowband 50.4mm
$ 930.00
ready for shipping in 1-2
weeks
Baader
Filters H-alpha/OIII/SII CMOS Ultra-Narrowband 1.25"
$ 890.00
ready for shipping in 1-2
weeks
Astronomik
Filters 2" OIII filter
$ 229.00
ready for shipping in 24
h
Astronomik
Filters H-alpha 4nm 36mm
$ 458.00
ready for shipping in 24
h
ZWO
Filters Narrowband Filter Set H-alpha, SII, OIII 2"
RRP:
$ 1,120.00
Our price:
$ 1,000.00
ready for shipping in 24
h
Omegon
Filters Pro 1.25'' H-Beta filter
$ 99.00
ready for shipping in 24
h
Astronomik
Filters H-alpha 12nm XT Clip Nikon Z XL
$ 435.00
ready for shipping in 1-2
weeks
Baader
Filters SII CMOS Ultra-Narrowband 2"
$ 445.00
ready for shipping in 1-2
weeks
Astronomik
Filters HSO 12nm 2"
$ 880.00
ready for shipping in 3-7
days
Astronomik
Filters H-alpha 12nm MaxFR XT Clip Nikon Z XL
$ 570.00
ready for shipping in 1-2
weeks
Optolong
Filters SHO 3nm 36mm
$ 960.00
ready for shipping in 3-5
weeks
Baader
Filters H-alpha CMOS Narrowband 1.25"
$ 171.00
ready for shipping in 1-2
weeks
Baader
Filters OIII CMOS Narrowband 50x50mm
$ 351.00
ready for shipping in 1-2
weeks
Baader
Filters H-alpha CMOS Narrowband 50x50mm
$ 346.00
ready for shipping in 1-2
weeks
Astronomik
Filters HSO 6nm 36mm
$ 860.00
ready for shipping in 3-7
days