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"
$ 468.00
Omegon
Filters OIII Filter 2"
$ 119.00
Omegon
Filters OIII filter 1.25''
$ 89.00
Optolong
Filters SHO Filter Kit 7nm 2"
$ 840.00
Astronomik
Filters 1.25" OIII filter
$ 116.00
Optolong
Filters SII Filter 2"
RRP:
$ 280.00
Our price:
$ 269.00
Omegon
Filters Pro 2'' H-Beta filter
$ 139.00
Optolong
Filters L-Ultimate 1,25"
$ 304.00
Omegon
Filters Pro 1.25'' H-Beta filter
$ 99.00
Omegon
Filters 7nm H-alpha 1.25 pro filter
$ 199.00
Omegon
Filters Pro 2'' SII CCD filter
$ 189.00
Astronomik
Filters H-alpha 12nm CCD Clip Canon EOS APS-C
$ 234.00
Baader
Filters H-alpha CMOS Narrowband 1.25"
$ 163.00
Omegon
Filters Pro 1.25'' OIII CCD filter
$ 129.00
Baader
Filters OIII CMOS Narrowband 1.25"
$ 160.00
Astronomik
Filters HSO 6nm 36mm
$ 880.00
Omegon
Filters Pro SII 7nm Filter 1.25"
RRP:
$ 179.00
Our price:
$ 149.00
Astronomik
Filters H-alpha 12nm CCD Clip-Filter Sony alpha
$ 398.00
Astronomik
Filters HSO 12nm 36mm
$ 590.00
ZWO
Filters 1.25" filter set: H-alpha, SII, OIII
RRP:
$ 670.00
Our price:
$ 548.00
Astronomik
Filters 2" OIII filter
$ 234.00
Optolong
Filters SHO Filter Kit 1,25"
$ 620.00
Astronomik
Filters H-alpha 6nm CCD Clip Canon EOS R XL
$ 620.00
Astronomik
Filters SII 6nm CCD MaxFR 2"
$ 800.00
Baader
Filters H-alpha/OIII/SII CMOS f/2 Highspeed 1.25"
$ 491.00
Baader
Filters H-alpha 20nm 2"
$ 194.00
Astronomik
Filters H-alpha 12nm MaxFR XT Clip EOS R XL
$ 580.00
Baader
Filters OIII CMOS Ultra-Narrowband 50.4mm
$ 451.00
Baader
Filters OIII CMOS Ultra-Narrowband 1.25"
$ 286.00
Astronomik
Filters OIII 6nm CCD 2"
$ 580.00
Astronomik
Filters H-alpha 12nm CCD MaxFR Clip Sony alpha 7
$ 530.00
Baader
Filters H-alpha 20nm 1.25"
$ 111.00
Baader
Filters H-Beta CMOS Narrowband 2"
$ 346.00
Baader
Filters H-alpha CMOS f/3 Ultra-Highspeed 1.25"
$ 312.00
Optolong
Filters SHO 3nm 2"
$ 1,510.00
