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)
ZWO
Filters H-alpha, SII, OIII 7nm 2"
RRP:
$ 1,040.00
Our price:
$ 1,010.00
ZWO
Filters 1.25" filter set: H-alpha, SII, OIII
$ 580.00
ZWO
Filters Filter set H-alpha, SII, OIII 31mm unmounted
RRP:
$ 670.00
Our price:
$ 580.00
Askar
Filters Color Magic D1 OIII/H-Alpha Duo 2"
$ 438.00
Astronomik
Filters H-alpha 12nm MaxFR XT Clip Sony alpha 7
$ 570.00
Baader
Filters H-alpha/OIII/SII CMOS Narrowband 2"
$ 820.00
Astronomik
Filters H-alpha 12nm MaxFR XT Clip Canon EOS R XL
$ 570.00
ZWO
Filters Filter set H-alpha, SII, OIII 36mm unmounted
$ 770.00
Baader
Filters SII CMOS Ultra-Narrowband 2"
$ 449.00
Astronomik
Filters H-alpha 12nm MaxFR XT Clip Nikon Z XL
$ 570.00
