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)
DayStar
Combo QUARK H-Alpha solar filter, for chromosphere
RRP:
$ 1,880.00
Our price:
$ 1,690.00
ready for shipping in 24
h
Coronado
Filters Double Stacking Etalon Solarmax II PST 40mm
$ 1,560.00
ready for shipping in 24
h
DayStar
Filters 0.5Å H-alpha Quantum PE filter
$ 27,400.00
ready for shipping in 3-6
months
Baader
Solar Filter SunDancer II TZ-3S H-alpha 2"/1.25"
$ 3,710.00
ready for shipping in 3-7
days
Baader
Filters K-LINE filter gestackt, 1 ¼ " (with 3,8 astro+solarly 200x290mm photo film)
$ 299.00
ready for shipping in 1-2
weeks
DayStar
QUARK Magnesium I (b2) Line
$ 2,190.00
ready for shipping in 24
h
Lunt Solar Systems
Filters H-alpha double-stack for LS60FHa
$ 3,560.00
ready for shipping in 24
h
Lunt Solar Systems
Filters H-alpha double-stack solar filter LS100FHa
$ 11,600.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Solar Filters Ca-K module for LS130MT/B3400
$ 3,670.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters LS50C double-stack filter
$ 1,780.00
ready for shipping in 1-2
weeks
DayStar
QUARK Sodium D-Line
RRP:
$ 1,880.00
Our price:
$ 1,690.00
ready for shipping in 24
h
DayStar
QUARK Calcium-H-Line
RRP:
$ 1,880.00
Our price:
$ 1,690.00
ready for shipping in 24
h
DayStar
QUARK H-Alpha solar filter for Nikon DSLR, chromosphere model
$ 2,190.00
ready for shipping in 3-6
months
Lunt Solar Systems
Filters H-alpha LS50FHa/B1800d1
$ 3,870.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters Ca-K module with 18mm blocking filter in star diagonal for 2" focuser
$ 3,040.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Ca-K Module with 18mm blocking filter in extension tube for 2" focuser
$ 3,040.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters LS 100F Ha H-Alpha filter set for 3400mm focal length
$ 14,700.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters H-Alpha Doublestack LS40F
$ 1,050.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters Ca-K module with 12mm blocking filter in extension tube for 2" focuser
$ 2,620.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters H-alpha LS50FHa/B600d1
$ 2,620.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters H-alpha LS50FHa/B600d2
$ 2,620.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters Ca-K module with 12mm blocking filter in star diagonal for 2" focuser
$ 2,620.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters LS 100F Ha H-Alpha filter set for 1800mm focal length
$ 13,100.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters 100mm Ha Etalon filter with B1800 for 2" focuser
$ 13,100.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters LS 100F Ha H-Alpha filter set for 1200mm focal length
$ 12,500.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters 100mm Ha Etalon filter with B1200 for 2" focuser
$ 12,500.00
ready for shipping in 1-2
weeks
DayStar
QUARK H-alpha filter, prominences
RRP:
$ 1,880.00
Our price:
$ 1,690.00
ready for shipping in 3-6
months
Lunt Solar Systems
Filters LS 100F Ha H-Alpha filter set for 600mm focal length
$ 11,900.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters 100mm Ha Etalon filter with B600 for 2" focuser
$ 11,900.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Ca-K module with 2", 6mm blocking filter in extension tube for 2" focuser
$ 2,100.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Solar Filters Ca-K Module for LS130MT
RRP:
$ 2,930.00
Our price:
$ 2,830.00
ready for shipping in 1-2
weeks
Lunt Solar Systems
Filters Ca-K module with 6mm blocking filter in star diagonal for 2" focuser
$ 1,990.00
ready for shipping in 1-2
weeks
DayStar
Combo QUARK H-Alpha solar filter, for prominences
RRP:
$ 1,880.00
Our price:
$ 1,690.00
ready for shipping in 3-6
months
Lunt Solar Systems
Filters LS60FHa B600 1.25" etalon filter system
$ 3,980.00
ready for shipping in 1-2
weeks
DayStar
QUARK H-alpha filter,chromosphere
$ 1,880.00
ready for shipping in 3-6
months
Lunt Solar Systems
Filters 50mm Ha Etalon filter
$ 2,040.00
ready for shipping in 1-2
weeks