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OIII filtersonly allow light with a wavelength of 501 nanometres through. This corresponds with the spectrum lines fordouble ionised oxygen. These lines are emitted by planetary nebulae and some emission nebulae, so these objects remain visible whilst the rest is blocked out by the filter. This effect increases contrast and makes faint nebulae visible for the first time.
Astrophotography:These filters are best used with monochrome cameras. Ideally, images taken using these filters should be combined with images using H-alpha and SII filters. This creates an image which depicts faint nebulae clearly and with high-contrast.