Colour filter
Also called: color filter, planetary filter, Wratten.
A tinted filter (light blue, orange, red…) that raises the contrast of particular features on planets.
Planet details have colour: Jupiter's red-brown belts stand out through a light blue (#80A) filter; Mars's dark markings through orange or red (#21, #23A); Venus's glare drops with a violet or dark blue one. The effect is subtle but real, especially in larger telescopes.