Hydrogen-Alpha H-Alpha12nm CCD Filter (1.25") by OVL
A good value, quality filter from OVL (Optical Vision Ltd, the official Skywatcher distributor in the UK and Western Europe).
This long-pass filter limits transmission of light with wavelengths longer than 650nm. Whilst suppressing all other polluting lights, the filter allows you to observe emission nebulae with greatly enhanced contrast. Ideal for observing emission nebulae or objects that emit in the H-alpha region. Suitable only for CCD photography.