The ALP-T dual-band filter transmits the deep red [S II] light (672.4 nm) from sulphur and the blue-green [O III] light (500.7 nm) from oxygen, whilst blocking all other wavelengths. In particular, it blocks the unwanted wavelengths caused by light pollution and airglow. This allows you to take high-quality deep-sky images even in moonlight and near urban areas. The signal-to-noise ratio is significantly improved, enabling even the faintest nebulae to be captured.
Outside these two spectral lines, the filter blocks wavelengths from 300 to 1100 nanometres with an optical density greater than 4.5.
