Narrowband (NB) filters enhance contrast of emission objects by accepting only a narrow range of wavelengths around the emission lines of hydrogen (H-a, 656 nm), oxygen (OIII, 501 nm), sulfur (SII, 672nm) and others.

They can be used to image when the moon is up, thereby extending imaging time. They can be used in light-polluted locations. The narrow range of wavelengths is defined as the FWHM (full-width at half- maximum intensity). Narrower filters decrease the background noise. However, narrower filters are more difficult to manufacture consistently, and are thus more expensive. Furthermore, it is difficult to maintain high transmission through the bandpass of the filter as it becomes narrower. If the peak transmission decreases as the filter is made narrower, the emission signal decreases and the gain in S/N (signal-to-noise) is not realized. Astrodon has achieved this goal of high transmission for narrower filters. Astrodon has evolved its narrowband product line from 6 nm to 5 nm FWHM, significantly lowered the prices on all filters, and added the ultra-narrow 3 nm FWHM filters for H-a, OIII and SII. We added a Red Continuum filter (645 nm, 5 nm FWHM) that produces a star map without the H-a or SII emission in order to subtract stars from emission images. Astrodon NB filters are renowned for not producing halos around stars and not leaking NIR light. The 3 nm FWHM filters are stocked and are not custom ordered. The 3 nm OIII filter provides the best protection from the effects of moonlight, producing half the background signal of a 90% T, 6 nm FWHM filter, resulting in an increase in S/N of ~21%. This will even be larger in comparison to wide 7 or 8 nm FWHM filters. * 28 mm dia (1.25" mounted)., 50.4 mm dia, 49.7 x 49.7 mm square unmounted * 3 mm (+/-0.025) thick * Parfocal to f/4 and parfocal with Astrodon LRGB depending on telescope optics * H-a, OIII, SII, NII, Red Continuum * 5 nm FWHM filters guaranteed >90% transmittance (typically 94-97%) * 3 nm FWHM filters typically 90-92% (scan included) * >4 O.D. (out-of-band blocking) 300 - 1150 nm * 1/4 wave, striae-free fused silica substrates * 30 arcsecond parallelism * sides are blackened * hardest available sputtered coating for durability * 3 nm filters can be used on systems to f/3 * designed to have lower blue spectral shift with faster (e.g. f/3) optics. Astrodon's Narrowband Filter FAQ