FILTRE ULTRA HIGH CONTRAST 1,25" / ULTRA HIGH CONTRAST 1,25" FILTER
This 1.25" Astro-Tech ultra-high contrast filter has a narrow 250 Ångstrom FWHM (Full-Width at Half-Maximum transmission) passband for visually observing emission nebulas from badly light-polluted areas. It typically has a peak transmission in the 95-96% region at the visually important emission lines of hydrogen-beta at 4861Å and both doubly ionized oxygen lines at 4959Å and 5007Å. This high transmission combines with the narrow passband to make emission nebulas stand out briskly against a dark sky. It can also be useful in hunting down and enhancing very faint nebulas from a dark sky observing site. It is not designed for astrophotography or for observing reflection nebulas or galaxies.
There is some minor leakage in the deep ultra-violet portion of the visible spectrum, but the transmission of the mercury light pollution lines near 4050Å and 4370Å is essentially zero. Only the youngest observers, those whose vision still extends into the deep ultra-violet, might see a trace of chromatic aberration if this filter is used on a low quality achromatic refractor. Older observers, whose ultra-violet vision started to degrade when they passed 25 years, are unlikely to see any trace of the leakage.
This Astro-Tech narrowband filter has a passband that is 15-20% narrower than some competitive filters, plus very steep passband cutoff slopes, to give it higher contrast on very faint objects. It has 2 to 4 times higher contrast than broadband filters, making it ideal for faint nebulas such as the Helix, Rosette, Veil, and North American. Brighter nebulas (such as the Omega, Dumbbell, Lagoon, etc.) stand out well under city lights.