| This filter threads onto the 2" rear cell of 5" and larger Celestron Schmidt-Cassegrain telescopes and 7" and larger Meade Schmidt- and Maksutov-Cassegrains. Its cell duplicates the telescopes’ 2" rear cell threads, allowing all visual and photographic accessories to be attached securely to the filter. It allows eyepieces to be changed quickly in a star diagonal without having to change the light pollution filter from one eyepiece to another. Because the filter mounts in front of any photo accessories, guide stars in an off-axis guider are visually dimmed in several magnitudes the guiding eyepiece by this type of filter. The filter has an extra-wide 900 Ångstrom visual passband designed to visually enhance both emission and reflection nebulas with all telescope types. It has peak transmission at the visually important emission lines of hydrogen-beta at 4861Å and doubly ionized oxygen at 4959Å and 5007Å. It also has high transmission at the photographically important ionized nitrogen and hydrogen-alpha lines near 6570Å. While its extra-wide passband makes it very good for reflection nebulas, it is somewhat less useful at severely light-polluted sites than a narrowband filter, since some blue-white mercury light pollution is allowed through. Brighter nebulas such as the Dumbbell, Omega, and Lagoon stand out well at suburban sites; faint nebulas such as the Veil, North America, and Pelican are not only visible from suburban sites, but have more detail and higher contrast from dark sky sites. Individually-measured transmission data is provided with each filter. Enter your telescope focal ratio in the formula box to the right to see what focal length eyepieces will work best with this filter.
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