The 1.25" Astro-Tech broadband filter has a moderately wide 690 Ångstrom FWHM (Full-Width at Half-Maximum transmission) passband for visual use on emission nebulas in slight to moderately light polluted areas. It typically has a peak transmission approaching 97% at the visually important emission lines of hydrogen-beta at 4861Å and both doubly ionized oxygen lines at 4959Å and 5007Å.
While it has acceptable performance on reflection nebulas due to its moderate passband, a wider passband filter with more transmission below 460nm would be more useful on this type of object, as the wider passband would show more reflected starlight detail. The Astro-Tech filter has very steep passband cutoff slopes that yield higher contrast than competitive filters of a similar passband width that have more gradual transition slopes.
Transmission at the photographically important ionized nitrogen and hydrogen-alpha lines near 6570Å is reasonably good (in the 80-85% transmission range). While this transmission value makes the Astro-Tech broadband filter acceptable for astrophotography, it is probably best suited primarily for visual use with 1.25" eyepieces.
Its performance compares favorably with that of the best-known broadband filter, but at only a fraction the price of the best-known filter. The filter comes packed in a foam-lined snap-lid plastic storage case.
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