Astro-Tech eyepiece color filters can be a valuable tool for the serious lunar and planetary observer. They reduce glare and light scattering; increase the contrast of surface features through selective filtration; increase definition and resolution; reduce irradiation; and lessen eye fatigue.
Each Astro-Tech filter.threads into the barrel of virtually any 1.25" eyepieces available today (except Vernonscope and Questar Brandons, which require Questar/Vernonscope brand filters). The filters are available separately to allow you to build a filter collection matched to your observing needs.
Each filter uses polished plane-parallel optical glass of excellent homogeneity to minimize internal reflections and ghosting. They are antireflection coated to prevent glaring and ghosting and are mounted in low reflectivity matte black cells with thin knurled grip rings for a more secure hold in cold weather. The filter number is printed on the side of each filter to eliminate guessing about which color you're looking at when examining a filter at night with a red flashlight. Each filter has dual threads to allow you to stack two filters together – to combine a neutral density filter with a color filter to simultaneously reduce glare and enhance contrast, for example.
Filters 8, 11, 12, 15, and 82A are generally used with small scopes in the 5” or 6” and smaller range. They can be used in larger scopes, but may require the addition of a neutral density filter with a larger scope.
Filters 21, 23A, 25A, 56, 58, and 80A are most often used with medium sized scopes in the 6” to 10” or 11” aperture range.
Filters 25A, 29, 38A, and 47 are typically best suited to large aperture scopes, although they can also be used with scopes in the 8” to 11” range.
Each 26mm clear aperture filter is packed in a foam-lined plastic storage case. Those who have more than one filter might find optional filter case #FP4 useful to keep all the filters in one convenient container.