The H-Beta 25nm filter allows you to visually observe the popular but elusive Horsehead Nebula with a large aperture telescope in dark skies, and can also be used to image the Horsehead Nebula and other nebulae emitting 486nm light (such as the California Nebula) and cocoon nebulae. Features: 2. Laser engraving to prevent fading 3. Apply multilayer anti reflective non adhesive coatings through electron beam gun evaporation and ion assisted deposition 4. The filter substrate undergoes fine optical polishing to achieve 1/4 wave and 30 parallelism on both surfaces 6. Each filter comes with a plastic casing lined with highpressure EVA casing Specifications: 1. The thin metal battery has a 26 millimeter aperture and is suitable for 1.25inch filters 2. Blocking area: 400-700nm 3. Transmittance: 80% at 486nm 4.0.1% transmittance mercury at 435nm and 578nm 5.0.1% transmittance of Na at 589nm 6. Half width: 25 ± 3nm 7. Thickness: 1.85mm 8. Surface quality: 60/40 (refer to MIL-O-13830) 9. Thread size: M28.5 * 0.6 10. Net weight: 5.4g/0.19oz Warning: Filters are not designed for sun observation. Do not use this filter to look at the sun. If you do not follow the warning, you will go blind. The package includes: SV132 1.25 H-Beta filter