A. About Optolong's S-II (6.5nm) FIlter

The Optolong S-II (Sulfur-III) extra narrowband filter is designed to improve the visibility of various deep sky objects. It does this by:

• Selectively reducing the transmission of wavelengths of light pollutants, specifically those produced by artificial lighting (including mercury vapor lamps, both high & low pressure sodium vapor lights, and the unwanted natural light caused by neutral oxygen emission in our atmosphere (i.e. skyglow)).
• Allowing 6.5nm bandwidth of light centered on a wavelength of 672nm to pass through, which corresponds to SII emission lines.

B. Main Uses and Performance

• Use with H-alpha and OIII-CCD narrowband filters (SHO Set) for tricolor CCD astrophotography.
• Narrowband imaging with SHO set can be done with the moon up in heavy light pollution, so your equipment is not sitting dormant for several weeks.
• Hubble look of images can be made by the combination of H-alpha, OIII-CCD and SII-CCD images, such as the famous "Pillars of Creation" (M16 Eagle Nebula).

C. Spectral Curve Transmission Characteristics

• 80% transmission at SII line 672nm
• 0.1% transmission of major transmission lines of artificial light pollution:
  • Hg (Mercury) at 435.8nm, 546.1nm, 577nm, and 578.1nm.
  • Na (Sodium) at 598nm, 589.6nm, 615.4nm, and 616.1nm.
• It also blocks out infrared wavelengths (700-1100nm) so a separate IR cut-off filter is not required for imaging applications.

D. Technical Specifications

• Both surfaces are fine optically polished, accurate to 1/4 wavelength or better, and parallel to within 30 seconds or arc.
• 2mm thick Schott B270 optical glass substrate material.
• Very high surface quality of 60/40 or better (US Military Spec MIL-O-13830).

E. Optical Coating Specifications

• Multi layer anti-reflection coated.
• Non-cementing optical substrate coating.
• Electron-beam gun evaporation with Ion-assisted deposition coating technology for durability and resistance to scratching, as well as stability on CWL (central wavelength) so that there is no deviation due to temperature changes during use.
• Precise and homogeneous coatings ensure high values of desired passband and blocking of unwanted offband.
• Every single filter is tested and passed by Optolong before shipping to dealers.

F. Other

• 1.25" astronomy filter with standard M28.5x0.6 male filter thread (no female thread, so it cannot be stacked with other filters).
• Ultra-thin filter cell to minimize vignetting; clear optical aperture of 26mm.
• Precision CNC-machined metal filter cell with sand blasted black anodized finish, anti-reflection treatment, and laser engraving.
• High quality protective plastic storage case.

WARNING: Optolong filters are not designed for direct solar observation. NEVER use these filters for observing the Sun, or permanent eye and equipment damage will result.