Filter ANTLIA SII 3nm model 50mm circular unmounted filter

The 3 nm Antlia Sulfur II (SII) filter is suitable for imaging SII emission from nebulae, planetary nebulae and supernova remnants.

The 3 nm bandwidth allows high transmission at a central wavelength of 671.6 nm. The extremely narrow bandwidth allows astronomical imaging from highly light-polluted locations, and provides better contrast in darker locations. The filter effectively suppresses interference from other wavelengths, particularly from artificial mercury and sodium lamps.

Narrow-band Antlia filters are designed to bring out the faintest details of nebulae by suppressing unwanted wavelengths in the background sky. The narrower the bandwidth, the more effective the filter. The transmission of the filter reaches 90% at 671.7 nm. The optical density outside the transmission zone is OD3 (0.1%), which provides an excellent signal-to-noise ratio and a clear cut-off of the other lines.

The clear cut-off of the signal outside the transmission zone is an important parameter because it determines the quality of the filtration. The higher the cut-off slope, the higher the contrast between the H-alpha signal and the sky background, which has become perfectly black. In the case of the H-alpha filter, this means that the signal of the NII line (658.4 nm), which is very close to H-alpha, is strongly excluded. In addition, a sharp cutoff minimizes halos around bright stars.

Antlia performs extremely strict quality control during the manufacturing process. Each filter is tested and measured to ensure that it meets the high performance requirements of the specifications. Tight tolerances require perfectly homogeneous substrates and uniform thicknesses; deviations from specifications should not exceed 0.05 mm.

Applications and performance

• Very narrow bandwidth of 3 nm, designed for astronomical imaging of diffuse nebulae, planetary nebulae and supernova afterglow, whatever the sky quality.
• It can be combined with the Antlia 3 nm H-alpha and OIII filters for SHO imaging in the Hubble palette.
• Sharp cut-off of unwanted lines around SII. The transmission of the NII line at 658.4 nm is greatly reduced (only 50% transmission), while it is fully included with other less selective filters. Transmission is 90% at 671.6 nm, which increases the contrast of emitting objects.
• All Antlia filters have blackened edges to eliminate stray reflections.

Technical Specifications

• Glass type: Schott Optical Glass
• Surface Quality: 60/40 (according to MIL-O-13830)
• Mounting diameter: 50 mm without mount
• Substrate thickness 2mm
• 90% transmission in the H-Alpha line of 671.6 nm
• Filter polished on both sides with λ/4" accuracy
• Multilayer and anti-reflective treatment on both sides