Antlia 3nm Narrowband SII Pro Highspeed Filter - f/2.6 to f/3.6

The Pre-Shifted 673.6nm Center Wavelength for f/2.6-f/3.6 Optics

In a fast optical system, the steep angle of the light cone causes a standard interference filter's passband to shift toward shorter (blue) wavelengths, reducing transmission of the target emission line. A simulation shows that at f/2.6 the angle of incidence is 10° and the band shifts 2.2nm toward shorter wavelengths. The Highspeed filter is pre-shifted 1.2nm toward the red, to 673.6nm, so the SII lines at 671.6nm and 673.1nm stay inside the band on f/2.6 to f/3.6 telescopes. There, these filters give much stronger and more contrasted images than the standard Pro version.

For optics slower than f/3.6, Antlia highly recommends the standard 3nm Pro filters; for optics faster than f/2.6 it calls its 4.5nm EDGE filters the good choice.

3nm Bandwidth and 90% Peak Transmission

The 3nm full width at half maximum (FWHM) passes much less city skyglow and moonlight around the SII lines than a wider band. Targets include emission nebulae, planetary nebulae, Wolf-Rayet nebulae and supernova remnants, and the filter can be used in light-polluted areas and at dark sites and extends imaging time when the moon is up.

Peak transmission is 90%.

OD5 Blocking from 300-1000nm

Blocking is rated at optical density (OD) 5 or better across 300-1000nm: no more than 0.001% of out-of-band light gets through.

This delivers an excellent signal-to-noise ratio and an effective cut-off that minimizes interference from other wavelengths, and its optical reliability reduces post-processing.

Lambda/4 Wavefront on a Single Substrate

The filter is built on a single, non-glued optical substrate, which eliminates internal reflections; the sharp cut-off of the 3nm Pro series minimizes halos around bright stars. The transmitted wavefront is Lambda/4 or better, parallelism is less than 30 arcseconds and surface quality is 60/40 scratch/dig.

• Thickness: 2mm ±0.05mm for 1.25", 2", 31mm and 36mm; 3mm ±0.05mm for 50mm.
• Stray Light Control: All unmounted filters feature edge-blackening to absorb stray light and prevent internal reflections within the filter itself.
• Mounted Cell: Ultra-thin, black anodized, male M28.5x0.6 (1.25") or M48x0.75 (2") thread; it minimizes vignetting by maximizing the clear aperture.

Antlia 3nm Highspeed vs. 3nm Pro: Choosing the Right Filter

Antlia matches its series to the focal ratio:

• This Filter (3nm Pro Highspeed): For f/2.6 to f/3.6.
• Antlia 3nm Pro (Standard): Not pre-shifted; Antlia highly recommends it for optics slower than f/3.6.
• Antlia 4.5nm EDGE: The recommended choice for optics faster than f/2.6.