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Antlia 3nm Narrowband NII Pro Filter - 2"
The Antlia 3nm Narrowband NII Pro Filter isolates the specific Nitrogen-II emission line at a central wavelength of 654.8nm, delivering maximum contrast for faint nebulae. With an extremely tight 3nm full width at half maximum (FWHM), this filter achieves an impressive 88% peak transmission while maintaining greater than OD5 out-of-band blocking to darken the sky background. Built on a single, non-glued substrate with a transmitted wavefront of Lambda/4 or better and less than 30 arcseconds of parallelism, the Antlia 3nm NII Pro can be used down to f/3 with minimal loss of signal.
3nm Bandpass: High-Contrast NII Imaging at 654.8nm
The core of this filter's design is its ultra-narrow 3nm bandpass centered on the 654.8nm NII line. This narrow bandpass enhances the contrast of emission targets by reducing unwanted background signals, and Antlia notes it can extend imaging time when the moon is up. The filter is only effective for deep-sky objects that contain NII emissions, and its data can be combined with other narrowband channels in post-processing.
Advanced Optical Design: Lambda/4 Substrate and <30 Arcsecond Parallelism
Antlia builds the 3nm Pro series on a single, non-glued optical substrate with a thickness of 2mm. Antlia uses a single substrate to eliminate internal reflections, and a steep spectral profile minimizes halos around bright stars. The filter is specified with a transmitted wavefront of Lambda/4 or better and parallelism under 30 arcseconds.
Usable Down to f/3 and >OD5 Blocking in a 2" Cell
Antlia states that its narrowband Pro filters can be used down to f/3 with minimal loss of signal. Its greater than OD5 blocking, which attenuates unwanted light by a factor of 100,000 across the 300-1100nm spectrum, ensures a high signal-to-noise ratio even under challenging conditions. The filter is housed in an ultra-thin, black anodized, laser engraved 2" (M48x0.75) cell that minimizes vignetting by maximizing the clear aperture.
3nm vs. Wider Filters: The High-Contrast Trade-Off
Choosing a 3nm filter over a wider 5nm or 7nm alternative prioritizes contrast. The tighter bandpass rejects more background light, which helps when imaging from light-polluted locations or when the moon is up. This increased contrast comes with a trade-off: a 3nm filter requires longer individual exposure times to achieve the same signal level as a wider filter.
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What is the Antlia 3nm NII Pro filter used for in astrophotography?
The Antlia 3nm NII Pro filter is a specialized narrowband filter designed to capture light specifically from the Nitrogen-II (NII) emission line at 654.8nm. It's used to isolate and image structures within emission nebulae that glow in this wavelength. Antlia notes it is only effective for deep-sky objects that contain NII emissions. The data can be combined with other narrowband channels, such as H-alpha, OIII or SII, in post-processing.
How does the Antlia 3nm NII Pro filter perform in a fast telescope such as an f/4 Newtonian?
Antlia states that all its narrowband Pro filters can be used down to f/3 with minimal loss of signal, so an f/4 system is within that range.
Can I use the Antlia 3nm NII Pro filter for visual observation?
No. Antlia lists night visual observation as not recommended for this filter. It is designed for astrophotography of deep-sky objects that contain NII emissions.
What is the advantage of the Antlia NII Pro's 3nm bandpass over a wider 7nm filter?
The primary advantage of the 3nm bandpass is higher contrast and better rejection of unwanted light. This is especially beneficial in areas with heavy light pollution or when imaging during a bright moon. The tighter bandpass results in a darker sky background and makes faint nebulosity stand out more clearly. The trade-off is that it requires longer exposure times compared to a 7nm filter.
Can the Antlia 3nm NII Pro filter be used on planetary nebulae?
Antlia states the filter is only effective for deep-sky objects that contain NII emissions. Planetary nebulae that emit in NII are suitable targets; objects without NII emission will not benefit.
Does the 2mm thickness of the Antlia 3nm NII Pro filter require refocusing?
The Antlia 3nm NII Pro filter has a filter thickness of 2mm +/-0.05mm. A focus adjustment may be needed when switching between filters of different thicknesses, so it is good practice to verify focus after changing any filter in the optical train.
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| Filter Type |
NII Narrowband (3nm Pro series) |
| Central Wavelength (CWL) |
654.8nm |
| Bandpass (FWHM) |
3nm |
| Peak Transmission |
88% |
| Out-of-Band Blocking |
>OD5 (0.001%) @ 300-1100nm |
| Recommended f-ratio |
Usable down to f/3 with minimal loss of signal |
| Filter Thickness |
2mm +/- 0.05mm |
| Substrate |
Single, non-glued optical substrate |
| Transmitted Wavefront |
Lambda/4 or better |
| Parallelism |
< 30 arcseconds |
| Surface Quality |
60/40 (MIL-O-13830) |
| Size |
2" Mounted |
| Filter Ring |
2" (M48x0.75), ultra-thin black anodized cell, laser engraved |
| Not Recommended For |
Solar imaging, night visual observation |
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3-years warranty against delamination.
