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Antlia 7nm Narrowband OIII Prime Filter - 36mm Unmounted
The Antlia 7nm Narrowband OIII Prime Filter isolates the critical 500.7nm oxygen-III emission line with a tight 7nm full width at half maximum (FWHM). With over 90% peak transmission, this 36mm unmounted filter is designed for OIII emission nebulae, planetary nebulae, Wolf-Rayet nebulae and supernova remnants, and can be used with nearly all systems as fast as f/2 with minimal loss in emission signal. It achieves its high signal-to-noise ratio through greater than OD4 out-of-band blocking, suppressing unwanted light across the 300-1000nm spectrum by a factor of 10,000.
7nm Bandpass with >90% Transmission at 500.7nm
This filter's design centers on a 7nm bandpass, which provides an excellent balance between signal strength and contrast. By achieving over 90% transmission at the 500.7nm OIII wavelength, it gathers the maximum amount of photons from faint emission structures. The 7nm FWHM is wide enough to deliver bright, high-signal images with shorter exposure times compared to ultra-narrowband filters, while still offering significant rejection of light pollution.
>OD4 Blocking for High-Contrast Imaging
The foundation of high-contrast narrowband imaging is the rejection of all unwanted light. The Antlia OIII Prime filter provides an optical density of greater than OD4, which corresponds to blocking 99.99% of light outside the 7nm passband. Antlia says this blocking from 300-1000nm delivers an excellent signal-to-noise ratio and minimizes interference from other wavelengths, for use in light-polluted areas and at dark sites.
Engineered for Optics as Fast as f/2
A key challenge with narrowband filters is band-shifting in fast optical systems. Antlia states that, with their wider FWHM, 7nm Prime filters can be used with nearly all systems as fast as f/2 with minimal loss in emission signal.
Lambda/4 Single Substrate Construction
Physical construction is critical to optical integrity. This filter is built from a single, non-glued substrate of optical glass, which Antlia uses to eliminate internal reflections. The filter is specified with a transmitted wavefront of Lambda/4 or better and parallelism under 30 arcseconds, and its edges are blackened to eliminate internal reflections from stray light.
Antlia 7nm Prime vs. 3nm Pro: When to Choose a Wider Bandpass
Antlia produces both 7nm Prime and 3nm Pro series filters, each suited for different conditions. The choice depends on your sky quality and optical system.
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Choose the 7nm Prime (This Filter): Ideal for faster optical systems (f/2 to f/5), darker sky sites, or when you want to prioritize shorter exposure times. The wider bandpass is more forgiving of slight band-shifting and gathers more signal in a given time.
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Consider a 3nm Pro Filter: Better suited for heavily light-polluted urban skies or when you want more contrast. The narrower bandpass rejects more background light but typically requires longer sub-exposures to achieve the same signal level. Antlia rates the 3nm Pro series as usable down to f/3.
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What is the main advantage of the Antlia 7nm OIII Prime filter's 7nm bandpass?
The 7nm bandpass balances light-gathering power and contrast. It allows for shorter exposure times than ultra-narrow 3nm filters while its OD4 blocking still minimizes interference from other wavelengths, which suits OIII targets such as planetary nebulae like the Dumbbell Nebula (M27).
How will the Antlia 7nm OIII Prime perform on the Veil Nebula (NGC 6960) with an f/4 Newtonian?
The Veil Nebula is a supernova remnant with strong OIII emission, a target type Antlia lists for this filter. Antlia rates the 7nm Prime filters for nearly all systems as fast as f/2 with minimal loss in emission signal, so an f/4 Newtonian is within that range. The OD4 blocking minimizes interference from wavelengths outside the passband.
Can I use the Antlia 7nm OIII Prime filter in an f/2 imaging system?
Yes. Antlia states that 7nm Prime filters are usable with nearly all systems as fast as f/2 with minimal loss in emission signal.
What does the Lambda/4 transmitted wavefront specification mean for the Antlia 7nm OIII Prime?
Transmitted wavefront describes how much a filter distorts the light passing through it. Lambda/4 or better means the wavefront error added by the filter is a quarter of a wavelength or less, which keeps its effect on image sharpness small.
Why shouldn't I use the Antlia 7nm OIII Prime filter for visual observation?
Antlia lists night visual observation as not recommended for this filter. It is designed for astrophotography of deep-sky objects that contain OIII emissions.
Is the Antlia 7nm OIII Prime filter parfocal with other Antlia filters?
The Antlia 7nm OIII Prime filter has a specified thickness of 2mm +/-0.05mm. The H-alpha and SII 7nm Prime filters have the same stated thickness, but Antlia does not give a parfocal specification for this series, so run a quick focus check when switching between filters, especially with very fast optical systems.
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| Filter Type |
OIII narrowband (7nm Prime series) |
| Central Wavelength (CWL) |
500.7nm |
| Bandwidth (FWHM) |
7nm |
| Peak Transmission |
>90% |
| Out of Band Blocking |
>OD4 (0.01%) @ 300-1000nm |
| Filter Size |
36mm Round Unmounted |
| Filter Thickness |
2mm +/- 0.05mm |
| Substrate |
Optical, single / non-glued |
| Surface Quality |
60/40 (MIL-O-13830) |
| Transmitted Wavefront |
Lambda/4 or better |
| Parallelism |
< 30 arcsec |
| Edge Treatment |
Edge blackened |
| Recommended F-Ratio |
Usable with systems as fast as f/2 with minimal loss of signal |
| Not Recommended For |
Solar imaging, night visual observation |
| Warranty |
3-years against delamination |
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Antlia 7nm OIII Prime Filter (36mm Unmounted)
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Antlia offers a 3-year warranty against delamination on this product.
