The Antlia ALP-T Dual Band 3nm SII & OIII Filter is a dual narrowband optical filter designed for astrophotography with color cameras. It isolates two specific emission lines, Sulfur-II at 672.4 nanometers and Oxygen-III at 500.7 nanometers, allowing users to image nebulae that emit strongly in those wavelengths. By targeting only these regions of the spectrum and blocking out everything else, including moonlight, airglow, and most sources of light pollution, the filter improves image contrast and signal clarity.
This particular model uses a 3nm Full Width at Half Maximum (FWHM) for each passband, with a tolerance of ±0.5nm. That narrow bandwidth helps suppress unwanted light across the 300nm to 1100nm range, achieving an optical density (OD) of 4.5 outside the passbands. The high OD rating indicates strong blocking performance across most of the visible and near-infrared spectrum, reducing the impact of city lights or moonlit skies during long exposures.
Peak transmission is listed as 90% for the SII line and 85% for the OIII line, meaning most of the light from nebulae emitting in these bands will pass through to the camera sensor. The filter is built with a 2mm thick optical-grade glass substrate and uses a single non-glued layer. Surface quality is rated at 60/40 scratch/dig per MIL-O-13830 standards. The transmitted wavefront is specified as λ/4 or better, and parallelism is under 30 arcseconds.
The housing is a 2-inch (M48 x 0.75) threaded ring, black-anodized with laser-engraved text. The filter ring is designed to be low-profile to reduce vignetting and increase the usable aperture area. This mounted version is priced at $390.00 USD.
The ALP-T SII&OIII version complements other dual-band filters from the same product line, such as the Ha&OIII and the SII&H-beta variants. All of these are made for use with one-shot color (OSC) cameras, but can also be paired with monochrome cameras to save exposure time or to create additional contrast separation when building up a false-color image stack.
Using the SII&OIII filter, astrophotographers can collect both red and blue-green emission data at the same time. This can be useful for producing bicolor images or supplementing data from other narrowband filters. The 3nm bandwidth also allows longer exposures under bright sky conditions without as much interference from ambient light. That’s especially helpful for users imaging from suburban areas or during times when the moon is present in the sky.
The filter uses double-sided multi-layer coatings applied via ion-assisted deposition. This method is designed to keep the spectral performance consistent across the entire surface. Internal reflections and halos are minimized through anti-reflective coatings and polished optical surfaces. The filter also blocks near-infrared wavelengths up to 1100nm, which helps reduce contamination from star bloating or off-axis reflections that are common in many camera sensors sensitive to IR.
Antlia recommends using this filter with optics that have an f-ratio slower than f/4. For faster optical systems, a high-speed version of the same filter is advised due to the potential shift in the effective bandpass that happens at steeper light cone angles. That shift can reduce the effective transmission or move the bandpass away from the target wavelengths.
This filter is intended for astrophotography only and is not suitable for visual observation. It is also not to be used for solar imaging under any circumstances. Doing so may result in permanent eye damage, as it does not have the necessary attenuation for solar observation.
Combo pricing is available when purchasing this SII&OIII version alongside either the Ha&OIII or SII&H-beta filters from the same series. These combinations allow for full false-color imaging with OSC cameras using three distinct narrowband filters: one targeting hydrogen-alpha and OIII, another targeting SII and OIII, and the third targeting SII and H-beta. This method allows for greater flexibility in assigning colors during post-processing, especially for those looking to emulate SHO (Hubble palette) or similar color schemes.
While not unique in concept, as dual-band filters are widely available across multiple manufacturers, this filter’s 3nm bandwidth in the SII and OIII regions puts it in the ultra-narrowband category. That allows more selective imaging of specific emission features in nebulae such as planetary nebulae, supernova remnants, and HII regions. The result is cleaner signal capture with less background interference.
The spectrum curve is not always included in the product documentation, but typically for filters like this, it shows two sharply defined peaks centered at 672.4nm and 500.7nm, with the rest of the spectrum flattened to near-zero transmission levels.
Build quality specifications show standard practices for premium filters. The 2mm thickness with a ±0.05mm tolerance ensures compatibility with most 2-inch filter wheels and holders. The edge blackening helps reduce stray light and internal bounce within the filter cell. The OD4.5 blocking ensures most wavelengths outside the target bands are attenuated by a factor of over 30,000.
There is no software or electronics associated with this product, and no calibration tools are required for use. It functions as a purely passive optical element. Users can incorporate it into any imaging setup that accepts 2-inch threaded filters, including DSLR adapters, cooled CMOS cameras, filter drawers, and filter wheels. It also works with refractors, Newtonians, and other telescope types as long as the focal ratio meets the recommended criteria.
The Antlia ALP-T 3nm SII&OIII filter does not include any accessories. No case or storage box details are listed. Buyers should plan to store it safely in a dry, padded container to avoid scratching the coatings or exposing the surface to condensation and dust. Routine cleaning should be avoided unless absolutely necessary and done with lens-safe methods.
This filter is widely available through online retailers specializing in astrophotography gear and ships internationally. Availability may vary based on regional distributors or stock levels. It is commonly purchased alongside similar filters for users building a full narrowband set, and fits within the general trend of OSC users adopting more advanced filters to replicate the benefits traditionally seen only with monochrome sensor workflows.
Users working in areas with higher Bortle scale readings will likely see the most benefit in terms of contrast gain. However, even in darker skies, this filter is useful for isolating emission features and cutting down on background noise from airglow or diffuse starlight. While not a required tool for all astrophotographers, it offers a way to add narrowband data to color camera images with more control and precision than broadband filters allow.
Overall, the ALP-T Dual Band 3nm SII&OIII Filter is a narrowband imaging accessory with defined performance targets. It isolates two key emission lines, blocks most unwanted light, and fits standard 2-inch filter holders. It is priced at $390 and is suitable for color and monochrome camera users working with telescopes slower than f/4, provided they want to extract SII and OIII data in one exposure. There are no software features or app integrations, and the primary benefit is simply physical light control. Buyers looking for other wavelength coverage will need additional filters from the same or other filter lines.
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