Light pollution filters fall into two general categories. Narrowband filters let through only narrow segments (typically 3nm to 12nm) of the optical spectrum that are emitted by nebulae, such as hydrogen-alpha, while blocking everything else, including light from terrestrial light fixtures. Multi-narrowband filters have more than one of these narrowband regions, making them useful for one-shot color cameras. Some examples include the Optolong L-eXtreme and Radian Triad Ultra. Galaxies, reflection nebulae, globular clusters, and star fields aren’t good targets for narrowband filters, though, since most of their light is blocked. Wideband filters, on the other hand, pass most of the optical spectrum while blocking regions of the orange and yellow segments where sodium and mercury vapor streetlight emissions lie. These are better for broad-spectrum objects but they let more light pollution through than narrowband filters do. Some examples include the Astronomik CLS-CCD and the Optolong L-Pro. With the increasing adoption of broad-spectrum LEDs without discrete regions to isolate, light pollution is becoming more difficult to block.
The Antlia Triband RGB Ultra filter merges these two approaches to fighting light pollution, passing bands about 25nm wide in the red, green, and blue portions of the spectrum. Several of the major emission lines are in these regions: Hα 656.3nm; the OIII 495.9/500.7nm doublet; the SII 671.6/672.4nm doublet; and even the NII 654.8/658.3nm doublet. A significant amount of light pollution is still blocked but enough light gets through these broader bands to image not only emission nebulae but also galaxies, reflection nebulae, star clusters, and just about anything else (dark nebulae, which have more of the blocked region of yellow, being an exception).
Because of the broad bandpasses, the Triband RGB Ultra can be used with optical systems as fast as f/2, unlike multi-narrowband filters. This filter can also be used with a monochrome camera as a luminance channel, although I did not test this.
Results
I tested the Triband RGB Ultra first with my Rokinon 135mm f/2 lens (at f/2) from my Bortle 7 backyard. It does indeed not suffer the loss in transmission that is common from multi-narrowband filters on fast systems.
Imaging large emission nebulae with wideband light pollution filters in star-crowded regions like the one above usually results in poor contrast of the nebula and heavy dominance of the stars, resulting in an unsatisfying image. The Triband RGB Ultra is narrow enough to mitigate the contrast problem while being broad enough to retain more star color and capture the reflection components often present in emission nebulae.
Star colors with the Triband RGB Ultra still take on the red-blue of a multi-narrowband filter but to a lesser extent. This is likely due to blocking the yellow portion of the spectrum, which is necessary to reduce light pollution.
The Triband RGB Ultra performed well on dim targets under heavy light pollution. It reveals the very dim tail on planetary nebula NGC 40, as seen in images with the very narrow-bandpass Optolong L-eXtreme, while showing more stars than the narrowband filter.
The Antlia filter was designed to have good color balnce and minimal color cast, which I found to be largely true. While my image did come out a little green, it calibrated nicely using PixInsight’s PhotometricColorCalibration process. With one-shot color cameras having twice as many green pixels as red and blue, I generally expect images to be more green at the start of processing.
As for galaxies, I selected a challenging target: the Fireworks Galaxy (NGC 6946). This object is challenging because it’s near the galactic plane, and thus appears much more yellow. As a starburst galaxy, it also has a lot of hydrogen-alpha emission because of its rapid rate of star formation, making it a good test for a filter that claims to emphasize emission lines while still capturing enough light to image galaxies.
Just two hours of data on this galaxy produced very interesting results.
The Antlia filter pulled out a lot of the hydrogen-alpha signal while also keeping some of the blue component. Just seeing it at all was a feat. With this filter, more exposure time may be needed on galaxies than imagers are used to but pulling this relatively low surface brightness galaxy out from behind the light pollution makes it worth it. Note that the color balance and levels weren’t adjusted in the image aside from color calibration and stretching.
I also imaged a bluer galaxy that has a nice amount of H-alpha: NGC 2841. This was taken under heavy moonlight.
I was able to pull out more red, and more color in general, than I often can using other light pollution filters. Again, more integration time is needed here, but that is to be expected under light-polluted skies and moonlight.
The Antlia Triband RGB Ultra filter I tested is not the final production version. Antlia expects to make some tweaks that will improve performance on galaxies before releasing it.
Conclusion
The Antlia Triband RGB Ultra is the most interesting filter I have used, walking the line between a light pollution filter and a multi-narrowband filter. The ability to pull out emission nebula like a narrowband filter but also still capture reflection nebulae and galaxies makes this filter unique. It is an effective filter that produces a beautiful image at an affordable price.
MSRP: $179 (2-inch mounted)
Website: www.antliafilter.com
