Radian Triad Ultra Filter for Fast Optical Systems Reviewed

In 2019, Oceanside Photo and Telescope (OPT) introduced the Radian Triad filter, an aggressive tri-band filter designed to allow one-shot color cameras to produce urban astrophoto results similar to traditional narrowband imaging using monochrome cameras, but without the complexity of combining images taken through multiple filters. Soon after, OPT released the Triad Ultra quadband filter, which adds an additional important wavelength band, for more realistic one-shot color images from heavily light-polluted urban locations.  The Triad Ultra quickly became the standard to which other light pollution filters are compared.

The Triad Ultra filter allows stunning photos to be taken under skies so bright that major constellations are hard to see visually. But when used with super-fast optical systems like the Starizona Hyperstar, the high incidence angle of some light reaching the filter creates an effect called spectral roll-off.  This effect shifts incoming wavelengths about 1.5 nanometers toward the blue end of the spectrum and the desired wavelengths are pushed near the edge of the filter’s bandpass, reducing the filter’s efficiency with fast optical systems. Thus, the Triad Ultra works very well with systems f/6 and above but is less efficient at passing select wavelengths at the Hyperstar’s native F/2. However, F/2 makes up for a lot of sins, and for two years I was thrilled with the Triad Ultra’s astrophoto results taken from deep within urban San Antonio, Texas.

All the images in this article were taken from deep within heavily light-polluted Loop 410 in San Antonio, Texas. Credit: Google Maps

Recognizing the limitations of spectral roll-off with fast optical systems, OPT tried a bold experiment; deliberately pre-shift the filter’s spectral roll-off in the H-Alpha wavelength so it again centers on the filter’s H-Alpha bandpass when used on very fast optical systems. This is no easy task, as each undesirable light pollution wavelength needs its own layer of interference filter, while still allowing desirable wavelengths through. With the critical optical manufacturing tolerances required, more than 90% of filters produced are rejected, leading to the filter’s high cost.

All Triad Ultra filter versions display an opaque, mirror-like finish that is very selective in the wavelengths it passes. Credit: OPT

With nebula photography, H-Alpha contributes the most image signal. Because of the complexities in fabricating filter layers that move a shifted desired wavelength back to the middle of its bandpass, only the H-Alpha wavelength is pre-shifted in the new third-generation Triad. Known as the Triad F3.0, the name relates to being both the third generation of Triad filter, and one effective with optical systems near F/3.

I can say confidently that OPT’s engineering worked! On a fast system like a Hyperstar, the H-Alpha signal with the Triad F3.0 is noticeably stronger than with the original Triad Ultra. The result is higher contrast and more colorful images of emission nebulae from bright urban locations. The Triad 3.0 filter passes only five percent of all wavelengths, but for deep-sky astrophotography, they are the wavelengths that count!

How good is the Triad 3.0? For the images displayed here, I decided to use a more common camera than a dedicated cooled astrocamera – a modified Canon 6D DSLR – on a C-14-mounted Starizona Hyperstar. Without the filter, my exposure at ISO 800 is limited to two seconds. With the Triad 3.0, my typical exposures range from 90 seconds under clear moonless skies to 45 seconds with a full Moon. The filter’s light pollution control is so aggressive that there is noticeable effect on subframes when my wife drives within feet of the telescope with the headlights on. Excellent images can be taken with the Hyperstar aimed within 30 degrees of the full Moon. These situations would have resulted in photo disaster just a few years ago.

Star clusters are imaged well with the Triad F3.0, although stars are dimmed due to the limited wavelengths passed by the filter. Credit: Robert Reeves

When used with a Hyperstar, both the original Triad Ultra and the Triad 3.0 produce halos around bright stars due to starlight bouncing between the filter and the imaging sensor. The 3.0 version, however, produces softer halos than the Schmidt camera-like sharp rings that surround bright stars with the standard Ultra filter. My assumption is the softer halos are created by the altered reflectivity of the modified filter layers used to shift the H-alpha wavelengths back to the middle of their bandpass. Sixty years of familiarity with Schmidt camera images have left me comfortable with the regular Ultra’s sharp star halos, but I find the softer halos produced by the F 3.0 version to be aesthetically pleasing. The telescope focus is also noticeably different with the F 3.0 filter. After swapping to the F 3.0 version, the star focus sharpness reference number in my camera control program jumped from the previous three to 10. A few pushes of the electric focuser button quickly refocused to the normal number.

Images of the Crab Nebula (M1), Orion Nebula (M42), Seagull Nebula (IC 2177), and Rosette Nebula (NGC 2237) taken with a fast F/ratio imaging system using the Triad 3.0 filter (left) and the standard Triad Ultra filter (right). The H-Alpha signal is greatly increased with the Triad 3.0 filter. Credit: Robert Reeves

The Triad 3.0 is available in 1 ¼-inch and 2-inch threaded mounts for use in fast optical systems like the Hyperstar. Admittedly, there is a bit of sticker shock when browsing the Triad 3.0 on the OPT website, but I think users will quickly find that this filter delivers the advertised performance and allows stunning one-shot color deep-sky photography from extremely light polluted urban locations. My own experience is the Triad has allowed me to take 200 deep-sky images from home – images that previously would have required 200-mile round trips to a dark site. Assuming six targets per dark site trip, the Triad has saved me 6000 miles of hauling a 300-pound astrophoto rig. The filter has paid for itself in gas savings alone and allowed me to enjoy my astrophoto passion on every available clear night, regardless of Moon phase.

The Triad 3.0 excels with emission nebulae, planetary nebulae, and supernovae remnants. It does a good job with star clusters despite relative star brightness being altered because of the limited wavelengths that are passed by the quadband filter. The filter is less effective with galaxies, but with extended exposures it will deliver satisfying galactic results. Neither version of the Triad filter works with dark nebulae. I have had limited success with reflection nebulosity like that around the Pleiades.

I highly recommend the Triad 3.0 to anyone doing one-shot color astrophotography with a fast optical system like a Hyperstar. The filter lives up to its hype – it just plain works! The Triad F3.0 will reward you with many evenings of celestial photography fun under bright urban skies.

Plus: Allows dark site-like one-shot color deep-sky imaging from bright urban locations.

Minus: High initial cost, though it can pay for itself by savings on travel to dark-sky sites.

Price: 2-inch $1075, 1 ¼-inch $780



About Robert Reeves

Robert Reeves has been exploring the Moon since 1958 and took his first lunar photograph in 1959. He began telescopic astronomy with a four-inch Criterion Dynascope. Today, Reeves uses a Celestron 11 Edge HD, a Sky-Watcher 180mm Maksutov, and a Sky-Watcher 20-inch Stargate Dobsonian for lunar photography, and a Celestron C-14 with a Hyperstar for deep-sky photography from his Perspective Observatory located in central Texas. Robert has published over 250 magazine articles and 200 newspaper columns about astronomy and has authored several books, including The Superpower Space Race, The Conquest of Space (co-authored with Fritz Bronner), Wide-Field Astrophotography, Introduction to Digital Astrophophotography and, most recently, Introduction to Webcam Astrophotography. Although Robert Reeves is an accomplished deep sky astrophotographer, his current passion is re-popularizing the Moon with the public and the amateur astronomy community. He enjoys speaking to astronomy conventions and spreading his passion for the Moon.

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