OPT Triad Ultra Filter
I’ve been lucky enough to test the OPT Triad filter (Radian Telescopes) for astrophotography in the backyard. This is a 2″ quad-band narrowband filter that collects light in 4 band passes, Ha, OIII, SII, and H-Beta.
The “Ultra” version of the filter improves upon the previous tri-band version, now with an even narrower band-passes, and the inclusion of Sulfur II (SII). In theory, this creates images with more contrast, and better separation between the H-beta and OIII channels.
The Triad Ultra filter was designed to be used primarily with color cameras, and I have found it to be an extremely useful DSLR filter.
OPT Triad Ultra Filter Review
If you are wondering what the breakdown of the band passes (in nanometers) are for this quad band filter, here it is:
- Hydrogen-beta: 5nm
- Oxygen III: 4nm
- Hydrogen-alpha: 4nm
- Sulfur II: 4nm
It’s worth noting that the OPT website states that telescopes with focal ratios faster than F/4 will move the transmission off-band. This doesn’t mean that if your telescope is F/2 (like my Celestron 8″ RASA), that it is not compatible with this filter, it just means that the images will have about am 0.8nm band-shift.
The following video provides some insight into the data collected using the Triad Ultra filter, and what it’s like to process an image captured in 4 isolated wavelengths. In this article, I will review the performance of this light pollution filter when used with a DSLR camera, and a dedicated astronomy camera.
As you may have noticed, I compared the Triad Ultra filter (Radian Telescopes) to the STC Astro Duo-Narrowband filter. Both of these filters allow you to capture full color images from the light they collect, but a little post processing is needed to balance the colors.
For example, images I capture using the ASI294MC Pro always have an overall green cast to them. This is totally normal, and can easily be adjusted by using levels in Adobe Photoshop. Setting the white balance of an astrophotography image in post processing is typical of any workflow.
As you can see from the transmission graph above, the OPT Triad Ultra filter isolates light wavelengths in the HB, OIII, Ha, and SII bandpasses. To understand what these mean in terms of your astrophotography images using a color camera, please see the following article:
The Omega Nebula
The first target I decided to shoot using the OPT Triad Ultra Filter was the Omega Nebula in Sagittarius. I chose this particular deep sky object (an emission nebula) because of it’s composition of gases. I used a one-shot-color camera (ZWO ASI294MC Pro) to collect 4-minute exposures on M17 using a Sky-Watcher Esprit 100 ED APO telescope.
The following image was captured using the OPT Triad filter with a color camera, under a nearly full moon in the city (Bortle Scale Class 6). I combined the data shot through a Celestron RASA 8 F/2 astrograph, and a Sky-Watcher Esprit 100.
In the case of the RASA, the Triad Ultra filter slid into a Starizona filter slider drawer, and sat at the objective of the telescope similar to the Hyperstar system. At a focal length of 400mm, the Omega Nebula jumped out of the image frame using this quad-narrowband filter.
Triad Ultra Filter Example Images
This image really blew me away. I used the 2″ Triad Ultra filter inside of my William Optics RedCat 51 to photograph this ultra-wide deep sky image of the Sadr region. The camera used was a Canon EOS 60Da, a dedicated astrophotography camera from one of the worlds biggest camera manufacturers.
The hydrogen-alpha clearly makes the biggest impact here, but the quad bandpass nature of this filter made it possible to capture a natural looking image of the area with a DSLR.
The Sadr Region using the OPT Triad Ultra filter with a Canon 60Da.
Next up, I pointed the RedCat 51 towards the constellation Sagittarius. The Lagoon Nebula and Trifid Nebula are well photographed by amateur astrophotographers, and for good reason. The image below uses 30 x 3-minutes (@ ISO 6400) for a grand total of 1 hour and 30 minutes of total integrated exposure time.
The Lagoon and Trifid Nebulae in Sagittarius. OPT Triad Ultra Filter and Canon 60Da.
Interesting to Note
I did experience some odd color banding with the OPT Triad filter with my ZWO ASI294MC Pro. These artifacts were isolated to the red channel of the color spectrum, where the majority of the narrowband details were collected.
I am not alone in this situation, as described in this Cloudy Nights forum post. It appears as though this scenario is only experienced by owners of the ASI294MC Pro, and it is due to the sensor design (Not the OPT Triad Filter).
To get around this, I used a number of gradient removal image processing techniques in Adobe Photoshop to correct the red channel. I will continue to investigate this issue, and update this article with my results. This issue did not arise when capturing images using my Canon EOS 60Da DSLR with the Triad Ultra filter.