Optolong 3nm Narrowband Filters: Review

Optolong 3nm Narrowband Filter Set. Credit: Optolong

I recently tested Optolong’s new 3nm hydrogen-alpha, oxygen-III, and sulfur-II filters and overall I am very satisfied, particularly for their price point ($439 each for 2”, $1239 for the kit). Until now, Optolong only offered 7nm-class filters (6.5nm for OIII and SII, 7nm for Ha), but for those who live under heavy light pollution and have well-guided mounts capable of 10-minute exposures, 3nm is an excellent option.

Frequently, 3nm OIII filters suffer from halo effects around stars, which is the main effect I was testing. I also acquired a new, improved Optolong OIII filter. I tested all three narrowband filters on both my Celestron 8” Schmidt-Cassegrain and my Takahashi FSQ-106N using a ZWO ASI1600MM Pro monochrome CMOS camera under Bortle 7 skies.

Below is a head-to-head comparison of two Ha filters, the Chroma 3nm vs the Optolong 3nm, on the “Heart of the Heart” Nebula. Each is 7 x 600 seconds captured on my Celestron C8 with a 0.63x focal reducer. The average sky quality from my Sky Quality Meter for the Chroma filter was 18.07 mag/arcsec (all from the same night), and 18.96 mag/arcsec for the Optolong filter (also from a single night), so the Optolong filter data had better contrast to start with due to the atmosphere being more transparent that night. The two images were processed with basic denoising and stretching in PixInsight.        

Optolong 3nm Ha filter (left) vs Chroma 3nm Ha filter (right). Credit: AstronoMolly

The stars in the Chroma filter are slightly smaller than those of the Optolong filter, but the difference was so slight that I had to blink the images rapidly in order to notice it. This may also be due to differences in seeing on the two nights, which can both cause blurring of the stars and reduce the quality of autoguiding. The transmission efficiency of the two filters appears comparable.

To push the limits, I aimed my camera at a bright star to look for halos – Polaris, magnitude +2. I used my Takahashi refractor instead of my Schmidt-Cassegrain to avoid its inevitable internal reflections from bright stars.

Polaris, Takahashi FSQ-106N, ZWO ASI1600MM Pro, Optolong Ha, OIII, SII 3nm filters, 7x300s each. Credit: AstronoMolly

The Ha and SII filters showed microlensing from my camera, though this was not the fault of the filters, but rather the long exposure times.  The diagonal spike is also from the camera and not the filters (it shows up in my images using other filters as well). The OIII filter shown here is the second one Optolong sent me, with improved halo performance compared to the earlier version. While there is still a halo in the OIII filter, it is well-controlled considering how oversaturated the star is with this long exposure time.

At the end of the day, how does a complete image look with the Optolong filters?

Celestron C8 (0.63x reducer), ZWO ASI1600MM Pro, Optolong Ha (53x600s), OIII (37x600s), SII (42x600s) 3nm filters, 22h total. Credit: AstronoMolly

I also tried out the filters with the same camera on my Takahashi. Even with a shorter exposure with low overall signal, the image of the Soul Nebula below is still a good example of what can be done with these filters.

Takahashi FSQ-106N, ZWO ASI1600MM Pro, Optolong Ha (13x600s), OIII (12x600s), and SII (7x600s) 3nm filters, 5h20m. Credit: AstronoMolly

I am very proud of my Heart Nebula image, and very impressed with the performance of all three filters in general on both telescopes. The difference between these Optolong narrowband filters and the higher-priced tier such as the Chroma filters ($1,300 for 2” each filter) is relatively small and the Optolong filters are well worth their much lower price. I currently own Chroma 3nm Ha and OIII filters and will be completing the trio by buying an Optolong SII filter when the test units are returned to Optolong.

MSRP:  Mounted set of 3 2″ filters $1239

About AstronoMolly

I got into astrophotography in July 2015 after receiving my first telescope as a gift, Much trial and error later, I now have three astrophotography rigs set up in my backyard just north of Berkeley, CA, in the San Francisco Bay area, including one dedicated to variable star and exoplanet transit observations. I love doing STEM and astronomy outreach, and I've accrued more than 150 hours of volunteer activities reaching over 20,000 people, both in-person and virtually. I am an AAVSO Ambassador (American Association of Variable Star Observers), an Explore Alliance Ambassador, and a panelist and broadcaster for The Astro Imaging Channel weekly YouTube show. I have a B.S. in Physics from Washington State University, and am currently pursuing my PhD in Physics at University of California, Berkeley, studying neutrinos with my two cats, Orion and Apollo.

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