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Review of Optolong 3nm Narrow Band Filters

As a long time Optolong product user myself, I always trusted their product quality. Optolong’s narrowband filters had helped me making imaging in the heaviest light polluted Bortle 9 possible. I have used their 7nm set filters version 1 with my 1600mm pro, and then upgraded later to version 2 filters with 294mm pro. The second iteration of the 7nm filters alleviated greatly the problem on the halo over bright stars, though a bit less, but still presented in Oiii Channel. That’s why I am thrilled to know they launched their version of 3nm narrow band filter set and solicited a review unit.

Theoratically, the 3nm band pass can help isolate even more the desired wavelength against the background light pollution, so it should be a game changer for people that imaging from high light polluted area. Fortunately or unfortunately, I moved from the center of Sao Paulo city (Bortle 9) to Ribeirao Preto (Bortle 7), so I had some doubt if the 3nm filter can still have its effect. After some thought, I decided to do the test with a 70% moon in the sky, to simulate the effect of heavy light pollution.

For the test I am using my Skyrover 70SA Astrograph, which is a quadruplet with a reasonable f5 focal ratio, not too fast not too slow. Camera I am using the newly acquired ZWO 2600MM Pro.

I put some thought for target selection according to my place and how to really put the filter to it’s test. Finally I decided to test the faint Vela SNR region, for its tenue structure and composition of Ha and Oiii sinals. Coupled with 70% moon in the sky, I think it will give the filter set of 7nm and 3nm a good comparation.

So first with a 300s exposure of Ha and Oiii channel. All are single frame without any calibration.

Ha channel, Optolong 7nm, 300s

Crop of Ha channel, Optolong 7nm, 300s

6.5nm Optolong Oiii, 300s

Crop of 6.5nm Optolong Oiii, 300s

You could see that the 7nm set did a great job on picking the emission signals on Ha and O3. O3 has a very slight gradient probably due to moon light but nothing with a simple DEB can not correct.

Now with the much expected 3nm set. Fristly ha channel.

Ha channel, Optolong 3nm, 300s

Crop of Ha channel, Optolong 3nm, 300s

The immediate impression is that the 3nm brings a much cleaner image, biggest impact is the smaller star size, and the filament of the SNR is more defined than the 7nm.

Let’s check the 3nm Oiii Channel

3nm Optolong O3, 300s

3nm Optolong O3, 300s

Oiii Channel continues the performance of Ha, stars are smaller, image cleaner, a bit more defined detail. The gradient cast by moon light in the 6.nm is also gone. Definitely a big improvement over the 6.5nm brother.

Now I put a side by side comparison for you to have a better idea.

Through the comparison, I am quite impressed with the ability of Optolong 3nm filter. It performed extremely well in my Bortle 6/7 site, I believe the difference even greater in Bortle 8/9 area. To my eye the 3nm have at least 4 advantages:

1. Smaller Stars

2. Better isolation for emission targets

3. More cutoff against light pollution

4. No need of increase exposure time. This is my misconception before hands-on. I used to think the 3nm will block more light so have less signal. The truth is that it will only block the light you do not want. For the signal that you are aiming for, it collects the same or even more signal comparing to the traditional 7nm set!

Unfortunately I don’t have final conclusion on the halo issue against brighter star because of target selection. But for the focusing on Sirius I did not notice any halo. I will give follow up report on the issue once I had the chance.

Conclusion, Optolong 3nm narrowband filter set is really a great upgrade comparing the traditional 7nm. Overall it offers better signal to noise ratio thus the final cleaner image, and could be a life saver for the imaginers in city center, especially on the Oiii channel that is more suspectable to light pollution and moon light. Certainly it is much more expensive than the 7nm, which is still a good product. But for the ones that can afford it, I highly recommend making the upgrade.

Finally, below is the final image for the session, just merely 85 minutes between Ha and Oiii on a F5 scope, which is far from enough for this faint target. But the final results delight me and I could imagine the beautiful results once I put more integration time on it.

OTA: SkyRover Super Astrograph 70/350 Camera: ZWO ASI2600MM Pro Mount: Ioptron GEM45 Frames: Optolong Ha 3nm 8x300, Optolong Oiii 3nm 9x300 Total Time: 85 minutos Bortle 6, Ribeirao Preto, Brazil 23/03/2022

Clear skys and a lot of photons colected!

Le Le

Fóton Astro

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Jason Dain
Jason Dain
08 de fev. de 2023

I'm looking into getting a set of these filters? Would you be willing to share the images with the 7nm and 3nm for comparison?


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