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I set out to answer a common question: which filter actually works best on emission nebulae under Bortle 5 skies—no filter, a broadband light‑pollution filter (Optolong L‑Pro), or a duo‑band (ZWO Duo‑Band)?
I ran this light pollution filter comparison for astrophotography using a ZWO ASI533MC Pro on a Celestron 130SLT, I captured the Tulip Nebula over several nights at the end of JULY/ beginning of August 2025 under a 40-50% moon with 300 second exposures at gain 100 for a total of 8-9 hours each filter setup. Below you’ll find side‑by‑side crops showing how the images, both single exposures and stacked exposures compared. This should help you choose the right filter setup for suburban conditions.
Watch this video from my channel @astroimagery on YouTube for explanation of the tests and overview of the results:
Test Setup and Conditions
This is a real‑world, suburban backyard test—no lab graphs, just practical results you can replicate.
Capture location: Cesme, Turkey. Bortle 5/6 sky.
Seeing was generally good and the sky was clear as it usually is around this time of year here. The Tulip sat roughly just before the meridian as I started to image it. It passed the meridian around midnight, give or take. Camera was cooled to -5°C; offset 50. I didn’t take calibration files as they are not normally needed with my low noise camera.
Stacked in Siril, finished in Siril, Photoshop and SETI Astro Suite with as near as identical stretches and color balance across all three datasets. If you also shoot from a driveway, balcony, or a nearby park, this is exactly the scenario you’ll face: gradients, moon glow, and the eternal trade‑off between star color and nebula contrast.
I also tested how the image came out using no filter even though I do suffer from light pollution gradients in my location.
I used the 1.25 inch versions of both filters.
No Filter vs Broadband (Optolong L‑Pro)
No filter preserves the most natural star color and overall brightness, but the background light pollution together with the bright stars subdue the nebula contrast and it is difficult to see the details under Bortle 5 skies. The moon was not a problem but at 40-50% illumination it added some additional light pollution to the sky.
I chose the Tulip Nebula for several reasons:
- It was high in the sky which would minimize the effects of light pollution.
- It is a purely emission nebula and I wanted to test this kind of target having tested reflection nebulae before.
- It is a nice and easy to image target that will give good comparison results with different filters.
- I didn’t need long exposure times to get significant and meaningful results.
L‑Pro does well in skyglow and cleans up gradients without throwing away the broadband look, though on Ha‑heavy targets like the Tulip it won’t pop the nebula as much as a duo‑band. The results do seem to confirm this.
I think it is not absolutely clear whether LPRO or No filter works best for an emission nebula like the Tulip, what do you think? Possibly the LPRO just defines more detail in the background but is it worth paying the extra for this slight improvement?
ZWO Duo‑Band vs L‑Pro for Emission Nebulae
The duo‑band isolates Hα/OIII, which boosts nebular contrast. This, like most narrowband filters, blocks out a lot of light pollution and moonlight. In this dataset, the Duo‑Band didn’t pick up much OIII because the Tulip Nebula is an emission nebula with mostly Hydrogen Alpha. Gradients were easy to remove, though star colors needed a bit of colour correction and boost in intensity.
Compared to the L‑Pro, there was more detail in the nebula itself, and the LPRO did a good job of reducing light pollution (not as much as the Duoband). The LPRO did seem to highlight dust in the background sky whereas the Duoband brought out the main nebula structure and detail and had a darker more neutral background.
It seems like a more obvious improvement can be seen between my images of the Tulip Nebula taken with a Duoband and an LPRO filter. I much prefer the narrowband image with the ZWO Duoband filter. Maybe it lacks some background detail in the dust there but the Nebula really pops! Even the details seen to the bottom left appear to be much cleaner. The stars are not so prominent but could be intensified. The results suggest the duoband filter for emission nebulae.
Results: Side‑by‑Side
Now let’s take a look at some of the images, both single exposures and stacked images side by side and look at the differences.
The single 300 second exposures are very different. I would say that at this stage the Duoband is clearly the winner because of the contrast in the Tulip Nebula and reduced light pollution. The colour of the stars is not the best and also they are very subdued in the image.
The L‑Pro offered a balanced image with better controlled background but the less nebular contrast on this emission target. The exposure I took without a filter has too much star intensity and light pollution which overwhelm the nebula and therefore it can hardly be seen. This could be fixed in post-processing and as we see in the stacked and processed images it is almost as good as the LPRO image.
The stacked and processed images are all much better which is normally the case as stacking increase Signal to Noise Ratio. In my opinion, the best is the Duoband filter for emission nebulae such as the Tulip Nebula. It is cleaner and has better contrast and detials in the nebula itself. These stacked images are 8-9 hours total integration time each composed of 5 minute exposures.
Recommendations by Sky Class
For Bortle 5,(like my sky), on emission nebulae, start with the Duo‑Band for the main integration and capture some short, no‑filter or L‑Pro subs for natural star color to blend. If the moon is not very bright and gradients are manageable, L‑Pro can work for a more broadband look with realistic star fields, but expect to invest more time in gradient removal. No filter is best reserved for darker skies or when you’re prioritizing star fields and broadband targets like galaxies and reflection nebulae.
I’ve tested the use of no filter at Bortle 5 for galaxies and the results were quite convincing. See how I tested the effect of the LPRO filter on galaxies (video).
Gear Notes
For the light pollution filter comparison for astrophotography test I used the following equipment:
- Telescope tube (OTA): Celestron 130slt Newtonian
- Camera: ZWO ASI533 MCPRO (OSC cooled camera)
- Mount: CEM26 Ioptron equatorial
- Guide camera: SV305 (SVBONY)
- Guide scope: SV106 60mm F4
- LPRO Optolong filter 1.25 inch version
FAQ
- Is L‑Pro good for emission nebulae? – Yes, as it reduces pollution and helps maintain star colour to a certain extent. It also keeps the background details visible.
- Duo‑band vs no filter under a bright moon? With a bright moon you want to use narrowband filters and these are so effective that I can even image under a full moon as long as I don’t point the telescope to close to the moon itself. No filter would not be a good idea because of increased skyglow from the moon.
- Does duo‑band kill star color on OSC? It doesn’t kill it but makes colour correction necessary and dims the stars quite a bit.
- What about galaxies or reflection nebulae? I have tested shooting galaxies (video) and found that it is best to use no filter rather than an LPRO filter. I shot M51 the Whirlpool Galaxy and compared stacked images taken with no filter against those taken with the LPRO filter. I saw quite a big difference in the detail obtained. It seems that a certain amount of light is blocked by the broadband LPRO filter than hurts the image of a galaxy rather than enhancing it.
Keep the Stars in Your Pocket
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