OSC narrowband filters

Optolong L-eXtreme
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John Tucker avatar
John Tucker avatar

I have an Optolong L-Extreme filter that I’ve used for many years to capture nebulae in my Bortle 8 skies, and less commonly, to reduce the dominance of stars in images of faint nebulae. Over the years quite a few other OSC narrowband filters have come out, and it gets pretty hard to sort them all out. I’m looking for modest budget filters that are optimal for three shooting scenarios:

  1. F/4 Newt in Bortle 8

  2. F/4 Newt in local dark site (Bortle 3-4)

  3. Small F/7 refractor at Starfront (Bortle 2?)

I’m mainly thinking of nebulae here, not sure there is much that adds value for other targets. Ideally I’d like to keep the cost of individual filters below $350.

I’m aware that there are a lot of tools for star reduction in processing, but for now, I’m just finding that using a filter gets me to an easier starting point.

thanks

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Tony Gondola avatar
Tony Gondola avatar

In Bortle 8 with an F/4 instrument the 6nm dual band filter you already have is probably the best bet. You could try something like the Antila 3nm DBF but because of the wavelength shift at F/4 (1 to 1.5nm) you may not gain anything.

Under B-4 or better you could still use the dual band to get Ha and Oiii but you now have the option of just running with a UV/IR cut for natural RGB.

Overall, I would strongly suggest that you get used to dealing with stars in post. It’s not at all difficult and it will give you more options. Your best stars even under B8 would come from short exposures with the UV/IR cut. Combining this with your narrowband data would work very well. I know there are scripts for creating RGB stars from narrowband data but in my experience, it doesn’t work as well as shooting straight RGB stars.

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John Tucker avatar
John Tucker avatar

Tony Gondola · Apr 10, 2026, 01:13 PM

In Bortle 8 with an F/4 instrument the 6nm dual band filter you already have is probably the best bet. You could try something like the Antila 3nm DBF but because of the wavelength shift at F/4 (1 to 1.5nm) you may not gain anything.

Under B-4 or better you could still use the dual band to get Ha and Oiii but you now have the option of just running with a UV/IR cut for natural RGB.

Overall, I would strongly suggest that you get used to dealing with stars in post. It’s not at all difficult and it will give you more options. Your best stars even under B8 would come from short exposures with the UV/IR cut. Combining this with your narrowband data would work very well. I know there are scripts for creating RGB stars from narrowband data but in my experience, it doesn’t work as well as shooting straight RGB stars.

Well, not really sure what “getting used to” entails. APP and Astroart allow me to reduce stars but beyond a certain point the star reduction algorithm starts leaving behind a lot of artifacts.

I’ve had varying degrees of luck with the various star removal scripts, but haven’t yet found a good way to reduce the star intensity in the stars-only image (for those scripts that generate them) and add it back. I spent about a day trying to clean up this image of the Andromeda Galaxy, eventually getting to the image on the bottom. But if you zoom in its a real mess.

I’ve played around endlessly with APP and AstroArt, tried some of the star removal programs like Starless, and tried a lot of things like masking in LIghtroom, including generating DSS star masks. What would really be nice would be a script that removed all stars below a certain size. Astroart has this capability in principle, but it doesn’t actually work.

📷 image.pngimage.png📷 image.pngimage.png

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Tony Gondola avatar
Tony Gondola avatar

I don’t have any experience with the programs you’re using but it sounds like it’s a lot more complicated then it needs to be. What you need to do is not star reduction but star removal. This is how it works in Siril:

While the image is still linear, you remove the stars from the image using Starnet++. This gives you two images, one with just the unstretched stars and one without stars. Once you’re done with processing the starless image you take the star image and stretch that separately. That gives you complete control of how prominent the stars will be in the final image. You can also get the star color balance just right. Once you have the stars the way you want them you can simply screen them back in. I think this is the simplest and most controllable way to do it.

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Addos avatar
Addos avatar

Your question seems to be split into - “Would getting more narrowband filters to pair with my OSC camera help with 1) better nebula photo’s, and 2) smaller stars”, so I’ll frame my answer around that understanding. If I’ve misunderstood, apologies for the off-track response.

1) No. The L-Extreme isolates the two main emission lines that are most prevalent / strongest we see out there, and has a narrow enough bands to work extremely well as a light pollution filter. In your bortle 4 and 2 environments, theres very little benefit in using it, so this really only relevant for the bortle 8 environment. Key thing to keep in mind, you’re using an older sensor and an OSC, so quantum efficiency isnt as high as with a newer camera, and bayer pattern means you have at least 1/3 less available pixel area to collect photons of a specific band. Using a narrowband filter cuts down the collection rate even further. So a narrower bandpass filter would require orders of magnitude more exposure time for the same SNR. Other emission line bands like Sii and Hb will have even higher diminishing returns. If your goal is to do more narrowband imaging or play around with less prevalent emission lines, this would start to build a case to switch to mono.

2) Not really. You could blow money on a 3nm dual band filter, but cost will start to escalate significantly, and per above, you’d also have some unintended downsides. There’s also no point in getting a filter to make smaller stars if your telescope optics and mount/guiding setup are limiting factors, and likely are. Software is the much much much better way to control stars, and my goodness cheaper. I have no idea about the software you use, I took the plunge on Pixinsight many many moons ago and it does the job extremely well, especially paired with a couple of Bill Blanshon’s scripts. Tony rightly also suggests, if you want super tiny stars with limited artefacts, go full star removal. If you’ve already de-linearised, use a histogram transform or curves to pull down the brightness of the stars, which will also shrink them. If you can start at linear, separate first and stretch independently like Tony suggests. Play around with iterations of this, and re-add to your starless image.

Good luck!

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