When photographing from bortle skies 1-4, Is It better to use 3 nm or 5 nm filters? Is It better to use the OIII 3nm, SII 3nm and Ha 5 nm filters?
From what i understand, the 5 nm halpha filters Is reccomended for capturing galaxies because It Is less subject to Red shift. It Is also raccomended for capturing the highest SNR on nebulose.
The OIII 3nm and SII 3nm filters, on the other hand, capture greater detail.
What do you think?
The narrower the better. Im using 3nm in bortle 1.
The spectral shift issue is only a problem if your scope is faster than around f4. Im thay case a wider bandpass is warranted, or filters that are designed for fast scopes.
For Red shift i intended the universe expand
It depends on what you are trying to do. Both filters will pass the same amount of the band of interest. The broader filters will pass a little bit more around your band of interest. You have decently dark skies so if it where me, I’d save some money and go with the broader filters, you’ll get great results.
3nm will cut the sky background almost in half, which for faint targets can make a pretty significant difference, even in dark skies. For Sii and Oiii I don’t think there’s any benefit in going for wider width filters.
For Ha in addition to what you said, there’s also another advantage (or disadvantage depending on your goals) of 5nm filters capturing Nii as well. On some targets with strong Nii signal this can provide a substantial boost to the SNR and on others, the exact opposite (due to higher background noise). On targets in dense starfields, the 3nm filter will also reduce star contamination more which helps quite a lot with extracting the structures.
I would personally go with the 3nm option but both are fine in my opinion.
Mikołaj Wadowski · Sep 21, 2025, 03:36 PM
3nm will cut the sky background almost in half, which for faint targets can make a pretty significant difference, even in dark skies. For Sii and Oiii I don’t think there’s any benefit in going for wider width filters.
For Ha in addition to what you said, there’s also another advantage (or disadvantage depending on your goals) of 5nm filters capturing Nii as well. On some targets with strong Nii signal this can provide a substantial boost to the SNR and on others, the exact opposite (due to higher background noise). On targets in dense starfields, the 3nm filter will also reduce star contamination more which helps quite a lot with extracting the structures.
I would personally go with the 3nm option but both are fine in my opinion.
3nm will cut the sky background almost in half, which for faint targets can make a pretty significant difference, even in dark skies.
What do you mean? 3 nm is better or worse for faint targets?
It decreases light pollution by almost half, so it’s certainly better in that regard.
However, for some specific targets with weak Ha and strong Nii it would be worse as the 5nm filter also lets Nii through. I don’t think these objects are that common though.
I think this is a confusing statement. Both will pass exactly the same amount of the signal of interest. The wider bandpass will pass more of the unwanted light that’s very close to the frequency of the wanted bandpass but if we are talking about sky background then it’s still a very tiny slice of what’s out there. Yes, that extra sky background will reduce SNR. How important that is in practice really depends on how high your sky background is compared to the strength of the signal of interest. It’s going to be a much bigger deal under B8 then it will be under B1.
Is there a downside to ultra narrow bandpass filters? Not in performance if the bandpass actually is as stated but F/number will be restricted. And of course, you are going to pay a higher price. Some will say, pay once, cry once but I think price verses performance in any bit of gear is something to be considered.
Mikołaj Wadowski · Sep 21, 2025, 09:48 PM
It decreases light pollution by almost half, so it’s certainly better in that regard.
However, for some specific targets with weak Ha and strong Nii it would be worse as the 5nm filter also lets Nii through. I don’t think these objects are that common though.
…within the total filter bandwidth.
Tony Gondola · Sep 21, 2025, 09:56 PM
Mikołaj Wadowski · Sep 21, 2025, 09:48 PM
It decreases light pollution by almost half, so it’s certainly better in that regard.
However, for some specific targets with weak Ha and strong Nii it would be worse as the 5nm filter also lets Nii through. I don’t think these objects are that common though.
…within the total filter bandwidth.
Yeah, I meant relative to the 5nm one. Should’ve made that clearer.
Tony Gondola · Sep 21, 2025, 09:54 PM
Both will pass exactly the same amount of the signal of interest.
Putting bandshift aside, both will pass the same amount of signal of interest, this is true. But the 5nm will pass more emission signal due to letting Nii through which can give higher final SNR if you don’t care about separating these bands. For some targets specific this is very significant, though most of the time it’s minor.
Tony Gondola · Sep 21, 2025, 09:54 PM
The wider bandpass will pass more of the unwanted light that’s very close to the frequency of the wanted bandpass but if we are talking about sky background then it’s still a very tiny slice of what’s out there. Yes, that extra sky background will reduce SNR. How important that is in practice really depends on how high your sky background is compared to the strength of the signal of interest. It’s going to be a much bigger deal under B8 then it will be under B1.
Yes, this is going to matter most on faint targets with skies this dark. That’s what I said.
Still, I think you’re underestimating how faint most object are. With 3nm filters under a 21.7mag/arcsec² sky, I measured a brighter part of the “cap” above the Pacman Nebula to be roughly 0.0042e-/px/s in Ha for my setup, while the sky background measured at 0.011e-/px/s. If I used a 5nm filter instead, the sky electron rate would’ve been 66% higher. So for Ha using a 3nm filter would give a ~16% SNR boost for that part of the nebula, compared to a theoretical 5nm. For the “main” part of Pacman it would’ve been smaller, for the background Ha - larger. That’s pretty significant already, even in very dark skies and on a moderately bright target.
+ keep in mind that roughly once a month a Bortle 1 sky turns into Bortle 7 or brighter for 10 days or so… The Moon is pretty nasty ;)
Frankly, how dark the skies are does not really factor into this decision. When imaging from B1 I always image broadband with the moon down, and narrowband when the moon is up. With 3nm you will be more productive during the bright moon nights.
So the bottom line is, it depends. On your target, on your sky conditions and what you are trying to accomplish. Plenty of amazing images are made with 6nm pass NB filters and plenty are made with 3nm NB filters. It’s sort of like the well worn OSC verses mono debate. We can talk about the numbers all day but after processing, how much of a difference does it make? I’d love to see someone actually do that test. It would be good to know if you have to use 3mm filters to make a great image or win an ITOD.
From what I understand, the 5nm Halpha filter is chosen only for photographing galaxies. Since they are farther away than nebulae, they are much more affected by the expansion of the universe, and therefore the Halpha wavelength will tend to shift away from 656nm.
The 5nm, being more tolerant than the 3nm, is better at capturing the red of galaxies.
I see that Kevin Morefield @Kevin Morefield uses 5nm with a long focal length telescope.
Considering my telescope has a 1000mm focal length, and therefore is used mostly for nebulae, perhaps the 3nm would be better?
Riccardo
Riccardo Civati · Sep 23, 2025, 02:17 PM
From what I understand, the 5nm Halpha filter is chosen only for photographing galaxies. Since they are farther away than nebulae, they are much more affected by the expansion of the universe, and therefore the Halpha wavelength will tend to shift away from 656nm.
The 5nm, being more tolerant than the 3nm, is better at capturing the red of galaxies.
I see that Kevin Morefield @Kevin Morefield uses 5nm with a long focal length telescope.
Considering my telescope has a 1000mm focal length, and therefore is used mostly for nebulae, perhaps the 3nm would be better?
Riccardo
But do you really want to be photographing faint Galaxies with a narrowband filter?
No, i never Say that
My thoughts on filter bandpass:
Unless you are trying to capture NB signal in distant galaxies or have a <f4 system, smaller bandpass is always better
A wider bandpass will capture more photons but the additional photons are not the NB signal you are trying to capture. The additional light is continuum signal which comes from different sources
That continuum signal reduces contrast and is effectively noise.
In addition, the narrower bandpass allows you to shoot in more light polluted areas when/if needed
Whether the more expensive, narrower bandpass filters makes sense for you depends on your budget and imaging goals. And I would say the same for higher and lower end filters with the same rated bandpass. More expensive filters will have a higher, flatter peak in transmission which means more signal and less continuum at the same bandpass width. They also usually have better anti-reflection coatings which, for me, is the best reason to pay more.
Kevin
Tony Gondola · Sep 22, 2025, 12:07 AM
So the bottom line is, it depends. On your target, on your sky conditions and what you are trying to accomplish. Plenty of amazing images are made with 6nm pass NB filters and plenty are made with 3nm NB filters. It’s sort of like the well worn OSC verses mono debate. We can talk about the numbers all day but after processing, how much of a difference does it make? I’d love to see someone actually do that test. It would be good to know if you have to use 3mm filters to make a great image or win an ITOD.
Hi Tony,
I knew I had seen a comparison that Don Goldman (AstroDon) did a few years ago on differing bandwidths. Finally found it. Here’s a link to the full presentation. It’s one of the most instructive I’ve seen even though it's 10 years old!
https://www.cedic.at/arc/c15/dwn/CEDIC15_W05_Don_Goldman.pdf
His 6 vs 3nm comparison:
📷 image.png
Kevin
Tony Gondola · Sep 22, 2025, 12:07 AM
So the bottom line is, it depends. On your target, on your sky conditions and what you are trying to accomplish. Plenty of amazing images are made with 6nm pass NB filters and plenty are made with 3nm NB filters. It’s sort of like the well worn OSC verses mono debate. We can talk about the numbers all day but after processing, how much of a difference does it make? I’d love to see someone actually do that test. It would be good to know if you have to use 3mm filters to make a great image or win an ITOD.
Its simply how much contrast you are going to produce in your narrowband data signal against background light pollution, whether that is due to city light or due to moonlight even if it is B1. The more light pollution you block, the better. How much better depends on how much light pollution is. On a moonless night in B1, not much. 60 degrees away from a bright moon, a lot more. The narrower the filter, the closer you can image to the moon and still be productive. You can make up for the contrast with extra integration time.
That being said bandpass width is not everything. I have for example tested 8nm and 5nm Chroma filters against 3nm Antlia filters and noticed that for the same integration time and sky conditions, I got better contrast on my Chroma filters than my Antlia filters. It has to do with the quality of the glass and the percentage of peak transmission being lower for the Antlia than the Chroma as far as I can tell.
As to distant galaxies and redshift, this will depend on your image scale, and how far are the galaxies you want to image. I image galaxies with Ha routinely to highlight the Ha structures and add them to the broadband data.
Ashraf AbuSara:
As to distant galaxies and redshift, this will depend on your image scale, and how far are the galaxies you want to image. I image galaxies with Ha routinely to highlight the Ha structures and add them to the broadband data.
Tony Gondola · Sep 22, 2025, 12:07 AM
So the bottom line is, it depends.
LOL. I remember a friend asking a question relevant to my profession (he was a math PhD but had little knowledge of health fields) and my answer started with “it depends”.
He commented that I must know the subject well because anyone that knows a subject knows that it almost always “depends”….
Arun H · Sep 24, 2025, 04:01 PM
Ashraf AbuSara:
As to distant galaxies and redshift, this will depend on your image scale, and how far are the galaxies you want to image. I image galaxies with Ha routinely to highlight the Ha structures and add them to the broadband data.
Since the redshift of any galaxy we want to image is a documented quantity, you can actually use this to calculate how much the H-alpha wavelength will shift and therefore how much transmission loss you can expect from a 3nm filter. Hubble redshift is not hugely significant in most cases, see the calculations that RJF-AStro did:
https://www.cloudynights.com/topic/695068-h-alpha-galaxies-with-a-3nm-filter-and-redshift-an-overview/
Also extensively discussed here:
https://www.astrobin.com/forum/c/astrophotography/deep-sky/narrowband-filters-nm-choice/
Great table and breakdown. I have been able to isolate Ha signal on galaxies as far as 50m ly like NGC 3079 with 5nm filters.