Dual-band vs. Mono imaging - Forums

Topic starter

1 day ago

Hi everyone,

I'm curious about the real-world difference between a dual-band Ha/OIII filter and separate Ha and OIII filters when imaging large faint planetary nebulae.

For example, if I collect 20 hours of data with a dual-band filter on an OSC camera, how would that compare to 20 hours with a mono camera split between 10 hours of Ha and 10 hours of OIII?

I'm especially interested in faint, large planetary nebulae. Has anyone imaged similar targets with both approaches? How big was the difference in the final result?

I'd love to hear about your experiences and see any examples if you have them.

Clear skies!

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andrea tasselli

Frequent contributor

1 day ago

I did some comparisons a while ago and, if we ignore resolution effects (and sometimes this can’t be done), DNB is around 80% more effective in collecting photons, if I remember the figure correctly. YMMV.

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Dave Stirling

1 day ago

I think you’d see the biggest difference in Ha as you’re red-pixel limited on the OSC. While you’re collecting both simultaneously, I suspect you’d want longer integration for faint Ha on the OSC compared to Ha on the mono.

That’s by the numbers, though… if you have limited clear nights from your location, knowing you got both Ha and OIII from the same clear night is beneficial.

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Alex Nicholas

1 day ago

I think either way will work well.. having done a lot of both though, there is no amount of extra SNR that will match what the mono will give you in resolution… perhaps thats just me, but Id prefer 10h H + 10h O over 20h H/O on an osc camera.

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Craig Towell

20 hours ago

@Vroobel images NB simultaneously with mono and OSC cameras and would have some good insight on this question

Vroobel

19 hours ago

Craig Towell · Jun 2, 2026, 05:38 AM
@Vroobel images NB simultaneously with mono and OSC cameras and would have some good insight on this question

Thanks, Craig. 😊

I think, reading the following thread may give you a clue.

https://stargazerslounge.com/topic/433215-osc-vs-mono/

After almost a year of operating with dual-setup based on two the same Askar FMA230 APOs with 2600MM and 2600MC respectively, I can say that it’s not exactly the same. I can see a difference in favor of mono, even if my SHO was based on a simple ~7nm Optolong set, while the OSC was coming from 4nm Altair Ha/OIII and SII/OIII combo. Yes, the 4nm filters are narrower, but the NB components from OSC have worse SNR. Simply saying, mono signal is smoother. I can say that in my case, mono NB creates a nice structure, while the NB component from OSC supports it for the better SNR.

In terms of the SNR, please take into account, that I operate under a Bortle 9 sky.

https://app.astrobin.com/i/0brbc4

https://app.astrobin.com/i/zu01z4

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AstroGadac

17 hours ago

I image in Bortle 8/9.

Having done both I can say that mono NB imaging is far superior in both signal and resolution (because no strange phenomenon around debayerisation occur, even if drizzle closes the gap a tad, and no cross bleeds between wavelength because of the relative response of the sensor to RGB which is not perfectly separated).

Even gradients are much more easy to correct when capturing data wavelength by wavelength.

The gap is much closer in broadband imaging though.

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Astro Hopper

15 hours ago

Alex Nicholas · Jun 1, 2026, 10:55 PM
I think either way will work well.. having done a lot of both though, there is no amount of extra SNR that will match what the mono will give you in resolution… perhaps thats just me, but Id prefer 10h H + 10h O over 20h H/O on an osc camera.

10 h of Ha and 10 h of Oiii taken with mono camera is actually 10 h of each. With OSC 20 hrs of data gives you 5 h of Ha and 5 h of Oiii since only 1/25 of sensor is collecting Red and Blue data and rest is grean. That’s because of Bayer matrix in front of sensor. So all do new OSC sensors are quite good they still are not match for Mono sensors. For Broadband is little different. Broadband is much closer to mono.

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andrea tasselli

Frequent contributor

14 hours ago

Astro Hopper · Jun 2, 2026, 11:08 AM
10 h of Ha and 10 h of Oiii taken with mono camera is actually 10 h of each. With OSC 20 hrs of data gives you 5 h of Ha and 5 h of Oiii since only 1/25 of sensor is collecting Red and Blue data and rest is grean. That’s because of Bayer matrix in front of sensor. So all do new OSC sensors are quite good they still are not match for Mono sensors. For Broadband is little different. Broadband is much closer to mono.

Nothing could be more wrong.

Vroobel

14 hours ago

Who said that Astro Hopper · Jun 2, 2026, 11:08 AM
Alex Nicholas · Jun 1, 2026, 10:55 PM
I think either way will work well.. having done a lot of both though, there is no amount of extra SNR that will match what the mono will give you in resolution… perhaps thats just me, but Id prefer 10h H + 10h O over 20h H/O on an osc camera.
10 h of Ha and 10 h of Oiii taken with mono camera is actually 10 h of each. With OSC 20 hrs of data gives you 5 h of Ha and 5 h of Oiii since only 1/25 of sensor is collecting Red and Blue data and rest is grean. That’s because of Bayer matrix in front of sensor. So all do new OSC sensors are quite good they still are not match for Mono sensors. For Broadband is little different. Broadband is much closer to mono.

Who said that OIII consist of blue only? 🙂

Alejandro Moreschi

14 hours ago

In my experience switching from a 294 MC PRO camera with a dual-band filter to a 294 MM PRO camera with narrow-band filters, the images from the monochrome camera were always better, even with shorter full integration times than the color camera. This is especially true for Ha; the color camera only utilizes 1/4 of the pixel because almost the entire signal is captured in the red part, whereas the monochrome camera captures the entire Ha band using the filter. O3 is visible in both B and G-G in color camera.

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urban.astronomer

13 hours ago

As mentioned several times in this thread before:

The color camera will only collect red light on a quarter of the sensor size due to the Bayer Matrix which is specifically critical for the Ha data.
For O3, the blue quarter + the two green quarters (thus 75%) of the sensor can be used, so the difference between mono and color camera for O3 is much less compared to Ha.

One should also note that mono sensors often have higher quantum efficiency compared to color sensors. All in all, I would prefer mono if you can.

To bring some real data on the table: Below, you will find two stacks of Sh2-129 using the same telescope and two different cameras utilizing the mono and color version of the Sony IMX571 sensor. Both stacks are taken from the same Bortle 8 balcony, same time of the year. Dualband from 2022 and mono from 2023.

Upper panel: 11 hours of mono data using an Antlia 4nm Ha-filter (Processing: Auto-strech + graxpert)

Lower panel: 16 hours of dualband data using an IDAS NBZ dual band filter (roughly 6nm for Ha), where only the red channel has been extracted (Processing: Auto-strech + graxpert).

As you can see, there is a lot more detail in the mono, and the noise is much less also.

I did not have meaningful data to compare also O3, but the difference is much less striking there.

11hr_mono.jpg 📷 16hr_dualband.jpg-Martin

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Astro Hopper

13 hours ago

Vroobel · Jun 2, 2026, 11:23 AM
Who said that Astro Hopper · Jun 2, 2026, 11:08 AM
Alex Nicholas · Jun 1, 2026, 10:55 PM
I think either way will work well.. having done a lot of both though, there is no amount of extra SNR that will match what the mono will give you in resolution… perhaps thats just me, but Id prefer 10h H + 10h O over 20h H/O on an osc camera.
10 h of Ha and 10 h of Oiii taken with mono camera is actually 10 h of each. With OSC 20 hrs of data gives you 5 h of Ha and 5 h of Oiii since only 1/25 of sensor is collecting Red and Blue data and rest is grean. That’s because of Bayer matrix in front of sensor. So all do new OSC sensors are quite good they still are not match for Mono sensors. For Broadband is little different. Broadband is much closer to mono.
Who said that OIII consist of blue only? 🙂

You are right, my mistake about OIII, never the less still norrowband in mono collects at least 25% more Oiii then dual narrowband, and in Ha is much bigger difference. That being said modern OSC cameras are much closer to mono then before but still there is quite big gap between them.

Interactive Sky

Topic starter

11 hours ago

Thanks, everyone, for your replies.

I really appreciate the comparisons and real-world examples.

Clear Skies!

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Vroobel

10 hours ago

urban.astronomer · Jun 2, 2026, 12:42 PM
As mentioned several times in this thread before:
The color camera will only collect red light on a quarter of the sensor size due to the Bayer Matrix which is specifically critical for the Ha data.
For O3, the blue quarter + the two green quarters (thus 75%) of the sensor can be used, so the difference between mono and color camera for O3 is much less compared to Ha.

For IMX571:

Ha/OIII

SII/OIII

The above equations came from an extensive ‘discussion’ with ChatGPT after feeding it with information about the sensor:

I did some experiments with PixelMath, and the results are consistent with PixInsight/DBExtract.

urban.astronomer · Jun 2, 2026, 12:42 PM
To bring some real data on the table: Below, you will find two stacks of Sh2-129 using the same telescope and two different cameras utilizing the mono and color version of the Sony IMX571 sensor. Both stacks are taken from the same Bortle 8 balcony, same time of the year. Dualband from 2022 and mono from 2023.

I don’t doubt that you have a proper intention, but another year, even if it’s exactly the same day in a calendar, doesn’t mean the same condition. This was mentioned in the linked thread or in many similar ones, I don’t remember. That’s why I use exactly the same optics and exactly the same cameras MM and MC (I mean the same manufacturer, not only the same sensor) at the time. Only such a dual-setup is able to provide a reliable dataset. It would be great to have the same-width filters, maybe one day… 😄

Tony Gondola

Frequent contributor

Exemplary contributor

9 hours ago

A lot of people have already quantified this so I’ll just echo the idea that mono will be more effective.

Arun H

Perfect post

9 hours ago

urban.astronomer · Jun 2, 2026, 12:42 PM
One should also note that mono sensors often have higher quantum efficiency compared to color sensors. All in all, I would prefer mono if you can.

While I agree that mono is preferable, I do not believe it is accurate to state that mono sensors have higher QE. Base sensors in each case have identical QEs. What’s different is the transmission of the dye based filters used in color cameras in the Bayer matrix versus the interference based filters used in mono imaging. The latter, especially high end ones like Chroma, can have near 100% transmission at the specific emission wavelength being passed through.

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urban.astronomer

5 hours ago

Arun H · Jun 2, 2026, 04:39 PM
urban.astronomer · Jun 2, 2026, 12:42 PM
One should also note that mono sensors often have higher quantum efficiency compared to color sensors. All in all, I would prefer mono if you can.
While I agree that mono is preferable, I do not believe it is accurate to state that mono sensors have higher QE. Base sensors in each case have identical QEs. What’s different is the transmission of the dye based filters used in color cameras in the Bayer matrix versus the interference based filters used in mono imaging. The latter, especially high end ones like Chroma, can have near 100% transmission at the specific emission wavelength being passed through.

You’re right, I should have been more precise. I meant the effective color QE peaks at roughly 70%–80% for Green, and drops closer to 55%–65% for Blue and Red compared to the undyed version.

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Mihai Barbu

New contributor

3 hours ago

I used color until I went to some place where I had 21.88 SQM and used mono. And then I changed to mono and never looked back.

It is quite obvious mono will always be better, but color has its advantages as well but it’s not regarding the question in the subject.

For example, if you’re at a star party you can shoot OSC and keep it simple. Usually star parties are in low light pollution areas and you will collect more photons that way. It is actually better to shoot broadband when you have the possibility.

If you’re just starting OSC is better. It’s cheaper. The cameras are cheaper, you need no filters or a couple, not 7.

From the city it’s a 50/50 thing because you might use a broadband light pollution if you don’t have LEDs near you. But mono you can only shoot narrowband.

Jared Willson

2 hours ago

So, here are some very rough approximations based on a QE graph for a 294 based sensor, both OSC with a dual band filter and mono with two different narrowband filters…

I’ll cut to the chase. For a given integration time, the mono camera will capture about 20% more photons overall from the two emission bands.

At the two frequencies in question (500.7nm for OIII and 656.3nm for Ha), here is how it would work out…

Between Bayer dyes and QE, the OSC camera would capture about 75% of the incident OIII light in the green channel and about 40% in the blue channel. Since half the pixels on the sensor are green and one quarter are blue, that works out to 47% of the OIII light incident on the detector for OIII.

For Ha, the OSC camera isn’t quite as good. The OSC would capture about 63% of the red light in the red channel once the dyes and quantum efficiency are accounted for, so that’s 16% of the total light from H-alpha. You’d also have a bit of contamination in the green channel since the green dyes pass a bit of red light, but not enough to really matter much. Luckily, most dual-band objects emit more strongly in H-alpha than in OIII, so a dual band filter with a OSC camera still does a really nice job.

With monochrome, you don’t have to worry about absorption from the dyes, so you are just looking at the QE of the sensor. A monochrome 294 camera should be around 88% efficient for OIII and 79% or so for Ha. Obviously, you can’t image with both at the same time, so cut those numbers in half for throughput and you get 44% and 39% respectively. With monochrome, you can pick a different balance than you are stuck with for OSC. If you subject was weak in OIII for example, you could spend more time imaging with that filter.

On balance, mono has about a 20% advantage assuming the same bandwidth filters for each. Enough to matter, perhaps, but not enough to suggest OSC is poor. I think the bigger advantage to mono is the ability to balance the two based on the particular subject’s relative strength at each frequency. Plus, Ha is less sensitive to contamination from the Moon and other sources, so you can use your time a bit more effectively with mono. Finally, there is no interpolation required with mono cameras, so there may be a slight resolution advantage.

Feel free to double check my math and let me know if I made any errors or missed any subtleties.

Jared

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Seung-Jun Kim

2 hours ago

Assuming that your color sensor consists of an RGGB Bayer pattern, OIII in OSC will receive signal through roughly 50% of the pixels, while Ha will receive signal through only 25% of the pixels.

So compared to mono Ha 10h + mono OIII 10h, a 20h OSC dual-band image is roughly equivalent to:

Ha:
20h × 25% = 5h mono-equivalent

OIII:
20h × 50% = 10h mono-equivalent

Since SNR scales with the square root of exposure time:

Ha SNR:
√(5 / 10) ≈ 0.71×
→ OSC Ha is about 70% of the mono Ha SNR

OIII SNR:
√(10 / 10) = 1.0×
→ OSC OIII is theoretically similar to mono OIII SNR

So in practice, OIII may be somewhat comparable, but Ha will be significantly weaker in OSC. On top of that, mono still has advantages from full-resolution sampling, better channel separation, no debayering loss, and generally cleaner narrowband data.

However, in reality, many nebulae are Ha-rich but relatively faint in OIII.

For example, with a mono setup, I might only need a few hours of Ha, while I often need 20–30 hours of OIII to bring out the faint oxygen structure. With mono, I can flexibly adjust the exposure ratio between Ha and OIII depending on the target.

But with OSC + dual-band filter, I cannot control that ratio. Every exposure always captures Ha and OIII at the same time, with the fixed Bayer pattern limitation: roughly 25% of the pixels for Ha and 50% for OIII.

So even though OSC dual-band is very efficient and convenient, it is less flexible. If the target needs much more OIII than Ha, mono has a clear advantage because I can dedicate most of the integration time to OIII instead of continuing to collect unnecessary extra Ha.

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