APP Ha OIII extract...
 
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APP Ha OIII extraction from OSC with Duoband filter - Unmixing Bayer colour leakage from duo band filters  

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(@williamshaw)
White Dwarf Customer
Joined: 2 years ago
Posts: 11
March 29, 2021 17:23  

I am trying to figure out how to best isolate pure Ha and Oiii signals from an OSC image using a duoband filter. In my case I am using the IDAS NB2 (also NB3 for Oiii and Sii) that does Ha and Oiii, and capturing on an Altair Hypercam 26C, similar sensor to APS-C OSC cameras from ZWO and QHY. I have been fiddling with this both by extracting R,G,B and combining, and with the APP tab 0 option for Ha and Oiii extraction. 

Due the the imperfections of filtering of the Bayer matrix, my Red Channel will have some leakage into it of Oiii, and there will be quite strong leakage of Ha into the Blue. Gets even worse with the green channel. The leakage will get even worse in the case of the NB3 filter, as the B and G sensitivities are actually creeping up by the time you get to Sii wavelength. The QHY RGB sensitivity plot is attached with the rough locations of the three main narrow bands. Apologies if this is fouled up - if I have put something in the wrong place it does not affect the principle. 

I can see various ways to try to unmix the leakage. Taking the simple case of the Ha Oiii duo NB2, which is similar to other duo filters, I can explore subtracting small amounts of R from B. R is probably close to pure Ha. That could be done by calibrating shooting with an NB2 in series with pure Ha or Oiii filters, or I can experiment. 

Then there is the interesting Tab0 option in APP that appears to extract the Ha and Oiii anyway. What is that doing compared to just extracting R and some combo of B&G? Is it already trying to unmix the effects of light leakage by optimising some function or making default assumptions about the mixing? Or is it just mixing B&G in some way for the Oiii and not addressing light leakage in any way.  

Any insights welcome, and apologies if I have missed an explanation elsewhere.

Note that this is a separate issue from the width of the transmitted narrowbands from the filter - this question will arise whether I am using filters with 12, 6, or 3nm widths. (Though in the red I realise 12 might increase the mix of Sii into Ha and vice versa - here I am trying to undo the imperfections of the Bayer matrix). 

I am fully aware of the benefits of going mono! I am keen to optimise the NB2/3 method as I can use the Bayer matrix at higher efficiency than using separate single band filters, as I can get all three main narrowbands and get a double dose of Oiii that is sometimes quite weak. Just using an NB2, or L-extreme for two-band HOO type imaging has the issue I am referring to.

Screenshot 2021 03 29 at 16.25.54

 

This topic was modified 7 months ago 2 times by William Shaw

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(@microastro)
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Joined: 2 years ago
Posts: 7
March 30, 2021 17:14  

@williamshaw I'm not an expert on APP, so take what I say with a grain of salt... However, I can't imagine how APP could deal with light leakage when it uses the Ha/OIII color, mono, extract Ha, extract OIII algorithms because 1) the relative amounts to subtract to get pure Ha and OIII depends upon the relative QE of the chip, which can vary considerably, and 2) although you say it's independent of the bandwidth, in fact whether the bandwidth of the OIII band is broad enough to include the Hb signal would have a major effect on the amount that needs to be subtracted to get a pure OIII signal. Without having inputs for your chip and filter, it can't imagine how it would be possible for APP to come up with proper scaling for "leakage". As you say, you can always determine your own values for Ha and OIII leakage and subtract using something like PixelMath but given that the amount of leakage is generally nearly a magnitude lower than the signal, I expect most would just ignore it. 

But those with more intimate knowledge of the algorithms, feel free to correct me. I'm interested in the answer as well.

 

Keith


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(@williamshaw)
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March 30, 2021 17:37  

@microastro Thanks for the feedback - Don't have an argument with any of that! I did not actually think that the details would be completely independent of the filter bandwidth, it was more than I was trying to make sure people understood that this matter would exist even if the filter bandwidths were tiny. Truth is, I do not know what that tab 0 option does at all, and would quite like to know! I did speculate that, on top of doing a basic job without leakage correction, it might do a generic remapping that might allow for a typical level of leakage, without necessarily being spot on for any particular filter and sensor QE combination. Another thought was that there could be an optimisation algorithm in there to adjust that typical level to something more optimal based on a fitness function - image contrast? - who knows? One final observation is that although the leakage indicated by the QE plot is small, the Ha coming in is often quite strong compared to the Oiii and Sii so the levels on the graph I showed could have uneven multipliers. Any further thoughts very welcome.


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(@microastro)
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March 30, 2021 17:49  

Good points. Hopefully someone with more detailed knowledge of the algorithms can enlighten us.

Keith


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(@vincent-mod)
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Joined: 4 years ago
Posts: 3810
March 31, 2021 09:06  

I would argue to use the extraction methods always as these are specifically meant for this (they also make it possible to retain the resolution of the separated channels), but to get a more detail answer I asked Mabula to chime in, he is busy till at least tomorrow, but I hope he can get back to you soon.


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(@williamshaw)
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Joined: 2 years ago
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March 31, 2021 09:14  

@vincent-mod thanks. I have mostly been using the tab0 Ha Oiii extraction setting as I realised it gave slightly cleaner data than eg just B and R. But this discussion of leakage made me realise I do not know how the extraction works in terms of using the RGB raw data. It would at least be good to know that as a starting point for perhaps refining a method to correct for Bayer leakage. Will check back for further insights. Ta


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(@mabula-admin)
Universe Admin
Joined: 4 years ago
Posts: 2715
April 8, 2021 12:24  

Hi William @williamshaw, Keith @microastro & Vincent @vincent-mod,

It is a very good question to ask this, thanks!

Extracting the pure H-alpha and Oxygen-III signals while compensating for bayer filter leakage is simply impossible if you would ask me. You can not determine per pixel how the ratio of incoming photons is distributed  from the varying signals...including noise as well. So I would not even attempt to go there 😉

APP's Ha & O3 extract debayer algorithms are created to give the user a very simply way splitting the channels while preserving the native sensor resolution. Depending on which extract algorithm you would use, APP uses the best possible debayer solution for the appropriate signal (Ha or O3).

For instance, if you would debayer with Adaptive Airy disc to get a RGB result that contains both Ha and O3, you will get a worse result than debayering with the Ha-O3 color algortihm.

If you debayer with Ha-O3 color and the split the channels of the RGB result, the red channel is the H-alpha signal, and the green and blue combined is the O3 signal. APP makes it easy, with the extract algorithms, to immediately provide you with these monochrome Ha & O3 signals without loss of resolution which has great benefits of course.

Let me know if this clarifies things 😉

Mabula

This post was modified 6 months ago by Mabula-Admin

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(@williamshaw)
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Joined: 2 years ago
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April 9, 2021 11:47  

@mabula-admin thanks for this information. Very useful clarification. I did imagine that it would be hard work at the very least to clean up any leakage automatically but thought it useful to check on this. 

You did not specify exactly how Ha Oiii extract works and what is meant by "best possible debayer solution" for it.  No problem if that is commercially confidential but I remain curious!

I think the one thing I can try is to try to calibrate an unmixing of the leakage myself for my particular filters. For example I have an Halpha filter from my older HaRGB days, so could actually shoot with the Ha filter in line with my duo and see exactly how much Ha leaks into the Oiii channel that your algo extracts, then explore subtracting it. That assumes a certain amount of linearity and might depend on the bandwidth of my Ha filter, but would at least give me a starting point for a simple one-sided unmixing. If that works I could then attempt a full matrix calibration. Thanks again.

 

 


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(@mabula-admin)
Universe Admin
Joined: 4 years ago
Posts: 2715
April 9, 2021 12:38  

Hi @williamshaw,

"You did not specify exactly how Ha Oiii extract works and what is meant by "best possible debayer solution" for it. No problem if that is commercially confidential but I remain curious!"

Well, simply said, a normal advanced debayer algorithm uses information from the green channel to reconstruct both red and blue. That is far better for regular for broadband RGB data. The key here is that for the H-alpha channel reconstruction, that is not the best method of course. The Red channel can only be reconstructed using information in the red channel 😉 For O3, the green channel which has more information can again be used for reconstruction of both green and blue channels after which APP directly created the monochrome O3 signal from the green and blue channels.

I think the one thing I can try is to try to calibrate an unmixing of the leakage myself for my particular filters. For example I have an Halpha filter from my older HaRGB days, so could actually shoot with the Ha filter in line with my duo and see exactly how much Ha leaks into the Oiii channel that your algo extracts, then explore subtracting it. That assumes a certain amount of linearity and might depend on the bandwidth of my Ha filter, but would at least give me a starting point for a simple one-sided unmixing. If that works I could then attempt a full matrix calibration. Thanks again.

Do let us know if you are able to get some interesting results 😉 I do plan to make it possible that camera matrices can be used and programmed even for processing...

Cheers,

Mabula

 


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(@williamshaw)
White Dwarf Customer
Joined: 2 years ago
Posts: 11
April 9, 2021 12:45  

Will do. Could be a while as I am doing galaxies right now but will attempt it when I go back to a suitable emission nebula with a good mix of H and O.


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(@microastro)
Brown Dwarf Customer
Joined: 2 years ago
Posts: 7
April 9, 2021 19:25  

@williamshaw This has been a very interesting thread. I have a filter wheel on my ASI2600MC camera and currently have Ha and OIII filters in the last two spots. I had been thinking of replacing them with a duo-band filter to speed up data collection, but this thread has caused me to reconsider that strategy. Especially your point about the impact of stronger Ha emissions leaking into the G & B channels and overwhelming the weaker OIII signal. I'm thinking now that my current setup with separate Ha and OIII might be the better strategy. Never would have thought about this if you hadn't brought it up. Thanks for posting.


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(@williamshaw)
White Dwarf Customer
Joined: 2 years ago
Posts: 11
April 9, 2021 19:41  

To be clear it wasn’t me who spotted the issue, it was Jean Dean, an exceptional astrophotographer who queried my use of pairs of dual narrowband instead of the individual band filters in a Facebook group.  I think it can be fixed by doing some calibration but I need to look into that. 


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1122
April 9, 2021 21:32  

@microastro Keith, the risk of Ha emission leaking into G and B very much depends on the filter used and may also happen when using an imperfect OIII filter. As can be seen in the diagram posted by William in his original post, the filters don't fully block Ha in G and B and OIII in R. But that simply applies to the filter he used. Other filters may very well have much better isolated pass bands and shouldn't run this risk. Many people on this forum use L-eNhance and L-Xtreme filters which perform very well in that respect.


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(@williamshaw)
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Joined: 2 years ago
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April 9, 2021 23:24  

@wvreeven I think you have really misunderstood the source of the problem, and also my graph. Those channel sensitivities are of the sensor colour channels, not of badly wide filter transmission bands. The issue is leakage caused by the Bayer matrix. The narrowness of the band pass in a Duoband type filter will affect the total transmission into each channel but the relative effect is a persistent problem.  Even even if the filters were a perfect pair of delta functions at the emission bands you would have the problem we are discussing here. The underlying issue is the non zero level of the Bayer matrix sensitivity of, e.g.  the blue and green channels to light at the Ha frequency. The narrower bands of something like an L-Extreme do a good job of blocking out background LP but do nothing to mitigate the problem here. I drew my graph with the emission lines as delta-functions rather to make that point. 

The real source of the problem is the fact that the colour sensitivities of the Bayer structure are not confined to the red green and blue zones the way R,G,B filters are. I believe this is because colour cameras are designed to mimic the properties of the colour receptors of the human eye in having wide sensitivities, rather than being designed for scientific purposes like our astrophotos. If the Bayer matrix filtered light as perfectly as R,G,B filters this problem would not arise and it is the fact that our Bayers do not that is the problem, not the details of the filter. 

This post was modified 6 months ago by William Shaw

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(@wvreeven)
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Joined: 3 years ago
Posts: 1122
April 9, 2021 23:53  

@williamshaw Thanks for the clarification William, I indeed misunderstood that. Still, this issue will also have effect on images taken with a Ha or an OIII filter so that's no reason to not use a dual-band filter.


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(@williamshaw)
White Dwarf Customer
Joined: 2 years ago
Posts: 11
April 10, 2021 00:10  

@wvreeven My initial reaction to hearing about this was the same as yours in fact! What would be really cool (and solve it) is an OSC camera with perfect RGB separation. Or even one with emission band filters on the sensor. Imagine OSC Hubble shots. 


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(@williamshaw)
White Dwarf Customer
Joined: 2 years ago
Posts: 11
April 10, 2021 12:47  

I also agree it is not a reason to stop using dual band with tools like the APP Ha-Oiii extract. I do think the images might be improved by cleaning up the leakage but I need to work on the numerics of that. But I also think the single band filters do a cleaner job - after all with a pure Oiii filter there would be no Ha coming in anyway to leak over to the B and G from which you would extract the Oiii. But with the duos you are using the Bayer at higher efficiency and get a double dose of Oiii from something like the NB2,3 combo (or L-Extreme+NB3), so there are very good reasons to persist with them. 

 


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
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April 10, 2021 13:01  

@williamshaw You're right of course that narrow band images will provide cleaner results. One might even try to compensate for the leakage by discarding the R channel in case of an OIII filter and discarding the G and B channels in case of Ha. On the other hand, many astro photographers suffer from lots of clouds and can save a lot of time by using dual-band filters. So whether or not to use a dual-band filter needs to be reviewed on a case by case basis and not discarded up front because of sensor leakage.


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(@williamshaw)
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Joined: 2 years ago
Posts: 11
April 10, 2021 13:11  

@wvreeven Well that's my view too! I'm rather minded to pursue the issue of how to squeeze the best out of one or more duos. And living in cloud-covered England I have a very strong incentive.


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(@fotoflo)
Hydrogen Atom Customer
Joined: 8 months ago
Posts: 1
April 23, 2021 20:18  

Hei everyone,

Thanks for all the valuable input. It was a very interesting discussion. Recently I started using a cooled OSC camera (QHY268c) and an Optolong L-extreme filter. That is why I was interested in how to extract Ha and OIII from an RGB images. I tried to compare the Ha and OIII extraction in APP with a manual method in PI. For the manual method I calculated the weights as described here. Curves were sampled from the datasheets provided by QHY and Optolong. Not 100% accurate.

APP vs PI Ha OIII Extraction 04

After doing the math, I came up with the following values to enter in PI's Pixel Math:

APP vs PI Ha OIII Extraction 05

I have to admit that I'm very new to APP and PI and I did not pay too much attention of what I'm doing. Therefore, the images are drizzled slightly different. So please take this into account when comparing the following images. Furthermore, the quality isn't the best. I took these images with a small MSM tracker which allowed me only to take 1min subs which is not really enough when working with the L-extreme. I didn't collect a lot of data either.

No DBE applied in both cases. Just the pure extraction of Ha and OIII:

APP vs PI Ha OIII Extraction 01

...and a crop on Orion and the Horsehead. Note that I didn't drizzle my APP data (or maybe not x2). Stars look a little squarish in APP ... don't know why. Noise looks quite similar.

APP vs PI Ha OIII Extraction 02

Then I stretched all images and combined them into a HOO image.

APP vs PI Ha OIII Extraction 06

... and another crop.

APP vs PI Ha OIII Extraction 07

Stars are still smaller in APP. Maybe I messed something up in PI. But the other all look and noise are quite comparable. So, in my opinion, both methods work quite well. What do you think?!

 

 


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