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QUAD band filter prosessing and more.  

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 Heno
(@heno)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 46
November 6, 2019 12:03  

1.
I'm using a quad band filter with my ASI294MC/RASA8 combination. It gives me Ha/SII and Hb/OIII in two passbands, each 35 nM wide. Which extract algoritm(s) should I use to assure all available data is extracted?
2.
The camera will not maifest itself as a color camera unless I use "force Bayer CFA" option. As I understand it this is because of missing information in the FITS header. What should that information be and who should provide it, SGP or ZWO? (Btw, the tool-tip for this is very cryptic for a non expert!)
3.
Could use of the "force Bayer CFA" option have any adverse effect on the processing and final result?

Helge


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(@vincent-mod)
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Posts: 2665
November 6, 2019 19:06  

1. There are multiple algorithms, in this case there is no 4-band filter option so you would need to select those separately and then process.

2. The one that creates or manipulates the fits should have that info in it, presumably ZWO in this case. But forcing it is the work-around for that.

3. None at all

 


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 Heno
(@heno)
Main Sequence Star Customer
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Posts: 46
November 7, 2019 11:03  

Vincent, thanks for your reply. Regarding 1, I discussed this with you in another thread some months back. But then I did not have the filter, I was just curious. Now I have tried most of the algoritms except extracting each band separately like for narrowband filters, and the  standard Airy disk algoritm seem to give the best result.

With tri band and quad band filters becoming increasingly popular, it might be worht looking into the "Extract Ha/OIII" algorithems if they could be widened to cover SII and Hb. It should not have any adverse effect for those who have two band filters because their passbands are covered anyway.
But I'm not sure how this extract of data is done. Maybe you or Mabula could shed some light on that?


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(@headworx)
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November 7, 2019 11:10  
Posted by: @heno

it might be worht looking into the "Extract Ha/OIII" algorithems if they could be widened to cover SII and Hb

+1 for that!

--Simon


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(@wvreeven)
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Joined: 2 years ago
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November 7, 2019 11:44  

Out of curiosity, what would the purpose of this widening be? Would you like to have 4 separate images (one for SII, one for Ha, one for OIII and one for Hb)? Because that, technically, is impossible. The camera has pixels that respond to R, G and B and nothing else. The R pixels catch both Ha and SII at the same time so they cannot be separated. And a similar situation exists for Hb and OIII. Depending on the wavelength sensitivity of the sensor of the camera, OIII and Hb may be caught by the G pixels, the B pixels or both and it would be impossible to separate both channels.

Wouter


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(@szymon)
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November 7, 2019 11:52  

Indeed.  From a scientific point of view, if you are trying to analyse individual emissions, a quadband or triband filter is useless -- you cannot differentiate between photons from individual channels.  However, from a purely artistic colour and detail point of view, your "red" channel (which will contain Ha and Sii) will have both merged together, and it will have extra detail which may work very well depending on the target.  Likewise you could merge the green and blue channels into a single image and have a combined Oiii/Hb channel, which again will have more detail than just the Oiii.  Processing these in say a HOO palette (which really would be a H+Sii/Oiii+Hb/Oiii+Hb) or similar would yield good results.  More detail is going to look good from a pure display point of view!  I've been considering getting one of these filters to use as a sort of "super luminance" for my narrowband images (and I use a mono camera)...

This post was modified 1 year ago 3 times by Simon Szymon

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(@headworx)
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November 7, 2019 11:54  
Posted by: @wvreeven

that, technically, is impossible

Right... Does that mean that using 4-band filter does not make sense with OSC or should we just be using the standard airy disk instead?

--Simon


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(@vincent-mod)
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November 7, 2019 12:16  

You're absolutely correct Wouter and Simon. So yes, then it doesn't make sense to have more bands as an algorithm. And @headworkx I indeed think it then won't have much use to have a 4-band filter (in the sense of wanting separate channels) as they will always be merged on either the R, G or B pixels of the sensor. You can still use the 4-band filter, it will benefit you to separate those 2 "groups" and process them independently. Like said above, it still only passes through certain bandwidths so for certain targets that will be really nice. A mono sensor with separate filters will be the only way to really separate the signals.


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(@wvreeven)
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November 7, 2019 18:48  
Posted by: @headworx
Right... Does that mean that using 4-band filter does not make sense with OSC or should we just be using the standard airy disk instead?

Using a 4-band filter with OSC certainly makes sense since it lets pass through almost only useful light and blocks almost all light pollution.


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(@headworx)
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November 8, 2019 15:24  
Posted by: @wvreeven
Posted by: @headworx
Right... Does that mean that using 4-band filter does not make sense with OSC or should we just be using the standard airy disk instead?

Using a 4-band filter with OSC certainly makes sense since it lets pass through almost only useful light and blocks almost all light pollution.

Agreed. But will Ha and Sii, and  Oiii and Hb be distinguished as different colors? Probably yes, as even if Ha and Sii are mostly red, they still are activate a bit of the green (and blue?) pixels...

 

--Simon


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(@wvreeven)
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November 8, 2019 16:23  
Posted by: @headworx

Agreed. But will Ha and Sii, and  Oiii and Hb be distinguished as different colors? Probably yes, as even if Ha and Sii are mostly red, they still are activate a bit of the green (and blue?) pixels...

No they will NOT be distinguished as different colours. This is what I wrote before:

Posted by: @wvreeven

Would you like to have 4 separate images (one for SII, one for Ha, one for OIII and one for Hb)? Because that, technically, is impossible. The camera has pixels that respond to R, G and B and nothing else. The R pixels catch both Ha and SII at the same time so they cannot be separated. And a similar situation exists for Hb and OIII. Depending on the wavelength sensitivity of the sensor of the camera, OIII and Hb may be caught by the G pixels, the B pixels or both and it would be impossible to separate both channels.

On top of that, Ha and SII only activate the Red pixels, not the Green and Blue ones.


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(@headworx)
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Joined: 1 year ago
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November 8, 2019 16:38  

I am confused... From what you are saying an OSC camera would be able to only reproduce 3 colors (R,G,B). But somehow it does reproduce the full color information. So why cutting out light pollution (by using the 4-band filter) are we using the color information for colors that pass through?

And a related question, has anyone tried using a Foveon sensor for astrophotography? It basically gives a full "analog" color information for each pixel.

--Simon

This post was modified 1 year ago by Szymon Slupik

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 Heno
(@heno)
Main Sequence Star Customer
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November 8, 2019 16:39  

 Guys, it was never my intention to have the data split into four different bands. If that can be done at all.

I don't know how the "Extract Ha and OII"  algorithm works at all. But I'm quite sure that when I use these algorithms I don't get the SII and Hb data. At least, that's what it looks like.  My data seem more complete when I use the Airy disk algorithm, Which make sense to me if SII and Hb is not included in the first mentioned. The downside of the Airy disk algorithm is that there is only one image, not two to play with and combine.
So my question still stands, can the "Extract Ha/OIII" algotihm be altered to also include Hb and SII?

 


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(@jw_duijndamhetnet-nl)
Red Giant Customer
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November 8, 2019 20:03  

@heno

Hi all,

 

I did read the whole post and also want to pitch in.

First off al when you use a OSC camera you've got a sensor with on top of it a Bayer mask. On every pixel is a color, 1 red 2 green and 1 blue that's why you get a RGGB mask.

The HA and S2 are both in the red spectrum of light, the O3 and Hb are both in the blue and green spectrum.

 

When you choose for the extract HA algorithm APP looks only at the RED pixels. And extrapolate the data of the RED pixel to "calculate" what the data would be in the other pixels (blue and green, the don't have any HA or S2 data at all).

When you choose for extract O3 algorithm APP will only look at the Blue and Green pixels and "calculate" what the data would be in the Red pixels (they also don't get any data because O3 and Hb are both in the blue green spectrum).

 

Now you can use the adaptive area disk, you get an instant color image. The downside of this algorithm is (only when you use an quad ore tri band filter) that the O3 and Hb data are very weak, so in the end result you get little blue and green color.

 

When you use a extract HA ore O3 algorithm you get 2 mono images.

The HA algorithm gives you a mono picture with all the HA and S2 data. They can't be split because they are both in the red spectrum!!!

Also for the O3 algorithm it gives you a mono image with only the O3 and Hb data.

 

If you would combine the 2 mono images you're able to increase the intensity of the blue and green channel, so in the end result you're picture would look more like a regular RGB or "one shot picture".

 

Hope this helps.

 

Clear skies Jan-Willem


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(@szymon)
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Joined: 1 year ago
Posts: 12
November 8, 2019 20:56  
Posted by: @headworx

I am confused... From what you are saying an OSC camera would be able to only reproduce 3 colors (R,G,B). But somehow it does reproduce the full color information. So why cutting out light pollution (by using the 4-band filter) are we using the color information for colors that pass through?

And a related question, has anyone tried using a Foveon sensor for astrophotography? It basically gives a full "analog" color information for each pixel.

--Simon

All colours are made up of a mix of those three colours; red, green and blue.  They are the "primary" colours, and when you mix them by adding them to each other, you get other colours.  For example, if you mix red and green you get yellow, if you mix red and blue you get purple, etc.  This is commonly known as "additive mixing".  With the right mix of each of these colours, you can make up any colour in your "full colour" package.

One important point is that your multi-narrowband filter isn't just blocking out "light pollution" -- it's cutting out _all_ light other than that which it explicitly allows through!  When you use the "4 band" filter, you are only allowing a subset of light frequencies to hit your sensor.  Your sensor (on an OSC camera) also has a built in filter in front of the pixels, which also only allows a subset of light frequencies to hit your sensor.  The built in sensor only allows through:

  • "red" light (roughly 600-700nm wavelength) to the "red" pixels
  • "green" light (roughly 500-600nm wavelength) to the "green" pixels
  • "blue" light (roughly 400-500nm wavelength) to the "blue" pixels.  

(Those are rough values, I don't remember the exact boundaries).  The four narrow bands that you are allowing through are roughly:

  • Hydrogen Alpha 650-660nm (centre 656nm, this is "red" light)
  • Sulphur ii 667-677nm (centre 672nm, this is "red" light)
  • Oxygen iii 491-506nm (centre 496nm and 501nm, this is "blue/green" light)
  • Hydrogen Beta 481-491nm (centre 486nm, this is "blue" light)

That is why those emissions get picked up by individual pixels -- they fall within the frequency for the colour filter.

I hope that helps.

-simon


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(@headworx)
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November 8, 2019 21:21  

Yes I think I know how this works in principle. But was thinking that Ha red is a bit different than Sii red, when captured by a bayer sensor (the NB filter does not change anything here...). From what you say both reds are indistinguishable. If it is so, the only function of a quad band filter is removal of a light pollution. IOW it is not possible to “see” the separated Sii and Ha colors  as different colors using OSC (with or without a filter). Correct?

 

- Simon


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 Heno
(@heno)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 46
November 8, 2019 23:06  

@jw_duijndamhetnet-nl

Maybe you are correct, maybe not. I'm thinking: If what you say is correct, and the Ha/SII is only selected based on the red pixel, what would be the point of having separate Ha and SII narrow band filters. The SII is "darker red" (have a lower frequency) than Ha. So if a nebula emits light in SII frequency you won't capture it with a Ha filter. Even though they both capture only red light.


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(@szymon)
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Joined: 1 year ago
Posts: 12
November 8, 2019 23:14  
Posted by: @headworx

Yes I think I know how this works in principle. But was thinking that Ha red is a bit different than Sii red, when captured by a bayer sensor (the NB filter does not change anything here...). From what you say both reds are indistinguishable. If it is so, the only function of a quad band filter is removal of a light pollution. IOW it is not possible to “see” the separated Sii and Ha colors  as different colors using OSC (with or without a filter). Correct?

 

- Simon

Right — you cannot separate the Sii from the Ha using a filter which passes both.  This is why people use monochrome and narrowband filters.  On the combined filter you will get both of them.  That can be cool too, and can produce some great images (I have seen many examples on the Altair Facebook group for example).


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 Heno
(@heno)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 46
November 8, 2019 23:15  

@szymon

I totally agree on this. But as long as we do not know how Mabula has designed the extraction algorithm of Ha and OIII, if it is based on pixel values or a passband around a central frequency (which may be sort of the same thing, I don't know) we will never be able to settle this discussion. That may be a good thing though, such discussions are great, and educating.

This post was modified 1 year ago by Heno

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 Heno
(@heno)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 46
November 8, 2019 23:19  

@szymon

An Altair quad band filter is exactly what I have bought. That is why I'm so anxious to find out how these extraction algorithms works. 
@Mabula: Where are you brother???? 🙂 


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