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Optimizing flats from OSC camera using NB filters?

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#1 George P Dunham

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Posted 20 March 2017 - 06:12 PM

I asked this before in more general terms.  I am asking again, more specifically.  Not interested in opinions about silliness of using NB filters with OSC camera's thank you.

I am curious about how to optimize flat frames in terms of exposure time vs mean and max ADU values, with NB H alpha or OIII filters.  I target mean ADU value of 30,000 with my flats without filters.  In obtaining flats with H alpha and OIII filters in place, using the QHY12 camera, I obtain a histogram with a modest peak of about 20,000 ADU and very small peaks of around 100-200 ADU.  I understand what these peaks are but I am wondering about how to optimize when calibrating NB images prior to de-bayering.  At this point I am not combining with RGB, L frames, just raw color images.
Bottom line question is how do you obtain and optimize flats with NB filters using OSC QHY12.



#2 srosenfraz

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Posted 20 March 2017 - 07:02 PM

My preferred method (when using a DSLR) for capturing flats was to only pay attention to the appropriate channel(s) - Red for Ha, Blue and Green for OIII.  So, when capturing flats for Ha, I only pay attention to the exposure for my red channel.  So, in your case, I'd pay attention the the 20,000 ADU peak for the red and ignore the 100 - 200 blips for each of the green and blue channels.

 

When you calibrate your data, your calibration software SHOULD be calibrating each light pixel based upon it's corresponding pixel in your calibration frames.  All of these files (lights and calibration files) are still monochrome at this point.  As such, if I'm using an Ha filter, then a red pixel in my light will be calibrated with the corresponding red pixel in my flat (and dark, bias, etc.).  Given this, your calibrated light should be correct.  The only difference from a broadband image is that the data (for an Ha image, as an example) in your blue and green channels is just useless noise.

 

The next step is the difference between calibrating broadband versus narrowband.  I do NOT debayer my calibrated lights.  This is because I don't want any debayering algorithm to try to estimate the values of each of my color pixels (as is normally done with debayering).  What I want is the useful data from the narrowband data.  So, I'll extract the red only pixels and end up with an image that has 1/2 the dimensions of my original light.  From that point, I normalize, align, and stack these 1/2 dimension images.

 

For Oiii, I extract the green and blue pixels and process the same as red pixels for Ha.  However, in this case, my green data has twice the SNR as the blue pixels.  So, when I have each of my stacked images for blue and green, I'll then copy the green channel and the stack the 2 greens plus blue using an average for these images.  This maintains the correct SNR ratios between the green and blue pixels.

 

Perhaps more complicated, but the results should be accurate.   Hope this helps.


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#3 billdan

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Posted 20 March 2017 - 07:26 PM

I wouldn't bother doing flats for Ha, as long as your optics are clean and therefore no dust bunnies, any vignetting can be eliminated in post processing.

 

 



#4 gezak22

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Posted 20 March 2017 - 07:47 PM

I wouldn't bother doing flats for Ha, as long as your optics are clean and therefore no dust bunnies, any vignetting can be eliminated in post processing.

If you dither and you don't do a flat calibration, then it will be next to impossible to properly normalize the image brightness of each sub before data rejection.



#5 George P Dunham

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Posted 21 March 2017 - 08:53 PM

My preferred method (when using a DSLR) for capturing flats was to only pay attention to the appropriate channel(s) - Red for Ha, Blue and Green for OIII.  So, when capturing flats for Ha, I only pay attention to the exposure for my red channel.  So, in your case, I'd pay attention the the 20,000 ADU peak for the red and ignore the 100 - 200 blips for each of the green and blue channels.

 

When you calibrate your data, your calibration software SHOULD be calibrating each light pixel based upon it's corresponding pixel in your calibration frames.  All of these files (lights and calibration files) are still monochrome at this point.  As such, if I'm using an Ha filter, then a red pixel in my light will be calibrated with the corresponding red pixel in my flat (and dark, bias, etc.).  Given this, your calibrated light should be correct.  The only difference from a broadband image is that the data (for an Ha image, as an example) in your blue and green channels is just useless noise.

 

The next step is the difference between calibrating broadband versus narrowband.  I do NOT debayer my calibrated lights.  This is because I don't want any debayering algorithm to try to estimate the values of each of my color pixels (as is normally done with debayering).  What I want is the useful data from the narrowband data.  So, I'll extract the red only pixels and end up with an image that has 1/2 the dimensions of my original light.  From that point, I normalize, align, and stack these 1/2 dimension images.

 

For Oiii, I extract the green and blue pixels and process the same as red pixels for Ha.  However, in this case, my green data has twice the SNR as the blue pixels.  So, when I have each of my stacked images for blue and green, I'll then copy the green channel and the stack the 2 greens plus blue using an average for these images.  This maintains the correct SNR ratios between the green and blue pixels.

 

Perhaps more complicated, but the results should be accurate.   Hope this helps.

Thanks, that is exactly what I was looking for.



#6 Jon Rista

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Posted 21 March 2017 - 09:27 PM

I asked this before in more general terms.  I am asking again, more specifically.  Not interested in opinions about silliness of using NB filters with OSC camera's thank you.

I am curious about how to optimize flat frames in terms of exposure time vs mean and max ADU values, with NB H alpha or OIII filters.  I target mean ADU value of 30,000 with my flats without filters.  In obtaining flats with H alpha and OIII filters in place, using the QHY12 camera, I obtain a histogram with a modest peak of about 20,000 ADU and very small peaks of around 100-200 ADU.  I understand what these peaks are but I am wondering about how to optimize when calibrating NB images prior to de-bayering.  At this point I am not combining with RGB, L frames, just raw color images.
Bottom line question is how do you obtain and optimize flats with NB filters using OSC QHY12.

If you are using an OSC for NB, you should be debayering with superpixel. There is no reason to debayer with any normal interpolation mechanism, as that will only add additional noise (since that is basically what the blue and green channels are...read noise and dark current noise, maybe with just enough of a smattering of photons to also add some more photon shot noise without any meaningful signal.) Additionally, you simply don't have the information for the interpolated resolution that normal debayering gives you...you only have the sparse resolution offered by the red pixels, which represent 1/4 of the spatial resolution the sensor actually has.

 

You can then simply batch-channel extract the red channel for pure Ha. If you debayer with superpixel, then those two extra channel peaks at 100-200 ADU in your flats simply won't matter. Here is an example of Ha OSC data, one done with a normal debayer and integration with extraction of a luminance channel, and the other done with superpixel and extraction of just the red channel:

 

jFq0cRF.gif


Edited by Jon Rista, 21 March 2017 - 09:30 PM.

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