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Does dithering reduce Bayer filter noise?

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#1 RealSorin

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Posted 25 December 2013 - 12:48 PM

I was just wondering about this based on another thread. I've noticed some distinct bayer filter pattern noise on my images when really stretched. While I expect it would, does anyone know if using dithering will substantially reduce this noise?

#2 Charlie B

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Posted 25 December 2013 - 12:58 PM

I don't understand! What method do you use to debayer the images? If properly debayered before combining, you should not need to dither and there should not be a debayer matrix visible.

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Charlie

#3 mmalik

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Posted 25 December 2013 - 01:12 PM

If properly debayered before combining, you should not need to dither and there should not be a debayer matrix visible.


Agree with Charlie; you might be wrongly referring to the pattern you see. Regards

#4 bilgebay

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Posted 25 December 2013 - 03:15 PM

If you are talking about the color mottle, yes, dithering helps. You must dither excessively, like 2-3 star distance. This will eliminate the color noise.

#5 Falcon-

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Posted 25 December 2013 - 03:22 PM

As Sedat said, yes the chrominance noise/mottle will be reduced.

That said, you mention "distinct bayer filter pattern noise" and that makes me wonder if you have some other issue, perhaps with your calibration frames or method of debayer. Can you post an example of this pattern noise?

#6 RealSorin

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Posted 25 December 2013 - 05:20 PM

Apparently I'm using the wrong term. The noise I'm referring to is distinct regions with red, green, or blue artifacting, which I assumed was an artifact from the Bayer filter on the sensor. I'm capturing directly into Nebulosity to FITS format. Here's a stretched/cropped example from an image.

Attached Files



#7 Falcon-

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Posted 25 December 2013 - 05:24 PM

Ya, that looks like the chrominance noise / colour mottle I would expect. So yes, dither should help reduce that.

You said you are capturing directly into Nebulosity/FITS - In the Nebulosity preffs under "Acquisition mode" are you set to "RAW Acquisition" or one of the two "Recon" types? It should be RAW type as "Recon" mode is actually JPEG image mode when Neb is controlling a DSLR.

#8 RealSorin

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Posted 25 December 2013 - 06:02 PM

Falcon, It was set to Recon. I wasn't aware of that setting before, thanks. That makes more sense now why I was getting full color FITS files directly. I thought just setting the camera to RAW would do it, but clearly not. I didn't realize I'd been capturing JPEGs! :foreheadslap: :p

#9 Falcon-

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Posted 25 December 2013 - 06:48 PM

Ya, I think it is Recon by default with colour cameras. This is unfortunately a terrible default for DSLRs. :(

Anyway, with it set to RAW mode you may find the mottle is reduced a bit further (and should definitely find an increase in the dynamic range out of the stacks).

#10 RealSorin

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Posted 25 December 2013 - 10:15 PM

Falcon,
Yes, I expect so. I was wondering why I wasn't able to stretch my images further, it didn't seem right compared to the RAW images I was taking before capturing with Nebulosity - now I know why. Again, thanks very much!

I'll also give the dithering a try to see how it affects the mottle. Now to hope for clear skies this weekend.

#11 Samir Kharusi

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Posted 25 December 2013 - 11:02 PM

We all try heroic tactics to eke out that little bit more out of our images, but NOTHING works better than increasing integration time. Think doubling, and doubling again, and doubling again :foreheadslap:

Nevertheless, dithering does help. But I have yet to experience a non-dithered stack! No matter how good the autoguiding, unless you are using a very short focal length, the stars span more than a single pixel and the atmosphere gives ample dither anyway. Nevertheless, when I did some rudimentary stellar spectroscopy I noticed noise from the Bayer array:
Posted Image

The middle strip is an undithered spectrum, in which noise is clearly visible, while the bottom is a vertically-integrated spectrum, sort of like a dithered spectrum.






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