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Astrophotography and Sketching >> DSLR & Digital Camera Astro Imaging & Processing

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RedLionNJ
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Reged: 12/29/09

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Re: DSLR 'long exposure noise reduction' new [Re: RedLionNJ]
      #5821762 - 04/25/13 10:43 AM

Point immediately above already covered...

Quote:

With you on most of your points, pfile. Additional thing worth pointing out:

Imagine you have a light pixel with a value of 5 and you subtract a single dark which happens (through statistical noise) to have a value of 5 or higher in that pixel - that calibrated light now has zero in that pixel. It's never going to have anything else in there. You've "lost" information. But say you averaged 20 darks and the average value of that pixel was 3 - subtracting 3 from that value of 5 will still leave information. Entropy is lower.






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pfile
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Re: DSLR 'long exposure noise reduction' new [Re: RedLionNJ]
      #5821837 - 04/25/13 11:29 AM

well, i missed it in all the action in this thread. it is never a good thing to destroy data. however, i assume that in normal usage you'd never zero out much data as you'll probably have some signal, DSO or skyglow, that gets the average ADU way above the dark signal.

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microstar
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Re: DSLR 'long exposure noise reduction' new [Re: pfile]
      #5822947 - 04/25/13 08:06 PM Attachment (16 downloads)

Here is my empirical test of the theory. I ran some lights the night before last with and without In-camera Noise Reduction turned on and followed up with dark frames. First, I planned to run more lights without ICNR, but due to a glitch in Sequence Generator Pro (who knew that if you disconnect the DSLR, as I did in order to access the Menu to turn off ICNR in the camera, then reconnect it resets the ISO to 100) I collected a bunch of frames at 100 ISO before I realized it had reset. Anyway, I was able to collect 16 x 5min frames at ISO800 with my DSLR with ICNR turned on and turned off. I then stacked these in different ways and analyzed them to see the effects on the stacked frames. The 4 treatments were:

1. ICNRx8: ICNR x 8 frames stacked in DeepSkyStacker with Average combine (to simulate the reduced number frames that can be collected due to the longer exposure time of ICNR)
2. ICNRx16: ICNR x 16 frames stacked in DSS with an Average combine (to compare with the Dark-subtracted and Bad Pixel Map methods).
3. DSx16: No ICNR x16 frames calibrated with x16 dark frames and Avg combine in DSS
4. BPM-Bx16: No ICNR x16 frames calibrated with a Bad Pixel Map generated from a single Dark and a Master BIAS frame created from 100 bias frames.

All frames were collected with Sequence Generator Pro and output as CR2 files with dithering turned on. All calibration (for the DS and BPM frames) was done with Nebulosity. Stacking was done with DSS.

Here are the results of analysis of the final stack produced by DSS in ImageJ:

Mean StdDev Min Max
ICNRx8 2077 648 109 65273
ICNRx16 2075 624 54 65239
DSx16 1627 399 96 65267
BPM-Bx16 1439 399 86 65267

The first thing you notice is that the Standard Deviation (Sq Rt of the variance) is considerably lower in the Dark-subtracted and the Bad Pixel Map stacks. However, the mean is also considerably lower in these frames. So I set out to see if I could normalize these in Nebulosity to reach an equivalent mean. I used the Match Histogram tool in Nebulosity with the BPM stack (lowest mean value) as the reference. Here is the result:

ICNRx8 1741 508 199 51259
ICNRx16 1711 504 77 52793
DSx16 1443 399 0 65217
BPM-Bx16 1439 399 86 65267

The Match Histogram tool brought the means closer together, and the SD of the ICNR stacks was lower, but the mean values were still considerably higher in the ICNR stacks. However, look at what happens to the Max Value as a result of the histogram matching, the ICNR frames have a Max about 14000 ADU lower than the DS or BPM stacks after trying to match the histograms to the BPM stack.

So what's happening? I can think of two possibilities, one of which hasn't been discussed in this thread. First, maybe the Canon ICNR isn't just subtracting the dark, it is also stretching it (this has been mentioned before and seems quite likely given other analyses of Canon frames, it is also supported by the fact that the ICNR frames were color balanced better - I shot with an IDAS LPS-V4 filter - and I think this could have only happened during the in-camera processing) and/or second, maybe this is an artifact of the Canon ICNR doing its calculations on 14bit files while the calibration in Nebulosity and DSS are doing the calculations in 32bit (this I'm not sure would be a factor, but I put it out there for discussion).

My take on these numbers is that the ICNR has higher variance among the pixels than BPM or DS calibration. If that's the case, this higher variance would be seen as higher noise in the images. Below is a jpg of the above stacked frames that have been color-balanced then linearly stretched in PhotoShop. I chose a region centered on the Cat's Eye Planetary Nebula -- the nebula itself is highly overexposed because of the length of exposure and the stretching, but a part of the very faint outer shell is visible and comparable between the images. I've tried to make the linear stretching as consistent as possible across frames. As predicted by the ImageJ analysis, the ICNR frames look noisier than the BPM and DS calibrated frames, and the stack of 8 ICNR frames is noticably noisier than the stack of 16 ICNR frames. So you definitely lose by shooting only half as many light frames as many on this thread predicted. However, based upon the discussion I would have expected the ICNRx16 frame to be comparable to the BPM-Bx16 and DSx16 stacks. But the ICNRx16 looks noticealy noisier to me than either the BPM or the DS calibration. Although not really reflected in the numbers so much, the BPM image looks a little noiser to me than the DS frame, but the SNR of these two is quite comparable. Given what the two difference calibration techniques do, perhaps that isn't all that surprising. The BPM made from the single dark frame only corrected 97 pixels by using the data from surrounding pixels, but these were the hottest pixels in the image. Once the Bias was subtracted only the variability in the rate of dark signal accumulation among the neutral to warm pixels remains. Dark subtraction based upon 16 dark frames removes more of the dark signal and seems slightly smoother to me.

My take on this (I'm sure others will chime in) is that if you like collecting lots of Darks, then this seems to be the best method to calibrate your images. If you are lazy (like me) then it can be quite effective (at least at the temperatures I shoot at: the range over the evening was +5C to 0C) to take as few as 1 Dark and make a Bad Pixel Map to remove the outlier hot pixels then rely upon dithering to average out the noise without a big cost in SNR. In my experiment at least, the ICNR was not a good option because you lose SNR by only gathering half as many light frames and (I suspect) because Canon doesn't just do a Dark subtraction when it processes the ICNR images -- it does some sort of stretch that also stretches the noise.

Now I leave it to those much more knowledgeable than me to tell me where my analysis went wrong!
...Keith

Edited by microstar (04/25/13 08:11 PM)


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Footbag
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Reged: 04/13/09

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Re: DSLR 'long exposure noise reduction' new [Re: microstar]
      #5823017 - 04/25/13 08:33 PM

Nice experiment. The only image I'd be happy with is the DSx16. That is what I would've expected.

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pfile
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Reged: 06/14/09

Re: DSLR 'long exposure noise reduction' new [Re: microstar]
      #5823045 - 04/25/13 08:41 PM

i would not be surprised if canon plays tricks when subtracting that in-camera dark. i think craig stark determined that they are already doing stuff to the raw frame even when ICNR is turned off to try to squash the dark signal.

who knows, it may have something to do with the problem i encountered in my experiment. perhaps they want to make sure somehow that they will never underflow a pixel on the dark subtraction.


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pfile
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Reged: 06/14/09

Re: DSLR 'long exposure noise reduction' new [Re: pfile]
      #5823056 - 04/25/13 08:45 PM

also, can i ask what your pixel offsets and rotation angles are in DSS? in DSS is 'black point correction' turned off in the DCRAW control? actually i can't even remember if DSS shows you that DCRAW control.

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microstar
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Reged: 01/05/08

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Re: DSLR 'long exposure noise reduction' new [Re: pfile]
      #5823127 - 04/25/13 09:02 PM

Quote:

also, can i ask what your pixel offsets and rotation angles are in DSS? in DSS is 'black point correction' turned off in the DCRAW control? actually i can't even remember if DSS shows you that DCRAW control.




Aren't the pixel offsets and rotation angles unique to each subframe? So are you asking for all of these for each subframe in each stack?

I can't find any black point correction in DSS.
...Keith


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microstar
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Reged: 01/05/08

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Re: DSLR 'long exposure noise reduction' new [Re: microstar]
      #5823197 - 04/25/13 09:36 PM

Quote:

Quote:

also, can i ask what your pixel offsets and rotation angles are in DSS? in DSS is 'black point correction' turned off in the DCRAW control? actually i can't even remember if DSS shows you that DCRAW control.




Aren't the pixel offsets and rotation angles unique to each subframe? So are you asking for all of these for each subframe in each stack?

I can't find any black point correction in DSS.
...Keith




I did have per channel background calibration turned on in DSS however.
...Keith


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pfile
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Re: DSLR 'long exposure noise reduction' new [Re: microstar]
      #5823247 - 04/25/13 10:08 PM

yes, it's per subframe. but since you were dithering the x/y offsets should be relatively predictable, except for the effects of field rotation and/or differential flexure.

i'm just curious if the frame-to-frame shift is 10 pixels or 1 pixel, or what.


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jgraham
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Re: DSLR 'long exposure noise reduction' new [Re: microstar]
      #5823251 - 04/25/13 10:10 PM

Wonderful experiment! Thanks for taking the time to do this. Very neat stuff.

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mmalik
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Re: DSLR 'long exposure noise reduction' new [Re: microstar]
      #5823339 - 04/25/13 11:02 PM

Quote:

the ICNR frames look noisier than the BPM and DS calibrated frames, and the stack of 8 ICNR frames is noticeably noisier than the stack of 16 ICNR frames. So you definitely lose by shooting only half as many light frames as many on this thread predicted. However, based upon the discussion I would have expected the ICNRx16 frame to be comparable to the BPM-Bx16 and DSx16 stacks. But the ICNRx16 looks noticeably noisier to me than either the BPM or the DS calibration.




One thing I would like to point out that first couple of frames taken at the start of an imaging session will always be noisier than the rest. I have done extensive ICNR imaging, I always discard first, sometimes first and second images of the ICNR session. In case you did your experiment with ICNR being the first couple of images of the session, that would explain bit more noise in them.

If you can repeat the experiment and discard first couple of images of the session, that may fix the "start of an imaging session noise phenomena". I don't know what causes this but ambient temp miss-equilibrium is the best explanation I could come up with. Although I have seen this phenomena in ICNR sessions, I am pretty sure it is true for out-camera NR sessions as well. In short, discard first few images of a session, regardless of the NR method to rule out this phenomena as a variable in the results. Regards


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jgraham
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Re: DSLR 'long exposure noise reduction' new [Re: mmalik]
      #5823361 - 04/25/13 11:20 PM

Good point. The temperature of the sensor is changing the fastest during the first couple of images to the point where even the internal darks can struggle to keep up with it.

Neat stuff.


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Falcon-
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Re: DSLR 'long exposure noise reduction' new [Re: jgraham]
      #5823395 - 04/25/13 11:50 PM

Regarding the different colour balances between the ICNR and BP/DS images: You said you used Nebulosity to do the BadPixel map and Dark Subtraction. I am guessing that means you exported FITS or TIFFs for DSS to stack in that run while DSS had CR2 files directly for the ICNR runs.

This would I suspect explain the variance in colour balance (especially if Nebulosity did the demosaic/debayer instead of DSS).

If you use nebulosity's Batch Conversion to convert the ICNR images to FITS files and then stack those with DSS we will likely see the same colour balance as the BPM/DS images. Would be interesting to see if that changes the Mean values as well.


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Falcon-
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Re: DSLR 'long exposure noise reduction' new [Re: Falcon-]
      #5823397 - 04/25/13 11:53 PM

Quote:

One thing I would like to point out that first couple of frames taken at the start of an imaging session will always be noisier than the rest. I have done extensive ICNR imaging, I always discard first, sometimes first and second images of the ICNR session.




Really? Given that sensor temperature tends to start low and go up over a couple frames before stabilizing I would expect those initial lower temperature frames to have *less* noise!

Do you perhaps tend to run a lengthy live-view focus session immediately before the imaging starts so that the camera starts out very warm?


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microstar
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Reged: 01/05/08

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Re: DSLR 'long exposure noise reduction' new [Re: mmalik]
      #5823401 - 04/25/13 11:55 PM

Quote:

Quote:

the ICNR frames look noisier than the BPM and DS calibrated frames, and the stack of 8 ICNR frames is noticeably noisier than the stack of 16 ICNR frames. So you definitely lose by shooting only half as many light frames as many on this thread predicted. However, based upon the discussion I would have expected the ICNRx16 frame to be comparable to the BPM-Bx16 and DSx16 stacks. But the ICNRx16 looks noticeably noisier to me than either the BPM or the DS calibration.




One thing I would like to point out that first couple of frames taken at the start of an imaging session will always be noisier than the rest. I have done extensive ICNR imaging, I always discard first, sometimes first and second images of the ICNR session. In case you did your experiment with ICNR being the first couple of images of the session, that would explain bit more noise in them.

If you can repeat the experiment and discard first couple of images of the session, that may fix the "start of an imaging session noise phenomena". I don't know what causes this but ambient temp miss-equilibrium is the best explanation I could come up with. Although I have seen this phenomena in ICNR sessions, I am pretty sure it is true for out-camera NR sessions as well. In short, discard first few images of a session, regardless of the NR method to rule out this phenomena as a variable in the results. Regards




Here are the ImageJ stats dropping the first 2 frames, so ICNRx14:

Mean StdDev Min Max
1774.147 589.470 108 65235

That is somewhat lower than ICNRx16:
2074.523 623.793 54 65239

but still quite a bit higher than the BPM and DS stacks.
...Keith


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Falcon-
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Re: DSLR 'long exposure noise reduction' new [Re: Falcon-]
      #5823407 - 04/25/13 11:58 PM

Hmm... thinking about that variance in Mean value between the ICNR and BPM/DS images... I wonder if Canon is doing a subtraction and then normalizing (adding the mean value of the dark frame to all pixels equally) to prevent the single-dark-subtract from decreasing image brightness/creating zero-value pixels.

Keith: Would you be able to run one of your dark frames and perhaps one of the Bias frames through ImageJ to get their Mean value? I would be quite interested to know if either turns out to have a mean of something close to 400...

Also - thanks for running that test for us all!


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microstar
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Re: DSLR 'long exposure noise reduction' new [Re: pfile]
      #5823415 - 04/26/13 12:03 AM

Quote:

also, can i ask what your pixel offsets and rotation angles are in DSS? in DSS is 'black point correction' turned off in the DCRAW control? actually i can't even remember if DSS shows you that DCRAW control.




Here are the pixel offset and rotation for the DS stack:

dx dY Angle
0.00 0.00 0.00 °
-5.04 3.36 -0.08 °
1.96 0.93 0.02 °
-2.27 0.75 -0.02 °
-2.98 1.10 -0.05 °
-3.79 2.08 -0.06 °
-8.80 2.63 -0.11°
-4.87 2.83 -0.07 °
-6.87 3.03 -0.10 °
1.20 0.85 0.01 °
-3.30 1.60 -0.04°
-6.75 2.56 -0.09°
-8.98 3.38 -0.12°
-3.16 1.18 -0.02°
-9.73 3.56 -0.13°
-9.91 3.39 -0.14°

...Keith


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microstar
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Re: DSLR 'long exposure noise reduction' new [Re: Falcon-]
      #5823418 - 04/26/13 12:06 AM

Quote:

Regarding the different colour balances between the ICNR and BP/DS images: You said you used Nebulosity to do the BadPixel map and Dark Subtraction. I am guessing that means you exported FITS or TIFFs for DSS to stack in that run while DSS had CR2 files directly for the ICNR runs.

This would I suspect explain the variance in colour balance (especially if Nebulosity did the demosaic/debayer instead of DSS).

If you use nebulosity's Batch Conversion to convert the ICNR images to FITS files and then stack those with DSS we will likely see the same colour balance as the BPM/DS images. Would be interesting to see if that changes the Mean values as well.




Good point, but I let DSS do all of the debayering. Still, you are correct, the ICNR files came into DSS directly as CR2 files and the BPM and DS files came in as FITS files from Nebulosity. I didn't test to see if that made a difference.
...Keith


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srosenfraz
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Re: DSLR 'long exposure noise reduction' new [Re: microstar]
      #5823470 - 04/26/13 01:10 AM

Keith - thank you so much for taking the time to do this experiment and for doing such a thorough and thoughtful analysis.

@Mike - even if any given sub(s) is somewhat noisier than the norm, I'm not sure it generally makes sense to toss them. Keep in mind that those subs are going to give you an improvement in SNR by virtue of increasing your signal. The subs would have to be awfully noisy for the increase in signal to be more than offset by a greater increase in noise. Modern cameras such as your 60Da have very low noise anyway, so a less than perfect dark subtracted light is going to still net improve your SNR.


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mmalik
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Re: DSLR 'long exposure noise reduction' new [Re: Falcon-]
      #5823493 - 04/26/13 01:43 AM

Quote:

Do you perhaps tend to run a lengthy live-view focus session immediately before the imaging starts so that the camera starts out very warm?




1. As most folks know, live-view focusing at times can take some time; it is possible as you point out could be causing session startup noise in first few images.

2. Other cause would be temp stabilization as you and I have surmised.

3. One other cause I could think of, a permutation of the previous, would be that generally my camera temp at the start of a session is warmer than the ambient before it cools down (i.e., ambient mostly cooler than the camera at the start of my session). Thx


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