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What Kind of Noise Is This?

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#1 Jim Waters

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Posted 04 June 2019 - 02:05 PM

This is an enlarged jpeg screen capture so the image and noise is compressed.  I was Dithering every Light sub.  Is this Walking Noise?  Isn't Dithering suppose to reduce or eliminate noise?

 

The only thing I can think of is that Dithering (DEC and RA) didn't move the camera enough to really have any benefit.  

 

50ea Lights, 30ea Darks, 30ea BIAS and 30ea Flats. Stacked using PixInsight.

 

Equipment and Software

  • Ha Mod Canon 6D
  • 200mm f/2.8L II lens at f/3.2
  • Losmandy GM811G
  • PHD Guide 2 (Dithering every sub)
  • BackyardEOS (Dithering every sub)
  • PixInsight

FOV is 10° 13‘ 44" x 6° 50‘ 20"

 

The screen capture was just after BPP and ImageIntegration.  Nothing else.

Attached Thumbnails

  • PI Noise.jpg

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#2 iwannabswiss

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Posted 04 June 2019 - 02:10 PM

To me it almost looks like moisture or condensation of some sort.



#3 einarin

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Posted 04 June 2019 - 02:43 PM

How about if you leave darks or bias (don't need both).



#4 Jim Waters

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Posted 04 June 2019 - 03:14 PM

To me it almost looks like moisture or condensation of some sort.

The sensor was 85F and the ambient was 67F.



#5 futuneral

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Posted 04 June 2019 - 03:14 PM

How much were you dithering? With a full frame camera and a modest FL of 200mm you should probably try something like "extreme dithering" option.

Also, worth checking if PHD2 is actually showing that it's dithering.



#6 Jim Waters

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Posted 04 June 2019 - 03:14 PM

How about if you leave darks or bias (don't need both).

I will try leaving out each.



#7 Jim Waters

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Posted 04 June 2019 - 03:22 PM

How much were you dithering? With a full frame camera and a modest FL of 200mm you should probably try something like "extreme dithering" option.

Also, worth checking if PHD2 is actually showing that it's dithering.

PHD was showing Dithering in RA and DEC.  Dither aggressiveness was set to 3 out of 5.  Settle Dither at was set to 0.25.



#8 futuneral

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Posted 04 June 2019 - 03:23 PM

I will try leaving out each.

You may need both though (depending on how you are calibrating). If you have a dark library and you use the "optimize" option, that scales darks to the exposure time equal that of the lights, then you need to take separate master bias and calibrate it out of darks. Otherwise your bias will be present in darks and will scale with them. 

 

If, however, you always take darks matching your lights, then you may not need the bias (but it shouldn't hurt or cause this kind of noise)



#9 futuneral

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Posted 04 June 2019 - 03:34 PM

PHD was showing Dithering in RA and DEC.  Dither aggressiveness was set to 3 out of 5.  Settle Dither at was set to 0.25.

 

From BYEOS manual: "The aggressiveness setting controls how far the mount pointing will move between exposures. You may want to increase this value a bit more if you are using a DSLR or one-shot color CCD camera to compensate for the 4-pixel Bayer color filter array overlaid on the detector."

 

I would probably try to set it to 5, and if that doesn't fully eliminate the pattern, increase the dithering scaling factor in PHD. 

 

What scope/camera do you use for guiding?



#10 Jim Waters

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Posted 04 June 2019 - 03:35 PM

I take all my Lights first.  After that I take Darks then BIAS.  I take Flats when I get home.  I do not have a Dark library.  Optimize Darks Frames is checked in BPP.

 

I use Winsorised sigma clipping.


Edited by Jim Waters, 04 June 2019 - 03:37 PM.


#11 Jim Waters

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Posted 04 June 2019 - 03:43 PM

I am using a 60mm f/4 scope and a QHY5L-II.  I can move up to a 80mm scope if necessary. 

 

I will set Dither aggressiveness to 5 next time.  How do you increase the Dithering scaling factor in PHD?

 

EDIT - I answered my own question.  Dithering scaling factor was set to 1.  Do you have a recommended setting?


Edited by Jim Waters, 04 June 2019 - 03:45 PM.


#12 futuneral

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Posted 04 June 2019 - 03:51 PM

I am using a 60mm f/4 scope and a QHY5L-II.  I can move up to a 80mm scope if necessary. 

 

I will set Dither aggressiveness to 5 next time.  How do you increase the Dithering scaling factor in PHD?

 

EDIT - I answered my own question.  Dithering scaling factor was set to 1.  Do you have a recommended setting?

 

Yeah, if I'm getting this right - your guide scope is _longer_ than your main lens, your guide camera pixel is _smaller_ than your main sensor's. This means an aggressiveness of 3px on the guider, will result in just up to about 1px shift on the main sensor (in fact, a random number between 0 and 1px), which is less than the size of the Bayer "unit".

 

I'd definitely try increasing the dithering amount.



#13 Jim Waters

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Posted 04 June 2019 - 03:58 PM

YES - the guide scope F.L. is longer than the imaging lens.  I will set BYE Dither Aggressiveness to 5 and PHD 2 Dithering Scaling Factor to maybe 10 next time.



#14 fmeschia

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Posted 04 June 2019 - 05:19 PM

Walking noise. Maybe the dithering radius wasn’t large enough for the focal length?

Francesco



#15 Jim Waters

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Posted 04 June 2019 - 05:31 PM

I think I agree.  I will increase the parameters next time.



#16 jerry10137

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Posted 04 June 2019 - 06:00 PM

This is called walking noise. It can be caused by a number of things or a combination of multiple...

 

dither aggressiveness, thermally unmatched darks and field rotation



#17 Jim Waters

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Posted 04 June 2019 - 07:32 PM

dither aggressiveness, thermally unmatched darks and field rotation

Are you saying it can be caused by Dithering?

 

The Darks were not matched to each Light.  Its not field rotation.


Edited by Jim Waters, 04 June 2019 - 08:21 PM.


#18 nyairman

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Posted 04 June 2019 - 08:45 PM

Jim, this is walking noise. I agree with those here that are saying it is a dithering issue causing the "raining" type noise. However, this noise is being "exaggerated" by use of dark and bias frame integration into post processing. This makes Noise Reduction that much harder in PP.

 

The 6D has on sensor dark current suppression already so, the use of darks and bias files are actually increasing the read noise in the image. Walking noise is also an indication of really good tracking (a great thing) and I have had this issue since I started with the Fornax LTII and Canon 7DMII. It was brought to my attention that my use of darks and bias frames were actually creating more noise in my images. I reprocessed those images without calibration frames and the "color, red, read noise" were much less prevalent. I was still left with walking noise on certain images but dithering really helped in creating a much cleaner less "rainy" image.

 

I just open PHD2 and leave all the defaults as is. Then in BYEOS, I try different subtle dithering settings until I see a difference in the walking noise pattern! It takes time to test these settings but, patience is a virtue and AP is king when it comes to patience!

 

I hope this helps.


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#19 Jim Waters

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Posted 04 June 2019 - 11:42 PM

I got the impression that I wasn't Dithering / moving DEC and RA enough because of the wide field of view and I needed to increase the parameters.



#20 betelgeuse91

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Posted 05 June 2019 - 12:50 AM

This is walking noise caused by not dithering. If you weren't dithering, try dithering.

 

If you were dithering, try dithering more often.



#21 Jim Waters

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Posted 05 June 2019 - 01:40 AM

I was Dithering every sub but wasn't moving the mount far enough.  Next time I will.



#22 fishonkevin

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Posted 06 June 2019 - 10:31 AM

I found something interesting this winter.  I set the pause between lights to 30sec.  PHD dithered for the whole 30sec.  My lights were very smooth.  Of course it could be the cold temps too,  but the increased time dithering couldn't hurt.  May be something to test the next time out.



#23 einarin

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Posted 06 June 2019 - 10:50 AM

Can't see why 30s dithering helps - too much dithering is not good either.



#24 bmhjr

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Posted 06 June 2019 - 10:59 AM

Agree with the comments on dithering.  Also you need to make sure you are getting enough signal to help bury the noise.  The 30sec exposures are relatively short and depending on your sky conditions you may need to increase the ISO for lower read noise.  I always use ISO 1600, dither very aggressively, and expose long enough so the skyfog peak is enough over from the left edge.  I dont have any issues with my 6D and walking noise, but my ambient temperatures are also cooler that what you state.


Edited by bmhjr, 06 June 2019 - 11:00 AM.


#25 entilza

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Posted 06 June 2019 - 12:27 PM

Simple to prove it to yourself, just blink through your frames watch how the image drifts in this direction causing this walking noise. Watch for this while imaging too to try to catch this.


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