Author Topic: Fixing Bad Pixel Columns  (Read 271 times)

Offline kerrywaz1

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Fixing Bad Pixel Columns
« on: 2018 September 13 22:53:44 »
Hello,

I have a 16803 CCD camera (Apogee F16M) that has some bad pixel columns. I tried calibrating these out with bias frames, but some are still there. Can someone please share a technique in PixInsight for getting rid of these artifacts?

Thanks,
Kerry
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Offline pfile

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Re: Fixing Bad Pixel Columns
« Reply #1 on: 2018 September 14 00:45:10 »
you can try CosmeticCorrection to repair bad/weak columns.

rob

Offline oldwexi

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Re: Fixing Bad Pixel Columns
« Reply #2 on: 2018 September 14 18:18:34 »
Hi Kerry!
CosmeticCorrection helps,  but it is not perfect because it averages the bad column with the neighbour column.
This is necessary if the colum defect is really black or complete white, but it will  leave an
averaged  visible column defect which maybe will be reduced by stacking dithered images.

Most of the time the defect columns are only a tiny bit brighter or darker.
Here it is better not to use CC.
You create first a vertical line mask (or more) with Pixelmeth.
And than yous test on one masked image with curves the improvement which makes the line artefact complete
invisible because the real data are a little brightened or darkened!
Than i create an image container with alle images showing the artefact and
repair the column(s) in one step using the process container with the masked Curves processes.

If this sounds to complex, so provide me 3 calibrated images with these artefacts and i will produce
a projectfile as a show case. This will include the mask generation, the curves process(es) process container
and the image container usage.

Gerald

Offline Geoff

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Re: Fixing Bad Pixel Columns
« Reply #3 on: 2018 September 15 03:44:00 »
That’s very clever Gerald!
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Offline pfile

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Re: Fixing Bad Pixel Columns
« Reply #4 on: 2018 September 15 04:22:44 »
another thing to try, as long as you have dithered, is to just write '1's over the bad columns using pixelmath and then the pixel rejection during imageintegration will take care of rejecting the columns. however, if some of the subs have equal X dithers then this may not work so well - you may end up with bright columns in the integrated result.

rob

Offline kerrywaz1

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Re: Fixing Bad Pixel Columns
« Reply #5 on: 2018 September 16 16:47:49 »
Hi Gerald,

Your method sounds interesting. I sent you a private message with Dropbox links to three cal'd images with the bad columns.

Thanks,
Kerry
Deep Sky Instruments RC14C
SBIG STL11000 w/MOAG + AOL + SBIG RGH
Pyxis 3"
Astro Physics AP1200
Tak FSQ-106EDXII
QSI683wsg
Hap Griffin modified Canon EOS 5D Mark II
PixInsight, MaxIm DL, PhotoShop CS5

Offline oldwexi

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Re: Fixing Bad Pixel Columns
« Reply #6 on: 2018 September 16 21:50:11 »
Hi Kerry!
Project for column correction without CC is already in my dropbox.
I sent the link to you just now.

Gerald

Offline ngc1535

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Re: Fixing Bad Pixel Columns
« Reply #7 on: 2018 September 19 05:15:03 »
Interesting:

1. What method does CC employ to substitute pixels in columns (median, mean? of neighbors?)? I have always wondered....
2. OldWexi: I have simply used DefectMap and choosing a method (such as mean or median). Does your way have a significant improvement on say mean or median of DefectMap?

I guess a side-by-side comparison would be really great. If both methods make an "invisible" result- I am going to use the one with less steps. :)

-adam

Offline oldwexi

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Re: Fixing Bad Pixel Columns
« Reply #8 on: 2018 September 19 17:59:42 »
Hi Adam!
CC and/or Defectmap is painting/inventing data in the bad columns by using the neighbouring pixels.
This is ok if the column defects are white or black only or with only one brightness.
But a line mask together with Curves increases or decreases the originnal data to a level which makes the column defect disappear but with "correct" data.

If Kerry allows me to show his images und the project i can make it visble to all.
As the used images belong to Kerry i cant go public with the project...

Gerald

Offline ngc1535

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Re: Fixing Bad Pixel Columns
« Reply #9 on: 2018 September 19 18:24:14 »
Yes, I understand what you are doing and I agree it will work just fine- but I wonder if you need to make a curve adjustment for each column. I am guessing you would. Biases usually have large scale gradients (sometimes in the horizontal direction) so that an adjustment you make for one column would not be valid for another column.

As Rob mentions, with a truly random dither and enough frames- no data needs to be invented at all through subsequent rejection. The only quibble for me there is I think writing zeros instead of ones is better since large outliers tend to be on the high end.

And just a step further... you might want to comment on whether you would make your adjustment before or after creating a "superbias."

thanks,
-adam