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Old 29-09-2014, 02:41 AM
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Weird discoloration on most of my astrophotos with Canon 6d

When I use Deepskystacker 3.3.4 to stack my images (the original .CR2 photos) I get a discoloration in the corners and the left and right edges when I edit the final image (autosave.tif). Regardless of the lens used: Canon 85mm f/1.8 squeezed to f/2.5, 70-300L (set to 300, f/5.6) or Televue Genesis prime focus. Using filters does not help: without filter or Hutech IDAS LPS-D1 or not does not make sense. Only using an Halpha filter does not show up the problem, but that is monochrome.
ISO setting does not influence either, whether 400, 1600 or 6400 in all cases it occurs.
Before saving to a file in DSS I align the RGB sliders over each other and, change the histograms a bit (midtones a bit steeper) and set saturation to +15-+20.
Then I use Photoshop CS6 or Darktable for finetuning.
The edge discoloration is always between blue-green and yellow-green (mostly).

When I edit the individual original CR2 subs with Darktable, Photoshop CS6 or Canon DPP 4.0 the problem does not appear.
Using flats in DSS worsens the problem, however when I skip flats I have to edit vignetting manually and I still get the discolored but in a lesser extent.

See on my website http://skysurfer.eu/skypics1.php

and the photos of Omega Centauri, Rosette Nebula, Running Checken nebula (starting 2nd of 5th row) and the east part of Cygnus (first of second last row in color) and the very last one (Andromeda Galaxy taken yesterday night) have an obvious sign of this UGLY edge dicoloration :-((

It is not the sensor (I had this problem already with the 7d last summer but due to the crop sensor it is less obvious). As said, single frame do not show this issue.

So crop them all to APS-C ? No that is not the solution.
Does somebody know what I am structually doing wriong ?
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Old 29-09-2014, 06:27 AM
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lazjen (Chris)
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What format are you exporting from DSS?

Try producing a TIFF is DSS and direct import that into PS without any further manipulation in DSS. See if that makes a difference. PS should be able to import 32 bit TIFF (you can switch mode to 16 bit when imported).

Another thing to check - what's your DSS settings for the camera? What settings do you use when stacking?

Finally, I'm not familiar with the lenses you've got - are they all full frame lenses?
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Old 29-09-2014, 06:42 AM
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Octane (Humayun)
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Discolouration at the edges would indicate potentially dodgy flat field frames?

H
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Old 29-09-2014, 07:27 AM
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I would not be adjusting colour before stacking. Wrong sequence. Stack then adjust for colour. You probably are saturating light pollution (its usually greenish). Also it looks like your flats are not flattening the image properly.

What is your procedure for doing flats? I have found that flats are more important in light polluted areas than in dark skies.

Another aid is the free HALVG plug for Photoshop from Andrea Rogelio that removes excess green from images. It would correct a lot of that.

Greg.
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Old 29-09-2014, 03:09 PM
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Quote:
Originally Posted by gregbradley View Post
I would not be adjusting colour before stacking. Wrong sequence. Stack then adjust for colour. You probably are saturating light pollution (its usually greenish). Also it looks like your flats are not flattening the image properly.
No I don't adjust any colors before stacking, I put the original CR2's into DSS and AFTER stacking I adjust colors.
An idea might indeed be to process the fresh 'autosave.tiff' with PS or DT. Indeed it is TIFF.

Quote:
Originally Posted by gregbradley View Post
What is your procedure for doing flats? I have found that flats are more important in light polluted areas than in dark skies.
Take a picture of a white wall from close distance but equally lit with the same aperture as the sky shots and focus to infinity and AF off to prevent any details. Only for showing vignetting.
Sometimes I put a white T-shirt over the lens.
Should I convert the flats to monochrome (i.e. B&W photos) ?
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Old 30-09-2014, 05:04 AM
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When I was shopping for a new DSLR, I did some experiments with dark frames that showed an interesting behavior with the 6D and I suspected that it might result in exactly what you are describing. I didn't take it any further as I decided that the 70D was a better overall camera for me.

Essentially I took a 30 second dark frame (LENR off) at each ISO setting for each camera and then looked at the mean and standard deviation of the noise in the RAW files. You can see the same information by looking at the histogram on the camera which moves towards the right and gets wider as the noise goes up, as you'd expect. With the 6D there was a point where the histogram suddenly goes back to the left axis but the width of the histogram (std dev) stays the same, inferring that the camera is doing a pseudo dark subtraction. This is happening prior to the RAW file.

I didn't do any tests with a constant ISO and increasing exposure time but if the same thing occurs, that would fit with what you are seeing. The camera subtracts a pseudo-dark for the light frames but not for the flat frames because they are a shorter exposure.

This may all be common knowledge or it may be a red herring. I don't know. You might want to experiment further.

Steve.
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Old 30-09-2014, 07:08 PM
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Steve, there's a second setting you might have missed. It's a high ISO speed NR setting. You have to turn both of them off. If you missed that, it's likely to cause the effect you saw.

If you did that setting as well... then that's quite bizarre and not something I'm seeing in a couple of quick tests here.
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Old 30-09-2014, 07:52 PM
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Quote:
Originally Posted by skysurfer View Post
No I don't adjust any colors before stacking, I put the original CR2's into DSS and AFTER stacking I adjust colors.
An idea might indeed be to process the fresh 'autosave.tiff' with PS or DT. Indeed it is TIFF.


Take a picture of a white wall from close distance but equally lit with the same aperture as the sky shots and focus to infinity and AF off to prevent any details. Only for showing vignetting.
Sometimes I put a white T-shirt over the lens.
Should I convert the flats to monochrome (i.e. B&W photos) ?

White T shirt is most likely going to work better. Monochrome? Yes probably as a flat is an image showing the unevenness of illumination across the frame from vignetting and perhaps uneven sensor performance.

Monochrome should show that.

With a CCD my procedure is to take several flats with a white t shirt and same focus as the imaging and same aperture. Then average combine them.

A shot of a white wall and one flat may be risky especially on light polluted main exposures.

Greg.
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Old 01-10-2014, 03:59 AM
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Quote:
Originally Posted by lazjen View Post
Steve, there's a second setting you might have missed. It's a high ISO speed NR setting. You have to turn both of them off. If you missed that, it's likely to cause the effect you saw.

If you did that setting as well... then that's quite bizarre and not something I'm seeing in a couple of quick tests here.
I thought that I had turned off everything but if you're unable to duplicate what I saw then it sounds quite likely that I missed that one.
Sorry 'bout that. Red herring it is. D'oh!!!

Steve.
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Old 01-10-2014, 05:14 AM
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Well, I found a slight fix (attachement 1) in the DSS settings.
But it still outputs a 32 bits TIFF which I have to convert with Photoshop to 16 bits with which I have to play with lots of parameters, but the result is better.
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