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Thread: Preprocessing artifacts

  1. #1
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    Preprocessing artifacts

    i've noticed repeatedly after preprocessing the resultant image looks like the pixel arrangement is always diagonal from bottom left to top right - see attachment
    Click image for larger version. 

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    Is this a result of the stacking algorithm and should I be worried?

  2. #2
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    No one...not even Fabio?

  3. #3
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    Hello, I missed the message.
    The cause seems to me an imperfect dark frame correction, which gets a diagonal shape cause the auto-alignment of several images. If you orient the camera with the equatorial axes the noise should get a horizontal shape.
    Remember that the master dark frame must be at least the average of 4 frames (8 or more is better).
    If you are using a bias as a dark frame, then again you should average several frames.
    The image also seems extremely oversampled: you may work in binning 2x2 without losing any information.
    Greetings,
    Fabio.

  4. #4
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    Here is a photo from a book about astro photography by Ron Wodaski that shows similar artifacts due to stacking very short sub exposures!

    Click image for larger version. 

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    Iver

  5. #5
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    thanks!

    Quote Originally Posted by facavi View Post
    Hello, I missed the message.
    The cause seems to me an imperfect dark frame correction, which gets a diagonal shape cause the auto-alignment of several images. If you orient the camera with the equatorial axes the noise should get a horizontal shape.
    Remember that the master dark frame must be at least the average of 4 frames (8 or more is better).
    If you are using a bias as a dark frame, then again you should average several frames.
    The image also seems extremely oversampled: you may work in binning 2x2 without losing any information.
    Greetings,
    Fabio.
    Thanks Fabio. I was using multiple dark frames. What do you mean by over-sampled?

  6. #6
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    thanks Iver.

  7. #7
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    An image if oversampled if it contains "too many pixels" (or too high resolution) for the information that it really contains, see:

    https://www.cloudynights.com/topic/3...ling-an-image/

    A test:

    1) Open (in any graphic software) a good image.
    2) Resize it to 50%.
    3) Now resize it to 200%.

    the result will have the same size, but it will be a blurred image, because step (2) decreased the information of the image.

    However, if at the step (1) you start with an oversampled image (e.g. blurred from the beginning) then after those steps, the result is as good as the original.

    This means that an oversampled image can be halved in size safely (e.g. work in binning 2x2) and processed at this size, without losing any information. Then, if needed it can be doubled in size as a last step.
    Working on oversampled images is a waste of processing time and/or disk space.

    This is the same thing of "megapixel myth" (Google it) that affected consumer cameras in the last years.

  8. #8
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    It is commonly called "walking noise" Try dithering your exposures and make sure you are well polar aligned.

    John
    CCD-Freak

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