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Image Comments posted by rotatux

  1. Thanks. When I'm there 3 or 4 times per year my house is actually already far from towns: nearest "big" town of ~600 people is at 6km, Limoges is really big but out of sight. The house is an old water mill burried deep down a valley, so I have to climb the hill up to the other side at 500m altitude to get an astronomic view. Clear Outside says my site is Bortle Class 3, which I believe is probably true given the rare SQM measurements I did.


  2. Not a focus problem, it was dead on as far as I can tell. Such saturated / somewhat wrong colors come from applying a star-based B-V color calibration on an image with stars bloated by halos of aberrant colors.

    Colors and halos are exactly the same without didymium filter, but the filter allows me longer subs without saturating the background.

    IMO my lens model is of the poorly-coated variant (Oly made another variant of same 200/4 but with MC). Hence CA produces strong halos of different size for blue (bigger) and red (smaller). The end result depends on original star colors, but typically shows outer blue then inner red or purple.

    I hope to try a technique based on star filters to remove the halos someday...

  3. 2 hours ago, Mr niall said:

    do you mean the length of the video or the shutter speed?

    Shutter speed. Each image in the animation was compiled from a separate video file, so there's many video files and each can have its own settings such as shutter speed -- though I would recommend to keep a constant exposure, it was an error of mine as I was searching for best settings.



    JPEG from the camera is strongly gamma-stretched from the raw image, so that the various color levels fit within the 8-bit range of JPEG, which means this kind of rendering is not linear to the signal actually recorded.

    Conversely, DSS works with the actual signal from your RAWs and produces an unstreched image in the TIFF. Additionally the TIFF has more "depth" being at least 16-bit if not 24 or 32. So when viewed directly the TIFF is a lot less bright, but should actually contain far more precise data from stacking.

    You must use a full-depth aware image editing application (Photoshop, Lightroom, Startools, Pixinsight, Fotoxx, Gimp 2.9+ come to mind, and many others I don't know of) to "stretch" the levels with software tools:

    - either a brightness tool, one that does'nt change the color balance

    - or a "levels" tool, to set the black/white/mid points (typically within an histogram), which is a gamma strech -- avoid if you can use a brightness stretch instead, since it will change colors

    You may also have to repeat the streching a few times to bring the faint signal into "acceptable" levels.

    And you have a very nice image, with very fine details.

  4. SGL has resized down the uploaded image (140K) contrary to most my other images.  Tried to reupload with no change, impossible to store the original (only 840K). Strange.

    Just checked most my other images are reduced too. And seems same for other Member's Albums. Must be SGL policy, but didn't notice when reading :(

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