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Re: [ccd-newastro] Dark frames create weird black spots.

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  • Ron Wodaski
    Dark spots like that typically correspond to bright spots in the darks. You can deal with the problem by taking multiple darks (10 might be a good number), and
    Message 1 of 9 , Jun 17, 2013
      Dark spots like that typically correspond to bright spots in the darks. You can deal with the problem by taking multiple darks (10 might be a good number), and then combining them (average, median, or one of the statistical combines to remove outliers, like these bright spots).

      Where do they come from? Often, they are cosmic rays that strike your sensor. As I noted, multiple darks are the best way to deal with this. But you should combine them in a way that removes the outliers if at all possible.

      Ron Wodaski



      On Jun 18, 2013, at 12:02 AM, Mark Striebeck <mark.striebeck@...> wrote:

      > I recently created new dark frames to improve my images. I put the camera
      > into our freezer to make sure that it stays at -10C (the temperature that I
      > usually image with).
      >
      > When I calibrated my first image, I noticed some strange dark spots:
      > https://lh6.googleusercontent.com/NLUOivvup_SB6tagAciZMLSFRIRhSIEcUCkU4lk4rvadShMfn2ylmIFtMEbmkyPIWg=s190<https://docs.google.com/file/d/0B0KcEqw-v71VOFFud3o1d1dfWmc/edit>
      >
      > Here is a cropped part:
      > https://lh5.googleusercontent.com/k6-3_33uIRL27H7Jwu2Cj7u8ajCfLDpVZ9ZVJN48kFeIkgMO1CzYTui5VdFEsWDEEA=s190<https://docs.google.com/folder/d/0B0KcEqw-v71VQ2R0Z05OMFpRams/edit?pli=1&docId=0B0KcEqw-v71VeWNuZ0kwRjhxeDg>
      >
      > When I calibrate with my older flats, I don't see any of these black spots.
      >
      > The two darks are here:
      > https://drive.google.com/folderview?id=0B0KcEqw-v71VQ2R0Z05OMFpRams&usp=sharing
      > (the
      > -5C is the one that doesn't create the black spots, the -12C is the one
      > that does).
      >
      > Any ideas what's going on?
      >
      > MarkS
      >
      > [Non-text portions of this message have been removed]
      >
      >



      [Non-text portions of this message have been removed]
    • Tim Stone
      Interesting. I have these on a few of my darks as well. I take series of 10, and use sigma clip for combining, so it s unlikely they re cosmic ray hits. I
      Message 2 of 9 , Jun 18, 2013
        Interesting. I have these on a few of my darks as well. I take series of
        10, and use sigma clip for combining, so it's unlikely they're cosmic ray
        hits. I suspect they're frost. If I look at them under high magnification,
        they bear some resemblance to frost patterns. Unfortunately, even the most
        sophisticated cameras can form frost under the right conditions. I'd say
        keeping your camera in the freezer only makes that worse. The freezer is
        typically a very high humidity environment, and frost readily forms in
        there.

        I use an Apogee Alta F16M, which uses an internal dessicant packet to keep
        the interior of the camera as dry as possible. This packet needs to be
        replaced on occasion. That's probably what's going on with mine.
        http://www.ccd.com/desiccant.html

        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        Tim Stone


        On Tue, Jun 18, 2013 at 1:32 AM, Ron Wodaski <yahoo@...> wrote:

        > Dark spots like that typically correspond to bright spots in the darks.
        > You can deal with the problem by taking multiple darks (10 might be a good
        > number), and then combining them (average, median, or one of the
        > statistical combines to remove outliers, like these bright spots).
        >
        > Where do they come from? Often, they are cosmic rays that strike your
        > sensor. As I noted, multiple darks are the best way to deal with this. But
        > you should combine them in a way that removes the outliers if at all
        > possible.
        >
        > Ron Wodaski
        >
        >
        >
        > On Jun 18, 2013, at 12:02 AM, Mark Striebeck <mark.striebeck@...>
        > wrote:
        >
        > > I recently created new dark frames to improve my images. I put the camera
        > > into our freezer to make sure that it stays at -10C (the temperature
        > that I
        > > usually image with).
        > >
        > > When I calibrated my first image, I noticed some strange dark spots:
        > >
        > https://lh6.googleusercontent.com/NLUOivvup_SB6tagAciZMLSFRIRhSIEcUCkU4lk4rvadShMfn2ylmIFtMEbmkyPIWg=s190
        > <https://docs.google.com/file/d/0B0KcEqw-v71VOFFud3o1d1dfWmc/edit>
        > >
        > > Here is a cropped part:
        > >
        > https://lh5.googleusercontent.com/k6-3_33uIRL27H7Jwu2Cj7u8ajCfLDpVZ9ZVJN48kFeIkgMO1CzYTui5VdFEsWDEEA=s190
        > <
        > https://docs.google.com/folder/d/0B0KcEqw-v71VQ2R0Z05OMFpRams/edit?pli=1&docId=0B0KcEqw-v71VeWNuZ0kwRjhxeDg
        > >
        > >
        > > When I calibrate with my older flats, I don't see any of these black
        > spots.
        > >
        > > The two darks are here:
        > >
        > https://drive.google.com/folderview?id=0B0KcEqw-v71VQ2R0Z05OMFpRams&usp=sharing
        > > (the
        > > -5C is the one that doesn't create the black spots, the -12C is the one
        > > that does).
        > >
        > > Any ideas what's going on?
        > >
        > > MarkS
        > >
        > > [Non-text portions of this message have been removed]
        > >
        > >
        >
        >
        >
        > [Non-text portions of this message have been removed]
        >
        >
        >
        > ------------------------------------
        >
        >
        >
        >
        >


        [Non-text portions of this message have been removed]
      • Mark Striebeck
        Hi Ron, Oh, sorry for got to mention. I took 50 darks (all in the dark freezer) and this is the master dark file (created with CCDStack). MarkS ... [Non-text
        Message 3 of 9 , Jun 18, 2013
          Hi Ron,

          Oh, sorry for got to mention. I took 50 darks (all in the dark freezer) and
          this is the master dark file (created with CCDStack).

          MarkS


          On Mon, Jun 17, 2013 at 11:32 PM, Ron Wodaski <yahoo@...> wrote:

          > Dark spots like that typically correspond to bright spots in the darks.
          > You can deal with the problem by taking multiple darks (10 might be a good
          > number), and then combining them (average, median, or one of the
          > statistical combines to remove outliers, like these bright spots).
          >
          > Where do they come from? Often, they are cosmic rays that strike your
          > sensor. As I noted, multiple darks are the best way to deal with this. But
          > you should combine them in a way that removes the outliers if at all
          > possible.
          >
          > Ron Wodaski
          >
          >
          >
          > On Jun 18, 2013, at 12:02 AM, Mark Striebeck <mark.striebeck@...>
          > wrote:
          >
          > > I recently created new dark frames to improve my images. I put the camera
          > > into our freezer to make sure that it stays at -10C (the temperature
          > that I
          > > usually image with).
          > >
          > > When I calibrated my first image, I noticed some strange dark spots:
          > >
          > https://lh6.googleusercontent.com/NLUOivvup_SB6tagAciZMLSFRIRhSIEcUCkU4lk4rvadShMfn2ylmIFtMEbmkyPIWg=s190
          > <https://docs.google.com/file/d/0B0KcEqw-v71VOFFud3o1d1dfWmc/edit>
          > >
          > > Here is a cropped part:
          > >
          > https://lh5.googleusercontent.com/k6-3_33uIRL27H7Jwu2Cj7u8ajCfLDpVZ9ZVJN48kFeIkgMO1CzYTui5VdFEsWDEEA=s190
          > <
          > https://docs.google.com/folder/d/0B0KcEqw-v71VQ2R0Z05OMFpRams/edit?pli=1&docId=0B0KcEqw-v71VeWNuZ0kwRjhxeDg
          > >
          > >
          > > When I calibrate with my older flats, I don't see any of these black
          > spots.
          > >
          > > The two darks are here:
          > >
          > https://drive.google.com/folderview?id=0B0KcEqw-v71VQ2R0Z05OMFpRams&usp=sharing
          > > (the
          > > -5C is the one that doesn't create the black spots, the -12C is the one
          > > that does).
          > >
          > > Any ideas what's going on?
          > >
          > > MarkS
          > >
          > > [Non-text portions of this message have been removed]
          > >
          > >
          >
          >
          >
          > [Non-text portions of this message have been removed]
          >
          >
          >
          > ------------------------------------
          >
          >
          >
          >
          >


          [Non-text portions of this message have been removed]
        • Mike Dodd
          Mark Striebeck wrote: ... Mark, When I try to view your images, Google tells me: ===== 403. That’s an error. Your client does not have permission to get URL
          Message 4 of 9 , Jun 18, 2013
            Mark Striebeck wrote:
            Here is a cropped part:
            > https://lh5.googleusercontent.com/k6-3_33uIRL27H7Jwu2Cj7u8ajCfLDpVZ9ZVJN48kFeIkgMO1CzYTui5VdFEsWDEEA=s190<https://docs.google.com/folder/d/0B0KcEqw-v71VQ2R0Z05OMFpRams/edit?pli=1&docId=0B0KcEqw-v71VeWNuZ0kwRjhxeDg>

            Mark,

            When I try to view your images, Google tells me:

            =====
            403. That’s an error.

            Your client does not have permission to get URL
            /NLUOivvup_SB6tagAciZMLSFRIRhSIEcUCkU4lk4rvadShMfn2ylmIFtMEbmkyPIWg=s190
            from this server. (Client IP address: xx.xxx.xxx.10)

            Forbidden That’s all we know.
            =====


            MODERATOR NOTE TO ALL:
            When posting links, please test to ensure that they work before hitting
            the Send button. (In this case, testing the link on a different PC, a
            tablet, or a smart phone likely would have revealed the problem.)

            I have run across several non-functioning links recently when approving
            postings (no, I didn't approve this one).

            Checking links benefits you directly, especially if you're asking for
            help. If someone can't see your image, your stunning nebula image would
            go completely unnoticed, of you could miss valuable information if that
            person gives up in frustration.

            So help yourself and the list members by testing your links.

            Thank you.

            --
            Mike

            Mike Dodd
            http://astronomy.mdodd.com
            Louisa County, Virginia USA N37.58.23 W77.56.24
          • Stan
            ... A large difference between ambient temps could possibly result in different actual temperatures of the CCD because the temp sensor is not directly attached
            Message 5 of 9 , Jun 18, 2013
              --- Mark Striebeck <mark.striebeck@...> wrote:
              > ... -5C is the one that doesn't create the black spots,
              > the -12C is the one that does

              A large difference between ambient temps could possibly result in different actual temperatures of the CCD because the temp sensor is not directly attached to the CCD itself.

              What was the CCD temperature? Same for each set? If CCD temp is very low (< -30) then the darks might be overly sensitive to the effects of traps and such.

              Do another round in the refrigerator to see if the results are the same.

              Stan
            • Stan
              I downloaded the dark FITS and see that you did mean CCD temp (not just ambient). Thus the CCD temp is hardly any different from the ambient. Why? Is the
              Message 6 of 9 , Jun 18, 2013
                I downloaded the dark FITS and see that you did mean CCD temp (not just ambient). Thus the CCD temp is hardly any different from the ambient. Why? Is the cooler not working properly? Most coolers will cool 30c below ambient.

                What was the CCD temp of the image? If it did not match the darks temp then such artifacts are to be expected.

                Presumably you used adaptive dark-subtraction, which works well for different exp times but not as well for different temperatures. The function of dark current by temp varies non-linearly with intensity, so hot pixels diverge from the main pixel population. The adaptive algorithm optimizes the dark fit for 99% of the pixels and a few hot pixels go crazy. But keep in mind that those pixels are easily rejected in the stack and thus the final image may be largely unaffected by different-temp calibration - so such calibration is feasible if not entirely optimal.

                Stan
              • Ron Wodaski
                Interestingly, the link did not work for you, but worked for me. Odd. (I approved it, since it worked.) If I click in the original message I received, it still
                Message 7 of 9 , Jun 18, 2013
                  Interestingly, the link did not work for you, but worked for me. Odd. (I approved it, since it worked.) If I click in the original message I received, it still works; if I click on yours below, it fails. So different mail clients may package the link differently - some get it right, some get it wrong.

                  Interesting. Here is where I wind up when I click on that link:

                  https://docs.google.com/folder/d/0B0KcEqw-v71VQ2R0Z05OMFpRams/edit?pli=1&docId=0B0KcEqw-v71VeWNuZ0kwRjhxeDg

                  I do note that there are actually two links below - one that is apparently wrong (the first part) and one that is right (the part in angle brackets). Apparently my email client always uses the one in angle brackets.

                  For other clients, it may be best to click on the links that show up between the brackets. That seems to work for me an in alternate client - the web. (I'm on a Mac.)

                  Ron Wodaski



                  On Jun 18, 2013, at 8:12 AM, Mike Dodd <mike@...> wrote:

                  > Mark Striebeck wrote:
                  > Here is a cropped part:
                  >> https://lh5.googleusercontent.com/k6-3_33uIRL27H7Jwu2Cj7u8ajCfLDpVZ9ZVJN48kFeIkgMO1CzYTui5VdFEsWDEEA=s190<https://docs.google.com/folder/d/0B0KcEqw-v71VQ2R0Z05OMFpRams/edit?pli=1&docId=0B0KcEqw-v71VeWNuZ0kwRjhxeDg>
                  >
                  > Mark,
                  >
                  > When I try to view your images, Google tells me:
                  >
                  > =====
                  > 403. That’s an error.
                  >
                  > Your client does not have permission to get URL
                  > /NLUOivvup_SB6tagAciZMLSFRIRhSIEcUCkU4lk4rvadShMfn2ylmIFtMEbmkyPIWg=s190
                  > from this server. (Client IP address: xx.xxx.xxx.10)
                  >
                  > Forbidden That’s all we know.
                  > =====
                  >
                  >
                  > MODERATOR NOTE TO ALL:
                  > When posting links, please test to ensure that they work before hitting
                  > the Send button. (In this case, testing the link on a different PC, a
                  > tablet, or a smart phone likely would have revealed the problem.)
                  >
                  > I have run across several non-functioning links recently when approving
                  > postings (no, I didn't approve this one).
                  >
                  > Checking links benefits you directly, especially if you're asking for
                  > help. If someone can't see your image, your stunning nebula image would
                  > go completely unnoticed, of you could miss valuable information if that
                  > person gives up in frustration.
                  >
                  > So help yourself and the list members by testing your links.
                  >
                  > Thank you.
                  >
                  > --
                  > Mike
                  >
                  > Mike Dodd
                  > http://astronomy.mdodd.com
                  > Louisa County, Virginia USA N37.58.23 W77.56.24
                  >
                  >
                  > ------------------------------------
                  >
                  >
                  >
                  >
                • Mark Striebeck
                  Sorry, looking at the link, this was my fault: I first inserted a link that didn t work. I actually checked it and then changed the link so that it would work.
                  Message 8 of 9 , Jun 18, 2013
                    Sorry, looking at the link, this was my fault:

                    I first inserted a link that didn't work. I actually checked it and then
                    changed the link so that it would work. But I only changed the underlying
                    target, not the displayed text. Check the text that Mike posted:
                    https://lh5.googleusercontent.com/k6-3_33uIRL27H7Jwu2Cj7u8ajCfLDpVZ9ZVJN48kFeIkgMO1CzYTui5VdFEsWDEEA=s190
                    <
                    https://docs.google.com/folder/d/0B0KcEqw-v71VQ2R0Z05OMFpRams/edit?pli=1&docId=0B0KcEqw-v71VeWNuZ0kwRjhxeDg
                    >

                    The first part is the displayed text (which doesn't work), the second part
                    (after the "<") is the underlying target (which doesn't work).

                    So, if you use a mail client that handles html links properly, then it
                    works. If not, then the link won't work.

                    Sorry for the confusion!!!

                    MarkS


                    On Tue, Jun 18, 2013 at 10:12 AM, Ron Wodaski <yahoo@...> wrote:

                    > Interestingly, the link did not work for you, but worked for me. Odd. (I
                    > approved it, since it worked.) If I click in the original message I
                    > received, it still works; if I click on yours below, it fails. So different
                    > mail clients may package the link differently - some get it right, some get
                    > it wrong.
                    >
                    > Interesting. Here is where I wind up when I click on that link:
                    >
                    >
                    > https://docs.google.com/folder/d/0B0KcEqw-v71VQ2R0Z05OMFpRams/edit?pli=1&docId=0B0KcEqw-v71VeWNuZ0kwRjhxeDg
                    >
                    > I do note that there are actually two links below - one that is apparently
                    > wrong (the first part) and one that is right (the part in angle brackets).
                    > Apparently my email client always uses the one in angle brackets.
                    >
                    > For other clients, it may be best to click on the links that show up
                    > between the brackets. That seems to work for me an in alternate client -
                    > the web. (I'm on a Mac.)
                    >
                    > Ron Wodaski
                    >
                    >
                    >
                    > On Jun 18, 2013, at 8:12 AM, Mike Dodd <mike@...> wrote:
                    >
                    > > Mark Striebeck wrote:
                    > > Here is a cropped part:
                    > >>
                    > https://lh5.googleusercontent.com/k6-3_33uIRL27H7Jwu2Cj7u8ajCfLDpVZ9ZVJN48kFeIkgMO1CzYTui5VdFEsWDEEA=s190
                    > <
                    > https://docs.google.com/folder/d/0B0KcEqw-v71VQ2R0Z05OMFpRams/edit?pli=1&docId=0B0KcEqw-v71VeWNuZ0kwRjhxeDg
                    > >
                    > >
                    > > Mark,
                    > >
                    > > When I try to view your images, Google tells me:
                    > >
                    > > =====
                    > > 403. That�s an error.
                    > >
                    > > Your client does not have permission to get URL
                    > > /NLUOivvup_SB6tagAciZMLSFRIRhSIEcUCkU4lk4rvadShMfn2ylmIFtMEbmkyPIWg=s190
                    > > from this server. (Client IP address: xx.xxx.xxx.10)
                    > >
                    > > Forbidden That�s all we know.
                    > > =====
                    > >
                    > >
                    > > MODERATOR NOTE TO ALL:
                    > > When posting links, please test to ensure that they work before hitting
                    > > the Send button. (In this case, testing the link on a different PC, a
                    > > tablet, or a smart phone likely would have revealed the problem.)
                    > >
                    > > I have run across several non-functioning links recently when approving
                    > > postings (no, I didn't approve this one).
                    > >
                    > > Checking links benefits you directly, especially if you're asking for
                    > > help. If someone can't see your image, your stunning nebula image would
                    > > go completely unnoticed, of you could miss valuable information if that
                    > > person gives up in frustration.
                    > >
                    > > So help yourself and the list members by testing your links.
                    > >
                    > > Thank you.
                    > >
                    > > --
                    > > Mike
                    > >
                    > > Mike Dodd
                    > > http://astronomy.mdodd.com
                    > > Louisa County, Virginia USA N37.58.23 W77.56.24
                    > >
                    > >
                    > > ------------------------------------
                    > >
                    > >
                    > >
                    > >
                    >
                    >
                    >
                    > ------------------------------------
                    >
                    >
                    >
                    >
                    >


                    [Non-text portions of this message have been removed]
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