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Saturation Signal

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  • night.skywatcher
    If I may ask, what exactly is saturation signal ? The Kodak 04022 has a saturation signal of 45,000; the 8300 is 25,500. Does that mean that the stars will
    Message 1 of 3 , Jul 6, 2011
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      If I may ask, what exactly is 'saturation signal'?

      The Kodak 04022 has a 'saturation' signal of 45,000; the 8300 is 25,500. Does that mean that the stars will quickly 'bloat' if there is a bright star in the frame? Would a higher saturation value usually result in skinnier, tighter stars?

      Then again, the pixel size on the 8300 is 5.4 versus 7.4 on the 04022. Do you have to merely compensate with shorter subs?

      Thanks

      Destrehan Dave
    • Ron Wodaski
      Can I assume that those numbers are in electrons? If yes, then they will be approximately equal to a 16-bit ADU (analog to digital unit) level, or about
      Message 2 of 3 , Jul 6, 2011
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        Can I assume that those numbers are in electrons? If yes, then they will be approximately equal to a 16-bit ADU (analog to digital unit) level, or about 65,000.

        What it means will vary from one chip to another, and will also vary somewhat from one camera design to another. In some cases, yes, stars will begin to bloat (particularly on an ABG camera). But the degree of bloat is highly dependent on the implementation; chips interact with the electronics in the camera, and how they are set up by the designer of the camera.

        And some chips will bloat easily, and some not so easily, due to differences in how the anti-blooming is implemented.

        The best way to get a sense of what happens to a particular chip in a particular camera is to look at images taken with that setup. If you see a lot of bloat, then you know that's what happens. If you see 30-minute exposures that look good, then you know that's what to expect.

        There is no simple rule that will cover all, or ever most, cases.

        Ron Wodaski



        On Jul 6, 2011, at 10:09 AM, night.skywatcher wrote:

        > If I may ask, what exactly is 'saturation signal'?
        >
        > The Kodak 04022 has a 'saturation' signal of 45,000; the 8300 is 25,500. Does that mean that the stars will quickly 'bloat' if there is a bright star in the frame? Would a higher saturation value usually result in skinnier, tighter stars?
        >
        > Then again, the pixel size on the 8300 is 5.4 versus 7.4 on the 04022. Do you have to merely compensate with shorter subs?
        >
        > Thanks
        >
        > Destrehan Dave
        >
        >
        >
        >
        > ------------------------------------
        >
        >
        >
        >
      • Stan
        ... Basic photon accounting reveals that the area-normalize saturation levels for these 2 detectors are virtually identical (saturation per square mm). So
        Message 3 of 3 , Jul 7, 2011
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          --- "night.skywatcher" <night.skywatcher@...> wrote:
          > The Kodak 04022 has a 'saturation' signal of 45,000;
          > the 8300 is 25,500...
          > Then again, the pixel size on the 8300 is 5.4
          > versus 7.4 on the 04022

          Basic photon accounting reveals that the area-normalize saturation levels for these 2 detectors are virtually identical (saturation per square mm). So unless the FL is extremely short and causes severe undersampling then this is a moot issue; e.g. for a given optic, a star that spans 4 pixels in the 04022 will span 5.5 8300 pixels and thus its photons will be distributed across nearly 2x more 8300 pixels, which means those smaller pixels will not saturate any sooner than the larger 04022 pixels.

          When comparing sensors with different sized pixels it is very important to normalize the signal and noise characteristics. Often, CCD lines exhibit stable area-normalized saturation and dark current; but read noise is usually constant per pixel and thus becomes larger per area for detectors with smaller pixels.

          Stan
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