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SXV faint object performance

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  • Terry Platt <tplatt@starlight.win-uk.net>
    Hi Ron, I m sorry to say that I think that you are completely wrong on this point :-) The pixel structure of the Sony devices includes a very thin P type
    Message 1 of 2 , Feb 1 3:17 AM
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      Hi Ron,

      I'm sorry to say that I think that you are completely wrong on this
      point :-) The pixel structure of the Sony devices includes a very
      thin 'P type' surface layer, which isolates the thermal electron
      generating surface layer (where nearly all thermal charge
      originates) from the electron storage well beneath. Almost 100% of
      photons pass through this layer, as it is virtually transparent, and
      do not generate photoelectrons until they are deep within the well,
      at which point they are trapped by the potential barrier at the 'P'
      layer junction. Absolutely no leakage of these electrons occurs,
      unless the well is nearly full and the ABG has started to take
      effect.

      You might argue that the P layer must scavenge some photons, but
      this would affect the blue and UV end of the spectrum more than the
      red, as red photons have a high penetration into silicon and blue is
      absorbed at shallow depths. However, the high blue response of the
      Sony chips shows that this is NOT the case.

      For comparison, the Kodak chips achieve a relatively low dark
      current by creating an even thicker surface barrier layer over their
      pixels, which is generated by a 'doping' of the substrate to give a
      charge 'inversion' to sweep off the thermal electrons. This has a
      greater blue light absorbing effect than that in the Sony chip, so
      that their blue response 'droops' more than the Sony devices. If
      your comments were correct, then the Kodaks would be worse than the
      Sonys for the charge loss effect in faint regions.

      I stand by the statement that the SXV-H9 has a better sensitivity
      than the ST7E-NABG and has much less thermal noise. There is no
      reason why an SXV would give worse results on a faint region of
      nebulosity, and, in practise, it gives much better results.

      Regards,

      Terry

      --- In ccd-newastro@yahoogroups.com, "Wodaski Yahoo Groups"
      <yahoo@w...> wrote:
      > No, these are two different things. ABG works by "topping off" the
      wells -
      > when the pixel accumulates a certain amount of electrons, often
      about half
      > the full well, the ABG circuits bleed off electrons. This involves
      pixels in
      > very bright areas of the chip.
      >
      > The thing I am talking about is what happens in very dim areas.
      Thermal
      > electrons are removed from the pixels to limit noise. Inevitably,
      there is
      > no difference between very low signal and thermal noise as far as
      these
      > circuits are concerned.
      >
      > Ron Wodaski
      > author of The New CCD Astronomy
      > http://www.newastro.com
      >
      >
      > -----Original Message-----
      > From: Paul K <pkane2001@h...> [mailto:pkane2001@h...]
      > Sent: Friday, January 31, 2003 6:51 PM
      > To: ccd-newastro@yahoogroups.com
      > Subject: [ccd-newastro] Re: SXV-H9 or ST-7XE
      >
      >
      > Ron,
      >
      > My understanding is that the "sucking away" of electrons is done as
      > part of the ABG process on Sony chips, not to reduce the dark
      > current. If this somehow does reduce the signal, it can be turned
      off
      > with a simple on-camera adjustment on the HX916 and very likely on
      > the SXV-H9.
      >
      > The full-well capacity I believe is another place where the two
      chips
      > differ, with ST-7E NABG having a larger well.
      >
      > Regards,
      >
      > -Paul
      > http://pk.darkhorizons.org
      >
      >
      > --- In ccd-newastro@yahoogroups.com, "Wodaski Yahoo Groups"
      > <yahoo@w...> wrote:
      > >
      > > * I have a different opinion about the low dark current in the
      > Sony (and
      > > some other) chips. The low dark current exists because the chip
      > design
      > > "sucks away" unwanted electrons from the pixels. However, if you
      are
      > > recording very dim subjects, some of your signal can get sucked
      out
      > as well.
      > > Thus the low-dark-current cameras tend to have low noise overall
      > but to have
      > > poor performance with very dim subjects (e.g., extended
      nebulosity,
      > outer
      > > areas of galaxies, etc.).
      >
      >
    • Terry Platt <tplatt@starlight.win-uk.net>
      Hi Ron, I m sorry to say that I think that you are completely wrong on this point :-) The pixel structure of the Sony devices includes a very thin P type
      Message 2 of 2 , Feb 1 3:18 AM
      • 0 Attachment
        Hi Ron,

        I'm sorry to say that I think that you are completely wrong on this
        point :-) The pixel structure of the Sony devices includes a very
        thin 'P type' surface layer, which isolates the thermal electron
        generating surface layer (where nearly all thermal charge
        originates) from the electron storage well beneath. Almost 100% of
        photons pass through this layer, as it is virtually transparent, and
        do not generate photoelectrons until they are deep within the well,
        at which point they are trapped by the potential barrier at the 'P'
        layer junction. Absolutely no leakage of these electrons occurs,
        unless the well is nearly full and the ABG has started to take
        effect.

        You might argue that the P layer must scavenge some photons, but
        this would affect the blue and UV end of the spectrum more than the
        red, as red photons have a high penetration into silicon and blue is
        absorbed at shallow depths. However, the high blue response of the
        Sony chips shows that this is NOT the case.

        For comparison, the Kodak chips achieve a relatively low dark
        current by creating an even thicker surface barrier layer over their
        pixels, which is generated by a 'doping' of the substrate to give a
        charge 'inversion' to sweep off the thermal electrons. This has a
        greater blue light absorbing effect than that in the Sony chip, so
        that their blue response 'droops' more than the Sony devices. If
        your comments were correct, then the Kodaks would be worse than the
        Sonys for the charge loss effect in faint regions.

        I stand by the statement that the SXV-H9 has a better sensitivity
        than the ST7E-NABG and has much less thermal noise. There is no
        reason why an SXV would give worse results on a faint region of
        nebulosity, and, in practise, it gives much better results.

        Regards,

        Terry

        --- In ccd-newastro@yahoogroups.com, "Wodaski Yahoo Groups"
        <yahoo@w...> wrote:
        > No, these are two different things. ABG works by "topping off" the
        wells -
        > when the pixel accumulates a certain amount of electrons, often
        about half
        > the full well, the ABG circuits bleed off electrons. This involves
        pixels in
        > very bright areas of the chip.
        >
        > The thing I am talking about is what happens in very dim areas.
        Thermal
        > electrons are removed from the pixels to limit noise. Inevitably,
        there is
        > no difference between very low signal and thermal noise as far as
        these
        > circuits are concerned.
        >
        > Ron Wodaski
        > author of The New CCD Astronomy
        > http://www.newastro.com
        >
        >
        > -----Original Message-----
        > From: Paul K <pkane2001@h...> [mailto:pkane2001@h...]
        > Sent: Friday, January 31, 2003 6:51 PM
        > To: ccd-newastro@yahoogroups.com
        > Subject: [ccd-newastro] Re: SXV-H9 or ST-7XE
        >
        >
        > Ron,
        >
        > My understanding is that the "sucking away" of electrons is done as
        > part of the ABG process on Sony chips, not to reduce the dark
        > current. If this somehow does reduce the signal, it can be turned
        off
        > with a simple on-camera adjustment on the HX916 and very likely on
        > the SXV-H9.
        >
        > The full-well capacity I believe is another place where the two
        chips
        > differ, with ST-7E NABG having a larger well.
        >
        > Regards,
        >
        > -Paul
        > http://pk.darkhorizons.org
        >
        >
        > --- In ccd-newastro@yahoogroups.com, "Wodaski Yahoo Groups"
        > <yahoo@w...> wrote:
        > >
        > > * I have a different opinion about the low dark current in the
        > Sony (and
        > > some other) chips. The low dark current exists because the chip
        > design
        > > "sucks away" unwanted electrons from the pixels. However, if you
        are
        > > recording very dim subjects, some of your signal can get sucked
        out
        > as well.
        > > Thus the low-dark-current cameras tend to have low noise overall
        > but to have
        > > poor performance with very dim subjects (e.g., extended
        nebulosity,
        > outer
        > > areas of galaxies, etc.).
        >
        >
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