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72500Daytime AI (was: Another Q: Dark Noise/Dark Current)

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  • Stan
    Mar 17, 2013
      --- "Stan" <stan_ccd@...> wrote:
      > might not be terribly difficult to image a galaxy in the daytime

      On second thought:

      There are some complications not considered in the first msg. The most serious problem is CCD PRNU (Photon Response Non-Uniformity), an inherent variation of pixel sensitivity (QE) within an individual CCD. Generally for AI PRNU is trivial and is easily calibrated out via flats. But the impact of PRNU increases with sky and object fluxes and it would be extremely difficult to produce a flat that could deal with the enormous sky flux of this situation.

      The pixel noise from flat fielding (pffn)=(s+sky)/flatSN

      A typical AI master flat made from 20 frames, each with avg 30ke-, has S/N near 700. Flat effect on the object varies by sampling (n pixels within object) but let's say the object subtends 100x100 pixels (10k) so if we used a typical AI flat (S/N = 700) in this situation then:

      pffn = (10,010,000,000/10k) / 700 = 14.3ke-/pix

      that is not at all trivial and must be included in the S/N eq:

      object S/N with flat effect = sqrt(t) * s / sqrt(s + sky + p*ffn^2)

      using the prior numbers:

      S/N with no PRNU/flat = sqrt(10k) * 1k / sqrt(1k + 1k*1m)
      = 100* 1k/31.6k = 3

      S/N with flat = sqrt(10k) * 1k / sqrt(1k + 1k*1m + 1k*14.3k^2)
      = 100 * 1k / sqrt(1b + 204b) = 0.0004

      It could be hopeless to create a flat with sufficient S/N to reasonably overcome this dynamic. You could make a 3 hour flat but the flat target would have to be impeccable because the slightest defects would "print thru" (e.g. you could not use the sky because at that level it would be too disturbed by "invisible" stars and such).

      So it may be completely hopeless to actually image a galaxy in daylight without significant offset/drizzle, which can overcome much of this problem by averaging out PRNU / flat error. I've not yet calculated how much dithering would be necessary but any dithering is very problematic because there are no guide stars and so it would require an ultra-precise mount with a coordinated control link to the image stream so that the stacking software could know how to align the frames.

      This would be a very difficult challenge!

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