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    Re: Digital camera: stars in daylight
    From: George Huxtable
    Date: 2010 Sep 12, 20:28 +0100

    Frank wrote-
    "Of the forty photos I took yesterday of the Moon, a slim which I really
    could not find without the pointing aid from the software on my phone, only
    a couple failed to show Venus clearly. It was bright and obvious in many of
    the photos but quite invisible to the naked eye. So how low can we go? What
    is the faintest star detectable from sea level in digital photos taken with
    an off-the-shelf digital SLR camera in good daylight conditions? If we can
    get down to magnitude 1.5, that's roughly ten to twelve stars and planets
    at a time. Four or five would be close enough to the horizon to image in
    the same frame with the horizon (and high enough to be clearly visible)
    without using exotic lenses."
    .The fact that Venus showed clearly on 38 out of 40 shots shows that it's
    sgnificantly above some signal to backgrond detectability threshold. But
    the fact that it failed to do so on 2 out of 40 shows that it wasn't very
    far above that threshold, and there isn't a lot in hand to play with.
    Now consider magnitudes. Venus on that day was about as bright as it gets,
    magnitude -4.5. Frank wants to get down to stars of magnitude +1.5. The
    difference is six orders of magnitude. Taking one stellar magnitude as
    equal to a brightness ratio of 2.5, such stars are down on Venus by the
    sixth power of 2.5, or 244. And that worsening, by 244, is in the
    signal-to-background ratio, so it's hard to see how that is to be overcome
    by technology.
    A touch of realism is called for.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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