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    Re: Digital Camera Celestial Navigation
    From: Frank Reed
    Date: 2008 Jul 04, 15:02 -0400

    I mentioned photgraphing a meter stick to get the angular scale, and Marcel,
    you replied:
    "Why not measuring the sun's horizontal diameter? This measure is (almost?)
    not affected by refraction. The value can be calculated quite accurately and
    only changes a few percent during the year. One only has to be careful that
    the sun is not overexposed."
    
    Yes. Certainly that will work. The advantages of the meter stick are small:
    1) you have multiple points that you can measure so you can potentially get
    a "sub-pixel" measurement. 2) It can be extended to longer distances (use a
    tape measure, but then you have to be careful to give it a curve so that it
    is an arc of a circle or you have to do a little math) and that can supply
    the angular scale all the way across the field of view, which is usually
    variable.
    
    I mentioned measuring the Sun's flattening as a surrogate for altitude, and
    Marcel, you replied:
    "This doesn't look to me as a reliable procedure. There are often
    situations where differential refraction produces odd shapes"
    
    I agree, and that's what I told Ken. He was annoyed. :-)
    
    But it's all a question of details. If you limit yourself to Sun altitudes
    above, let's say, two degrees, most of the complexity in refraction goes
    away. Then it's just a question of accuracy. How reliably can one measure
    the flattening using a digital camera? A few tenths of a minute of arc?? How
    much of a change in altitude corresponds to a tenth of a minute of arc
    change in the flattening? Naturally, if you can see the horizon, you would
    get better results directly measuring the distance to the horizon. So this
    is a procedure specifically designed for those cases where the horizon is
    obscured.
    
    For more background on Ken Gebhart's "sun squash", read here:
    http://fer3.com/arc/m2.aspx?i=027082&y=200601
    
    And you wrote:
    "BTW: using a digital camera has yet an other advantage. Sometimes the
    sun can't be seen with the eyes. However, if the clouds in front of
    the sun are sufficiently thin the sensor of the digital camera still
    can see the IR image of the sun. In those cases it's possible to take
    "shots" without actually seeing the sun (except on the screen of the
    camera)."
    
    Very cool! (pardon the pun) I'll have to try that.
    
     -FER
    
    
    
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