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    Re: Digital Camera Celestial Navigation
    From: Frank Reed
    Date: 2008 Jul 04, 05:11 -0400

    Greg, you wrote:
    "Measurements are made using a ruler directly on the laptop screen."
    Does your software show pixel positions (x,y) when you move your mouse
    around? That should be a little more accurate than measuring with a ruler.
    If not, just a little fishing around for graphic software should turn up
    something that does display the exact pixel location.
    "The almanac diameter of the sun is a given at
    approximately 32 minutes of arc. Knowing this will allow the ratio of
    the laptop screen measurements to yield an altitude of the sun's lower
    limb above the horizon in minutes of arc."
    For a given level of zoom in your camera, the angular size of pixels (at
    least near the center of the field of view) is a fixed quantity which you
    could measure quite accurately. Place a meter stick 34.38 meters from your
    camera and perpendicular to the line of sight. Take a photo. Then the
    centimeter marks will be minutes of arc. The "pixels to minutes of arc"
    ratio will not change (unless you select a different zoom setting). You
    could get really fancy and measure the slight variation in angular scale
    across the field of view --there's a little distortion like this.
    "Perform normal sight reduction for the GMT of the sun's photo to get an
    azimuth and intercept."
    Very nice! There's something else you can try with photo navigation. Have
    you ever heard Ken Gebhart talk about getting Sun altitudes by the
    refractional flattening of the Sun? Suppose you take a digital photo of the
    Sun a few degrees above the horizon when the horizon is obscured somehow. If
    you very carefully measure the vertical diameter of the Sun and compare it
    with the horizontal, you can work backwards from the refraction tables (or
    formulae) to determine the Sun's altitude. It works, but it requires very
    accurate measurements. Since many modern digital cameras have fairly high
    optical zoom magnifications, you might be able to get good results.
    Navigation List archive: www.fer3.com/arc
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