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    Re: camera sextant?
    From: Greg Rudzinski
    Date: 2010 Jul 6, 10:45 -0700


    What you see is what you get (Flip Wilson quote). There is no intention to extrapolate the graphs. To extend the 50mm and 100mm lenses to lower altitudes requires a set of plotted data from zero degrees to 12 degrees for the 50mm and zero degrees to 6 degrees for the 100mm. In practice what I do is use the 100mm from 6 degrees to 12 degrees then switch lenses to the 50mm for 12 degrees to 22 degrees. For zero to 6 degrees I have been using the 200mm with the Sun's diameter as reference. So far I have no complaints with results. All intercepts by photograph have been observed under 2.5 nm from gps out of 50 plus photos taken on different days from shore and at anchor. This matches what I normally get out of my Plath.

    The graphed data for both lenses is only good to plus or minus 0.8' moa because of pixel measurement, refraction, dip, horizon quality, and time uncertainties. Going after moa tenths may not be worth the extra formula calculating effort.

    3888 pixels by 2592 are the dimensions of my Canon Rebel DSLR 10MP images as shown on iPhoto editing software.

    There may be a way to tighten things up some for determining moa/px but that would involve multiple sunrise and sunset data over varying conditions plotted and averaged. Lots of work for a small improvement.

    The 2700 pixel point could be an outlier. I'll give it a look.

    Greg Rudzinski
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