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    Re: NG's "Midnight Fun"
    From: Jeremy C
    Date: 2010 Jun 17, 10:14 EDT
    A few comments on this one as well.
     
    1) There have been very few sunspots of late, with many days having none at all as we are at the very beginning of the 24th sunspot cycle.  The HF radio propagation conditions have been terrible which is too bad as I am trying to operate from the relatively rare VQ9 Prefix area, but I digress.
     
    2) Photographic lenses are measured in focal length but angle of view specified by various makers are for diagonal angles.  Depending on the sensor size, a 50mm lens has the same diagonal field of view as about a 75mm lens on a 35mm film camera.  The distortion inherent in a lens will remain unchanged by the sensor size.
     
    3) I would discourage any sort of zoom lens for this kind of application because as a lens zooms, the distortion across the frame changes, sometimes dramatically.  In order to get repeatable measurements for this kind of accuracy, we should stick with "calibrated" prime lenses.
     
    4) Lens selection will be a compromise of field of view and magnification.  The wider the angle, the higher the body can be from the horizon, but the smaller the body in the frame.  The longer the lens, the larger the body size, but the lower it will have to be to measure Hs.
     
    5)  I would also go for high mega-pixel cameras as the more pixels to count, the finer the resolution.
     
    When i get home in a few weeks, i might pull out my DSLR, a few primes, and my tripod and try it out.
     
    Jeremy
    In a message dated 6/15/2010 7:38:25 P.M. Bangladesh Standard Time, gregrudzinski@yahoo.com writes:

    George,

    It is my understanding that the modern digital 50mm SLR camera produces an image equivalent to a 75 or 80mm film version. Also the diagonal is probably not a good way to measure with sensors arrayed in a rectangular grid. I stay in the middle of the frame rotating the camera 90 degrees to take advantage of the wide width of the rectangular frame. The Sun (or Moon) and horizon are viewed equally distant from the frame center using the frame center marking in the camera view reticle. No clamping of the camera is needed with shutter speeds over 1/500 of a second. At these shutter speeds camera shake is not a factor and will allow use on a small craft. Sun limbs are surprisingly sharp even with a hand held telephoto. See attached 200mm example. No Sun spots seen in this example nor were any viewed through the 7 x 35 sextant scope. When some Sun spots show up I'll see if they are useful as an exposure tool.

    Greg Rudzinski
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