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    Re: Visual satellite position fix
    From: Peter Monta
    Date: 2013 Aug 22, 20:00 -0700

    Frank writes:
    
    > ... If a satellite passes 250 miles overhead, a shift in its angular
    > position of one minute of arc (easily attainable with a camera)
    
    Indeed, one arcminute should be very easy.  I don't remember the
    internal accuracy of the astrometry.net-reduced images I did a few
    years ago, but a more recent point-and-shoot, a Canon SX160, is giving
    me 6-arcsecond-rms internal error with handheld shots.  Limiting
    magnitude is about 5.5 with 0.5-second exposures (which are inevitably
    a trifle smeared, but I take the best ones, and of course image
    stabilization greatly helps).  The challenge, of course, is to use a
    camera system that could be used onboard ship, not a DSLR on a tripod.
    
    The nice thing about the SX160 is that it has reasonable plate scale
    (3.6 arcsec/pixel at maximum zoom), reasonable FOV (3.5 x 4.5 degree),
    optical image stabilization, manual focus and exposure, and is cheap
    ($145, about 1/3 the cost of a metal sextant).  Unfortunately no raw
    mode, just JPEG.
    
    So with a camera firmly in the cheap-and-cheerful consumer class, one
    ought to be able to do handheld few-arcsecond astrometry down to
    magnitude 5.  A fixed, small, sextant-like beamsplitter can be used
    for moon shots, with a neutral-density filter in the straight-ahead
    moon path and an offpoint angle of, say, 45 degrees in the star path
    to allow for baffling.  This angle could easily be calibrated with
    starfield-starfield shots.  For satellites, of course, there is no
    need for high dynamic range and a single image can be used directly.
    
    I still have to catch up on the archived posts over the last few years
    related to camera CN.
    
    Cheers,
    Peter
    
    ps: speaking of Moon cameras, after reading about the remarkable
    Markowitz dual-rate moon camera, it strikes me that something similar
    could be done with small cameras.  Merely fashion a selective
    neutral-density filter on a small glass plate, just larger than the
    imaging chip.  The filter is clear outside of the lunar diameter, to
    capture stars, but has transmission factor 0.001 or so inside the
    lunar diameter.  After cementing this to the chip, moon shots can be
    taken directly, with no nuisance sextant-angle parameter to estimate
    and without the need for multiple shots to constrain the lunar
    position in both dimensions.
    

       
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