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    Photographic lunars
    From: UNK
    Date: 2009 Dec 11, 23:31 -0800

    Hi all,
    
    I've recently become interested in celestial navigation and
    thought I'd try a lunar distance measurement via digital camera.
    It seems to work.  After skimming the interesting archives of this
    group, perhaps it's worth sharing the workflow for this.
    
    I'm using a Canon 40D with an 85mm lens at f/4 and ISO 800.  No
    single exposure time gives both lots of stars and a well-defined
    lunar limb, so the camera was set to exposure-bracket by +- 2
    stops (which is just barely enough).  The two exposures at 1/80
    sec and 1/5 sec were used and the center one at 1/20 sec discarded
    (being the worst of both worlds).  All images were taken on a
    tripod less than a second apart in time (by virtue of the
    automatic bracketing), so the camera pointing should change very
    little, limited only by the stiffness of the tripod and camera
    body under the stress of the shutter operation.  Image scale is
    about 20 arcseconds per pixel when considering only the green
    pixels (raw images were used and the G pixels extracted from the
    Bayer matrix, discarding R and B).
    
    So the image at 1/80 sec shows a considerably overexposed Moon
    (with nevertheless a pretty sharp limb) and no stars, and the 1/5
    sec shows a number of stars in the field of view with a hopeless
    blob of an overexposed moon at the center.
    
    Having a little experience with the free CCD astrometry pipeline
    from Astromatic (http://www.astromatic.net/) and the cool work at
    astrometry.net, I thought the easiest way to reduce the images
    would be not to try for any "lunar distances" but just to go
    directly for a lunar position in global coordinates.
    
    astrometry.net can take any image and find out where it's pointed
    in the sky with no prior information whatsoever.  Quite amazing.
    Feeding it the 1/5 sec image results in a world coordinate system
    for the whole image, mapping (x,y) pixel coordinates onto
    (ra,dec).  It also estimates lens distortion, which is about 1%
    in this case.  About 50 stars are detected with good coverage over
    the whole field (except near the moon).
    
    Then it's a matter of using the 1/80 sec image to estimate the
    center of the moon in pixel coordinates, then using the other
    image to translate that to the moon's right ascension and
    declination.
    
    The results are:
    
    estimated moon center from images:
      ra 2h 31' 7", dec 19d 46' 42"
    
    moon's position at my location using planetarium program (Stellarium):
      ra 2h 31' 18.0", dec 19d 46' 40.8"
    
    Woohoo---11 arcseconds error.  With well-exposed stellar fields
    I usually get errors around 2 arcseconds rms (~0.1 pixel) against
    good star catalogs like UCAC-3, so perhaps most of that 11
    arcseconds is the error in estimating the subpixel position of the
    lunar limb (which could maybe be improved with better image
    processing).
    
    Now I'm sure one objection to all this is that it requires a
    stable platform.  But with further playing around, maybe some
    handheld images would be usable if the short exposures were used
    to derive a "track" of the pointing instability using the sharp
    lunar images.  Could take a stream of, say, 100 images over the
    course of a minute or so, locate the images with the smallest
    image-to-image movement, then look for (possibly somewhat
    streaked) star images in the interspersed longer exposures.
    
    I'd be glad to upload the images if there's interest.
    
    Cheers,
    Peter Monta
    
    -------------------------------------------
    [Sent from archive by: pmonta-AT-gmail.com]
    
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