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    Re: Digital camera: stars in daylight
    From: George Huxtable
    Date: 2010 Sep 11, 17:20 +0100

    Frank wrote-
    
    " suppose we replace the eye with a camera. Now Venus is easy every time.
    With a decent camera and the right settings, how many stars and planets can
    you see in daylight? Jupiter? Yes. Sirius? Probably. How about Vega,
    Arcturus, Capella...? How faint can you go? If the number of visible stars
    averages more than three at a time, that completely changes the name of the
    game. Imagine getting three-star fixes all day long."
    
    Is Frank, here, discussing what's presently possible, in a navigational
    context, from the deck of a vessel, in ordinary sea-conditions? Is he
    claiming that daylight shots are possible, in such conditions, that show
    such stars or planets when at a respectable altitude, with a clear horizon
    below in the same shot? With angular accuracy and resolution of, say, a
    (very) few arc-minutes, in the angle between them? In ordinary, clear-sky,
    weather, with no special requirement for extraordinary crystal-clarity of
    the sky?
    
    If so, then he is right, that a useful tool is becoming available. But if
    not, before being carried away with enthusiasm, it would be worth
    considering any such limitations in terms of the restrictions they impose
    on its use, for a navigational purpose.
    
    ========================
    
    I agree with Frank's assessment of the calibration of lens / array
    geometry, when applied to a non-zoom infinity-focussed system. A lens will,
    in general, be axially symmetrical, in an (r, theta) plot, and the only
    thing that really matters for astro is the plot of off-axis image angle
    against off-centre pixel radius. However, this differs from the way that
    lens distortions usually seem to be expressed, as a plot of off-centre
    radial displacement on a distant plane object, against displacement from
    the centre of the pixel array.  Those two plots differ by a factor of
    exactly tan (angle), which should be easy to allow for. Being axially
    antisymmetric in an (x,y) plot, it can involve only odd powers of pixel
    off-centre displacement.
    
    But, to use such a camera for our astro purposes calls for highly accurate
    calibration, perhaps more accurate than the camera maker ever envisaged.
    Although the maker's calibration curve might well be sufficient in terms of
    its shape, if the aim is to even approach the precision of a sextant, it's
    likely to be necessary to calibrate the overall scale-factor of each
    individual instrument. Even if the pixel array is exactly uniform, is the
    array pich exactly the same in x and y directions? Is the lens-to-array
    spacing exactly reproducible, between cameras? Here, we're seeking an
    exactness of one part in many thousands. And as temperatures change, does
    that spacing change exactly in proportion to dimensions of the array
    itself?
    
    These are matters which over many years have been though about, and dealt
    with, in the case of the sextant, but may never have even been considered
    seriously for common  usage of a camera.
    
    But such a calibration should not be difficult to make A single shot of a
    night-sky image should provide all that's needed.
    
    George.
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    ===================
    ----- Original Message -----
    From: "Frank Reed" 
    To: 
    Sent: Saturday, September 11, 2010 12:12 PM
    Subject: [NavList] Digital camera: stars in daylight
    
    Paul Jackson's daytime Venus observations reminded me of a topic I meant to
    bring up back in July. Finding Venus by eye in daylight is a real
    challenge. Without a pre-set sextant or a mounted telescope, you can stare
    right at it for minutes and never "see" it. But suppose we replace the eye
    with a camera. Now Venus is easy every time. With a decent camera and the
    right settings, how many stars and planets can you see in daylight?
    Jupiter? Yes. Sirius? Probably. How about Vega, Arcturus, Capella...? How
    faint can you go? If the number of visible stars averages more than three
    at a time, that completely changes the name of the game. Imagine getting
    three-star fixes all day long... In order to be useful, this would require
    some sort of software that can scan through an image looking for the
    "little white dot". That's easy enough in principle. Does that software
    exist anywhere? This would probably require analyzing "raw" image files
    rather than compressed jpegs, but most mid-range "prosumer" digital SLRs
    can output those easily.
    By the way, a few months ago when I did not have much time for NavList,
    there was another running discussion of camera calibration. I think it
    might be worth noting that is a solved problem. People have been making
    highly accurate angular measurements with digital cameras for over a
    decade. You can even use a fisheye lens and still get accurate angles from
    the images. This is not rocket science. The trouble, from the perspective
    of a navigation enthusiast, is that these "well-known" algorithms are
    generally deeply buried in various software tools, from standalone products
    to photoshop plug-ins. The tools will spit out a calibration matrix (or a
    set of coefficients) when fed a series of standardized calibration images
    like photos of a checkerboard in various orientations, but if you want to
    do it yourself, you'll probably have to dig through the original technical
    literature. Some useful search terms: "camera calibration" (but that also
    includes things like color calibration), "camera resectioning",
    "photogrammetry" and "digital photogrammetry" (this is the process of
    producing 3d models from a series of photos taken from various
    angles --one-tenth of a minute of arc accuracy was considered reasonable
    even seven or eight years ago), and "amateur astrometry"
    (professional-quality astrometric results have been possible with mid-range
    digital cameras and backyard telescopes for over a decade). The problem of
    calibration for celestial navigation involves large angles and a very small
    number of objects all at effectively infinite distance visible at any one
    time. This problem falls somewhere in between the photogrammetry problem
    (which deals with objects in a wide field of view at various distances from
    the camera while celestial deals only with objects at infinity) and the
    astrometric problem (which usually depends on field stars with known
    angular positions in a smaller field of view).
    
    
    
    
    

       
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