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    Re: Digital camera: stars in daylight
    From: Hewitt Schlereth
    Date: 2010 Sep 11, 12:06 -0400
    Speaking of dim, I have a vague notion that at one time Air Force fighter/bombers were equipped with a navigation device that automatically tracked three stars day or night to give a cockpit position readout.

    Anybody else ever hear of such a thing?

    Hewitt

    On Sat, Sep 11, 2010 at 7:12 AM, Frank Reed <FrankReed@historicalatlas.com> wrote:

    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).

    -FER


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