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    Re: Digital camera: stars in daylight
    From: Marcel Tschudin
    Date: 2010 Sep 13, 16:37 +0300

    George,
    
    IMHO, the question should not be whether a camera can replace a
    sextant one to one, but rather what can be done with a camera, and
    only then you may eventually find what this can be useful to.
    
    Marcel
    
    
    On Mon, Sep 13, 2010 at 2:34 PM, George Huxtable  wrote:
    > It's useful to have been able to see one of Frank's daylight pics of Venus.
    > And indeed, Venus is clearly visible there, well above any detection
    > threshold (and rather more so than I expected, indeed).
    >
    > Now, it provides a bit more information to go on, in the quest for a bit of
    > realism about what Frank is proposing.
    >
    > =============
    >
    > But first, a digression. Nobody disputes that with the right optics, stars
    > can be seen and used in daylight. As far back as the 1670s, in the early
    > days of Greenwich, Flamsteed had clamped a telescope firmly in place, such
    > that Sirius passed its crosshair every (sidereal) day. He used it, day or
    > night, as long as the sky was clear of cloud, to correct his sidereal
    > clock, and from that his mean-solar clock also.
    >
    > A century later, Maskelyne had a rota of 31 "clock-stars", chosen to be
    > bright enough to be seen in daylight, which were used to keep the Greenwich
    > clocks in order.
    >
    > The important qualities for such a purpose are aperture and magnification
    > (which generally go together) and resolution. As you increase
    > magnification, a star-image remains a point (until the stage where that
    > gets spoiled by poor resolution), but the amount of light from a star
    > falling on that point increases with the area of the telescope. However,
    > the sky-brighness of that image is not increased by magnification, so the
    > signal-to-background ratio improves accordingly.
    >
    > =================
    >
    > Now go back to Frank's photo. Unless it's been heavily cropped, it's clear,
    > from the apparent size of the Moon, that it's been taken with quite a lot
    > of magnification. The �apparent distance between Moon and Venus was then
    > about 3.5�, so the diagonal span across the frame wasn't much more than 5�,
    > which may well be maximum-zoom of the camera. That setting may well be just
    > what's needed to show up the close gap between Moon and Venus, but it would
    > be of little use in determining the altitudes of sky-objects above the
    > horizon. Ideally, an altitude instrument would have an angular span of 90�,
    > as an octant does. But that's asking a lot of a wide-angle lens system, and
    > perhaps a limit of, say, 50� might be acceptable; to take in most, though
    > not the upper part, of the sky, together with the horizon.
    >
    > Changing the magnification to cover a span of 50� rather than 5� implies
    > that each pixel now covers nearly 100 times the sky-area it did before,
    > collecting correspondingly more light, and at best, all the light from a
    > star would be collected in just one pixel. So that change, on its own,
    > could worsen the signal-to-background ratio, by a factor of getting on for
    > 100. This is what has to be considered, on top of the factor of 244
    > difference in brightness between Venus and a magnitude 1.5 star. That's a
    > big deficit to make up. Sirius, by the way, the brightest star in the sky,
    > is magnitude -1.5, about a factor of 15 down on what Venus was in that
    > picture.
    >
    > Frank points to the shortcomings of his camera- "It has no manual focus,
    > minimal ISO range, no raw file output, and no ability to be controlled by
    > software". These may indeed make it awkward to use for such a purpose, but
    > (except for the raw-file ability) impinge little on this question of
    > signal-to-background.
    >
    > And -"we might not even get to magnitude 1.0. We won't know without some
    > experimentation." There should be plenty of opportunity for that. Bright
    > stars come over pretty often. I wonder if any listmember can photograph
    > even the brightest of them, in daylight, with a camera spanning 50� (or
    > so). Even from a firm footing on dry land.
    >
    > ===================
    >
    > On camera calibration, Frank wrote, on 12 Sept-
    >
    > "Yes, I'm not talking about something that the camera manufacturer
    > provides. There is a well-established system used in photogrammetry and
    > computer vision applications. Here's ONE example of this sort of thing:
    > http://www.vision.caltech.edu/bouguetj/calib_doc/
    > While the details differ, the basic procedure seems fairly similar. You
    > photograph a standard target, frequently resembling a checkerboard, a few
    > dozen times from various angles, and then the software generates a
    > calibration at the sub-pixel level. Even ten years ago, these folks were
    > generating 3d models with photogrammetric methods that had accuracies
    > across the line of sight of one part in 30,000. That is, a point in the
    > model would be correctly placed +/- 1cm at a distance of 300 meters. This
    > ratio, you'll note, implies an angular accuracy of about 0.1 minutes of
    > arc --and that includes the inaccuracy resulting from the 3d modelling
    > algorithms."
    >
    > But we're not discussing imaging 3d objects, which was what that example
    > appears to be all about. Any method which involves photographing some sort
    > of checkerboard target requires refocussing of the camera from its infinity
    > setting onto that target, and must therefore change the very quantity
    > that's required to be measured. So I suggest that the appropriate way to
    > calibrate a camera, for this purpose, is to use the test-card at infinity
    > that has been provided by the star pattern overhead.
    >
    > Finally, Frank hasn't explained yet how such a camera is to provide its
    > images under at-sea conditions.
    >
    > 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.
    >
    >
    >
    >
    >
    
    
    
    

       
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