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    Re: Digital camera: stars in daylight
    From: George Huxtable
    Date: 2010 Sep 13, 12:34 +0100

    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 3.5º, so the diagonal span across the frame wasn't much more than 5º, 
    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 90º, 
    as an octant does. But that's asking a lot of a wide-angle lens system, and 
    perhaps a limit of, say, 50º 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 50º rather than 5º 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 50º (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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