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    Re: Digital camera: stars in daylight
    From: George Huxtable
    Date: 2010 Sep 14, 08:08 +0100

    Marcel wrote-
    
    "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."
    
    ==============
    
    Presumably, that relates to my recent postings on the subject of whether a 
    camera can make a useful observing instrument at sea, and specifically 
    about Franks notion that it could be used to obtain 3-star fixes at sea in 
    daylight.
    
    I was doing my best to bend over backwards in relaxing the standards that 
    would normally be expected from a sextant, in these respects-
    
    1. Angular range. "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."
    
    Giving up a sextant's ability to measure altitudes of objects above 50� is 
    quite a sacrifice, but one that seemed worth making if it might allow a 
    camera to be used. If the altitude range was reduced much below 50�, that 
    would make Frank's proposal for 3-star daylight fixes even less plausible. 
    But if Marcel considers that I'm being unreasonable, perhaps he will 
    suggest a practical limit on angular span, which would still allow such a 
    sea-camera to be an instrument of general use, rather than one that only 
    comes out of its box in special circumstances.
    
    2. Angular resolution. In an earlier posting, on 11 September, I had 
    written-
     "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? ..." Indeed, Frank's 
    answer was "yes", so he appeared to consider those as reasonable 
    requirements.
    
    Marcel may note that the suggested precision, of "a (very) few 
    arc-minutes", is relaxed, considerably, from the fraction of a minute that 
    we all expect of a sextant. Is even that asking too much? If he thinks it 
    is, then perhaps he will give us a notion of what we should expect from 
    such a camera.
    
    To perform the task that Frank was proposing, a camera would need many of 
    the properties of a sextant. However, that proposal seems to me fanciful. 
    If Marcel sees a more limited role for a sea-camera (which might well 
    apply), perhaps he will tell us what it is.
    
    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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