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Re: Digital camera: stars in daylight
From: Marcel Tschudin
Date: 2010 Sep 14, 12:14 +0300
From: Marcel Tschudin
Date: 2010 Sep 14, 12:14 +0300
George, regarding your last post. We are at the very beginning. We only just started to see what can be measured with a camera. Greg had his 50mm calibrated for the centre line only. Frank showed that a photo can be taken from Venus during daytime. There is still a lot to be explored. Let's advance step by step and see what we arrive at. Marcel On Tue, Sep 14, 2010 at 10:08 AM, George Huxtablewrote: > 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. > > > > >