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Re: Digital camera: stars in daylight
From: George Huxtable
Date: 2010 Sep 12, 20:28 +0100
From: George Huxtable
Date: 2010 Sep 12, 20:28 +0100
Frank wrote- "Of the forty photos I took yesterday of the Moon, a slim which I really could not find without the pointing aid from the software on my phone, only a couple failed to show Venus clearly. It was bright and obvious in many of the photos but quite invisible to the naked eye. So how low can we go? What is the faintest star detectable from sea level in digital photos taken with an off-the-shelf digital SLR camera in good daylight conditions? If we can get down to magnitude 1.5, that's roughly ten to twelve stars and planets at a time. Four or five would be close enough to the horizon to image in the same frame with the horizon (and high enough to be clearly visible) without using exotic lenses." .The fact that Venus showed clearly on 38 out of 40 shots shows that it's sgnificantly above some signal to backgrond detectability threshold. But the fact that it failed to do so on 2 out of 40 shows that it wasn't very far above that threshold, and there isn't a lot in hand to play with. Now consider magnitudes. Venus on that day was about as bright as it gets, magnitude -4.5. Frank wants to get down to stars of magnitude +1.5. The difference is six orders of magnitude. Taking one stellar magnitude as equal to a brightness ratio of 2.5, such stars are down on Venus by the sixth power of 2.5, or 244. And that worsening, by 244, is in the signal-to-background ratio, so it's hard to see how that is to be overcome by technology. A touch of realism is called for. 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.