A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Digital camera: stars in daylight
From: George Huxtable
Date: 2010 Sep 13, 12:34 +0100
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 email@example.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.