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Re: Digital camera: stars in daylight
From: Marcel Tschudin
Date: 2010 Sep 13, 16:37 +0300
From: Marcel Tschudin
Date: 2010 Sep 13, 16:37 +0300
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. Marcel On Mon, Sep 13, 2010 at 2:34 PM, George Huxtablewrote: > 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 3.5�, so the diagonal span across the frame wasn't much more than 5�, > 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 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. > > Changing the magnification to cover a span of 50� rather than 5� 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 50� (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. > > > > >