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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Visual satellite position fix
From: Peter Monta
Date: 2013 Aug 22, 20:00 -0700
From: Peter Monta
Date: 2013 Aug 22, 20:00 -0700
Frank writes: > ... If a satellite passes 250 miles overhead, a shift in its angular > position of one minute of arc (easily attainable with a camera) Indeed, one arcminute should be very easy. I don't remember the internal accuracy of the astrometry.net-reduced images I did a few years ago, but a more recent point-and-shoot, a Canon SX160, is giving me 6-arcsecond-rms internal error with handheld shots. Limiting magnitude is about 5.5 with 0.5-second exposures (which are inevitably a trifle smeared, but I take the best ones, and of course image stabilization greatly helps). The challenge, of course, is to use a camera system that could be used onboard ship, not a DSLR on a tripod. The nice thing about the SX160 is that it has reasonable plate scale (3.6 arcsec/pixel at maximum zoom), reasonable FOV (3.5 x 4.5 degree), optical image stabilization, manual focus and exposure, and is cheap ($145, about 1/3 the cost of a metal sextant). Unfortunately no raw mode, just JPEG. So with a camera firmly in the cheap-and-cheerful consumer class, one ought to be able to do handheld few-arcsecond astrometry down to magnitude 5. A fixed, small, sextant-like beamsplitter can be used for moon shots, with a neutral-density filter in the straight-ahead moon path and an offpoint angle of, say, 45 degrees in the star path to allow for baffling. This angle could easily be calibrated with starfield-starfield shots. For satellites, of course, there is no need for high dynamic range and a single image can be used directly. I still have to catch up on the archived posts over the last few years related to camera CN. Cheers, Peter ps: speaking of Moon cameras, after reading about the remarkable Markowitz dual-rate moon camera, it strikes me that something similar could be done with small cameras. Merely fashion a selective neutral-density filter on a small glass plate, just larger than the imaging chip. The filter is clear outside of the lunar diameter, to capture stars, but has transmission factor 0.001 or so inside the lunar diameter. After cementing this to the chip, moon shots can be taken directly, with no nuisance sextant-angle parameter to estimate and without the need for multiple shots to constrain the lunar position in both dimensions.