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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Photographic lunars
From: UNK
Date: 2009 Dec 11, 23:31 -0800
From: UNK
Date: 2009 Dec 11, 23:31 -0800
Hi all, I've recently become interested in celestial navigation and thought I'd try a lunar distance measurement via digital camera. It seems to work. After skimming the interesting archives of this group, perhaps it's worth sharing the workflow for this. I'm using a Canon 40D with an 85mm lens at f/4 and ISO 800. No single exposure time gives both lots of stars and a well-defined lunar limb, so the camera was set to exposure-bracket by +- 2 stops (which is just barely enough). The two exposures at 1/80 sec and 1/5 sec were used and the center one at 1/20 sec discarded (being the worst of both worlds). All images were taken on a tripod less than a second apart in time (by virtue of the automatic bracketing), so the camera pointing should change very little, limited only by the stiffness of the tripod and camera body under the stress of the shutter operation. Image scale is about 20 arcseconds per pixel when considering only the green pixels (raw images were used and the G pixels extracted from the Bayer matrix, discarding R and B). So the image at 1/80 sec shows a considerably overexposed Moon (with nevertheless a pretty sharp limb) and no stars, and the 1/5 sec shows a number of stars in the field of view with a hopeless blob of an overexposed moon at the center. Having a little experience with the free CCD astrometry pipeline from Astromatic (http://www.astromatic.net/) and the cool work at astrometry.net, I thought the easiest way to reduce the images would be not to try for any "lunar distances" but just to go directly for a lunar position in global coordinates. astrometry.net can take any image and find out where it's pointed in the sky with no prior information whatsoever. Quite amazing. Feeding it the 1/5 sec image results in a world coordinate system for the whole image, mapping (x,y) pixel coordinates onto (ra,dec). It also estimates lens distortion, which is about 1% in this case. About 50 stars are detected with good coverage over the whole field (except near the moon). Then it's a matter of using the 1/80 sec image to estimate the center of the moon in pixel coordinates, then using the other image to translate that to the moon's right ascension and declination. The results are: estimated moon center from images: ra 2h 31' 7", dec 19d 46' 42" moon's position at my location using planetarium program (Stellarium): ra 2h 31' 18.0", dec 19d 46' 40.8" Woohoo---11 arcseconds error. With well-exposed stellar fields I usually get errors around 2 arcseconds rms (~0.1 pixel) against good star catalogs like UCAC-3, so perhaps most of that 11 arcseconds is the error in estimating the subpixel position of the lunar limb (which could maybe be improved with better image processing). Now I'm sure one objection to all this is that it requires a stable platform. But with further playing around, maybe some handheld images would be usable if the short exposures were used to derive a "track" of the pointing instability using the sharp lunar images. Could take a stream of, say, 100 images over the course of a minute or so, locate the images with the smallest image-to-image movement, then look for (possibly somewhat streaked) star images in the interspersed longer exposures. I'd be glad to upload the images if there's interest. Cheers, Peter Monta ------------------------------------------- [Sent from archive by: pmonta-AT-gmail.com] -- NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList+@fer3.com