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Peter,

Can you tell us more about sub pixel processing? I'm not so sure sub
pixel processing is a very good way to add precision to angular
measurements on a digital image. Better to stick with whole pixels.
Experiment with partial field neutral density filtering of the Moon
when bracketing exposures. Give black and white a shot to improve
sharpness.

Greg

On Dec 11, 11:31�pm, NavL...@fer3.com wrote:
> 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]

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