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    Re: Photographic lunars
    From: George Huxtable
    Date: 2009 Dec 12, 11:46 -0000

    Peter Monta has explained well his procedure for obtaining a precise
    position of the Moon from digital photographs.
    I, for one, would very much like to see his photos. It seems to be a method
    which can produce good results, with a tripod, from on land. I'm a bit
    doubtful about handheld shots on land providing sharp enough images, if they
    need 1/5 second. Maybe his proposal for storing a stream of many such
    images, in the hope that some will show little motion, will work, if the
    camera is capable of processing and storing those high-resolution images
    fast enough.
    As is my wont, I will ask a few questions, and point to some drawbacks.
    First, we have to be aware that, if stars need a 1/5 sec exposure to show
    them up, there's no hope of taking such a picture from at sea, with the
    inevitable motion underfoot. That's where lunar distances were originally
    called-for. It would call for an improvement in imaging light-sensitivity by
    several orders of magnitude to make short-enough exposures practicable at
    sea. However, in time, that may come. So we have to confine ourselves to
    land-based observations, which is fair enough.
    Peter has described a single observation that provides excellent agreement,
    which is a very promising start. However, it's always possible to hit a
    bull'e eye with your first dart. I hope he will follow-up by analysing
    several other shots, taken with different Moon phases, and different star
    backgrounds, to see what scatter results. And, to be absolutely fair, the
    Moon's position on the array should be deduced before its astronomical
    position is known, in a proper blind-test (not that I'm suggesting that
    Peter's position estimates are influenced by his expected result). Perhaps
    he will consider passing full-resolution images to this list for others to
    have a go at, with details of location and precise time. List-members might
    offer more old-fashioned techniques to do a similar job.
    Peter wrote- " astrometry.net can take any image and find out where it's
    pointed in the sky with no prior information whatsoever.  Quite amazing."
    Indeed, from that description, it seems powerful, and clever. But how many
    stars need to appear in shot for it to do so? It has to determine many
    parameters: altitude, azimuth, orientation, angular scale, distortion, and
    arrive at an unambiguous solution which is unique to that bit of the sky.
    How many bright stars does it need to identify, to do all that?
    And what's the angular size of the patch of sky, in the photo, from which it
    has to do so?. My guess would be about 5 degrees across, but, not being a
    photographic expert, I'm unsure what a 85mm lens implies when used with a
    CCD array. If I understand it right, that patch has to contain sufficient
    bright-enough stars for the program to do its thing. There may be parts of
    the sky that are easily rich enough in stars for that to happen, but again,
    there may be many others around the ecliptic, in which bright stars are
    sparse. It depends on the brightness-threshold that is detectable; there's
    no shortage of faint stars! Peter's photos may answer the question.
    A question about overexposure. How sure can Peter be that his Moon image
    isn't overexposed, which, if it was, could significantly affect its edge? If
    he can make out the Moon's features, its "seas", that answers the question.
    Can he estimate the Moon's diameter from its pixels, and does that result in
    the correct answer? If it is less than full Moon, he has only one limb to
    work from in estimating its centre. How does he go about that? Does he use
    its tabulated semidiameter?
    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.
    ----- Original Message -----
    Sent: Saturday, December 12, 2009 7:31 AM
    Subject: [NavList 11095] Photographic lunars
    | 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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