Re: Nanosail-D2 best color result

Date: Tue May 10 2011 - 21:54:23 UTC

  • Next message: Paul Grace: "RE: Nanosail-D2 best color result"

    Hi Paul,
    Sorry to butt in so to speak but with respect I think you have got the wrong 
    end of the stick as far as Thierry L.'s comments go.
    I believe he is being scientifically objective and that there is no 
    intention for there to be any personal slight here.  When folk apply a 
    logical approach to some questions it can indeed sound 'petty and 
    uncharitable' but scientists are used to it and respond in a similar 
    somewhat abstract style of writing.  Think of it as putting forward 
    challenging arguments which intend to be helpful in the long run - a devil's 
    advocate if you will.
    Thierry Legault has a great deal of experience (as does Damian Peach, who 
    Legault quotes) in high-res imaging so it's always worth listening and 
    taking heed of their comments.
    My two English penneth,
    Richard Miles
    ----- Original Message ----- 
    From: "Paul Grace" <>
    To: <>
    Sent: Tuesday, May 10, 2011 9:32 PM
    Subject: RE: Nanosail-D2 best color result
    Mr. Legault, good telescopes should have a Dawes resolving limit in
    arcseconds of 134/D mm. (see
    A good 10" scope should yield about 0.45 arcsecs.  This is better than
    needed to resolve a 2m object 740Km distant, Not small details of course,
    but possibly the difference between a circular and a rectangular plan.  At
    least it's not impossible.
    I agree with you, there must be shape and color artifacts in the image, and
    that the image is not free of these distortions.  Mr. Vandebergh makes no
    claim that the image was free of error though (note he says "color tones
    actually exist in the image." he didn't say the color exists on the
    satellite), so I guess I don't really understand your position.  The sarcasm
    you employ (e.g. "unimportant things") makes you sound petty and
    uncharitable though.  I don't think any reasonable viewer would assume that
    the nanosail looks exactly like that image of it, in color or shape.
    I think without constructive comments to make, you could let Mr.
    Vandebergh's photos stand without taking the effort to sarcastically piss on
    another person's earnest efforts; there is nothing positive to gain.
    -----Original Message-----
    [] On Behalf Of
    Thierry Legault
    Sent: Tuesday, May 10, 2011 13:39
    Subject: re: Nanosail-D2 best color result
    hi all
    well, let's talk one second about object size,
    atmospheric dispersion and other unimportant things.
    First, an object whose size is 3 meters, seen at
    740 km (or farther: 740 km was the minimum
    distance but we do not know exactly what was the
    real distance for this precise image),
    corresponds to an apparent diameter less than one
    arcsec (about 0.8 arcsec). This is less than the
    size of the Airy disc, whose diameter is 1.1
    arcsec for a 10" telescope. Thus, in the
    comparison of size with the Airy disc shown
    above, the size of Nanosail in the image is not
    mainly related to its real shape but caused by
    atmospheric turbulence or other spreading causes
    (manual tracking, defocus etc.).
    But the most interesting comparison is with
    atmospheric dispersion. We all know the
    atmospheric refraction, which changes the
    apparent altitude of an object above the horizon.
    Actually, this effect depends on the wavelength:
    blue rays are move deviated than red ones. The
    consequence is that colors are spread like a
    spectrum, as in the image of Venus below. Just
    like in this Nanosail image: red on one side,
    white (or green) in the center and blue on the
    other side (orientation of colors depend on the
    orientation of the camera and telescope). The
    diagram below, made by Jean-Pierre Prost, shows
    that at an altitude of 50, the spreading across
    the visible spectrum is already 1.2 arcsec. That
    is to say, more than the apparent size of
    Nanosail itself. Thus, any color variation on the
    sail would be hidden by dispersion.
    The dispersion is well known by top planetary
    imagers, who use either filters (green or red) or
    an ADC (Atmospheric Dispersion Corrector) for their luminance images.
    Of course, a spreading of colors may also come
    from atmospheric turbulence (we all have
    experienced the variations of colors of twinkling
    stars), and/or Bayer matrix sampling: an object
    which falls on a very few numbers of color pixels
    (here, the processed image is a huge enlargement
    of a raw image covering very few pixels), even if
    it is resolved, has different parts falling on
    red, green or blue pixels. Not to mention noise,
    unavoidable in such raw images. These effects are
    very clear on the example images I have put on
    this page:
    A simple experience that many of us can do is
    taking a video of Io (apparent diameter 1.1
    arcsec, larger than Nanosail) with a consumer
    camcorder behind an eyepiece on a 10" telescope
    tracked by hand, extract one single 8-bit
    compressed image from the video and try to draw
    details on the surface of Io. I'll be telling to
    Damian Peach that it's his next challenge, and wish him good luck   ;-)
      In short, obtaining such color info on an
    object of this size, in these atmospheric and
    instrumental conditions, is physically impossible.
    At 12:37 06/05/2011, Ralf Vandebergh wrote:
    >The image set shows how the color tones actually
    >exist in the image. Left is original color,
    >right is auto-color correction, this process
    >increases contrast between the subtle color
    Thierry Legault
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