re: Nanosail-D2 best color result

From: Thierry Legault (legault@club-internet.fr)
Date: Tue May 10 2011 - 20:38:33 UTC

  • Next message: Thierry Legault: "re: Nanosail-D2 best color result"

    hi all
    
    well, letís 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 10Ē 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 50į, 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.
    
    http://astrosurf.com/prostjp/images/Atmospheric%20dispersion%20effect%20small.jpg
    
    http://legault.perso.sfr.fr/dispersion.jpg
    
    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: http://legault.perso.sfr.fr/bad_astrophotography.html
    
    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 10Ē telescope 
    tracked by hand, extract one single 8-bit 
    compressed image from the video and try to draw 
    details on the surface of Io. Iíll be telling to 
    Damian Peach that itís 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 
    >dyes. http://freeimagehosting.nl/pics/562a0c0ea73a6b3811d80c3226860812.jpg
    
    Thierry Legault
    www.astrophoto.fr
    
    
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