Iridium Flash Reflection

From: juliette (fbpsjuliette@wanadoo.fr)
Date: Thu Nov 07 2002 - 17:12:31 EST

  • Next message: paul: "8305PG obs 08nov"

    Hello,
    
    Sky and Telescope December 2002 publishes, page  143,  a picture by
    S.M. SABATINI  showing an Iridium flare reflection on the River
    Colorado in Arizona.
    I send to the author my best congratulations.
    That is the picture I dreamed of for many years, tried again and again 
    and never succeeded.
    
    This question had been widely discussed on this list under various subjects
     particularly " Iridiums Flares ground projection" ( february 98 )  and 
    " Zigzagging flare coordinates " (may 98) and I took part of it.
    
    Rob. Matson, Bjorn Gimle and many others wrote on this subject.
    Here is a small extract :
    
    >RJ@RMATSC.RIEM.COM schrieb:
    > The width and length of a perfect Iridium flare can be calculated as
    > follows:
    >
    >     dist = distance from the observer to the sat
    >     alt = angle from observers horizon to the sat
    >     1/2 = angular diameter of the sun
    >
    >     width = dist * tan(1/2)
    >     length = width / sin(alt)
    >
    > For an Iridium whose distance is 1200 km and 30 degrees
    > above the horizon the flare width would be 10 km and
    > length 20 km.
    >
    > Real flares are far from perfect however.  There is no sharp line
    > between light and dark and the actual flare spot would be both
    > larger and fuzzier.
    >
    > The velocity of the flare is much harder to calculate but it would
    > be of the same order as the velocity of the spacecraft.
    >
    
    I got the impression that almost everybody thought my aim of
    photographying a flare from a hill, reflecting on the sea surface 
    close to  the shore, was a mere utopia.
    The success of S. M. Sabatini shows at least that I was not completely
    wrong and  that the width of the  light line seems far smaller than 10 Km.
    Why did I not succeed ?  My explanation is as follows :
    Because of my location I pointed my camera to the place where the projection 
    of the flare would appear on the sea  (to the North-West)  while the satellite 
     would flare  in the sky from the South West.
    SM Sabatini has succeeded because he pointed to the satellite and  to its
    reflection at the same time, and  ACROSS THE WATER PLANE.
    
    That is my impression, but I would ask advice from the members skilled
    in optics and mathematics.
    Is the light projected by the iridium antenna really a beam ?  and  what
    is  it like  in 3 dimensions through the space ?
    
    Jean Monseur
    N 49.305     W 0.067, Alt 74 m  
    Cliff along  the English Channel Shore, France.
    
    
    
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