Re: Magnitude estimates on Heavens-Above

From: Allen Thomson (thomsona@flash.net)
Date: Mon Apr 23 2001 - 12:52:11 PDT

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    Rik Hill from LPL.Arizona, where, all these years ago, I spent some time as
    a grad
    student in the presence of Dr. Kuiper, noticed,
    
    > I too have noticed discrepencies and mentioned them on this list.
    > Since I am often doing visual photometry on variable stars on the
    > same night I have a pretty good idea what a given magnitude looks
    > like. While I would expect variations of 1-2 mag. just due to
    > presentation of a satellite, still on several I have noted 4 mags.
    
    The visual magnitude of an object, like its radar cross section, is a very,
    very complicated function of the physical characteristics of the object, its
    orientation in space, the distribution of illuminating sources in space,
    their spectrum, polarization, and the location of the observer in space.
    
    A good example are the Iridia: most of the time they're not too bright, but
    given the right combination of satellite location, observer location, sun
    location, and satellite orientation, they are quite striking.
    
    It's actually kind of amazing that estimates of visual magnitudes are as
    useful as they are, not that they are sometimes noticeably incorrect.
    
    "BDRF" is a good term to look up for more information.
    
    See http://www.sciam.com/2000/0200issue/0200dorseybox2.html.
    
    and http://geography.bu.edu/brdf/brdfexpl.html , which helpfully says that,
    
    "The BRDF is the 'Bidirectional Reflectance Distribution Function'. It gives
    the reflectance of a target as a function of illumination geometry and
    viewing geometry. The BRDF depends on wavelength and is determined by the
    structural and optical properties of the surface, such as shadow-casting,
    mutiple scattering, mutual shadowing, transmission, reflection, absorption
    and emission by surface elements, facet orientation distribution and facet
    density."
    
    
    
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