Re: Light Curve from Satellite Trail

From: Björn Gimle @GlocalNet (@GlocalNet)
Date: Sat Jan 12 2008 - 18:09:48 UTC

  • Next message: Marco Langbroek: "Obs ML (4353) CAM, 12 Jan 2008: Lacrosse 2 is early (?)"

    I apologize if I unintentionally introduced the "cloaking" side-track.
    I was referring to "natural" shading effects of geometry, but I agree
    that just orbital motion is not sufficient to explain Lacrosse's rapid 
    changes.
    (Maybe if it needs to rotate the spacecraft, or reposition a SAR panel)
    > ...
    > Briefly back to the original topic - my reason for raising it was to point
    > out that just about anyone can produce a light curve from an electronic
    > image. And that includes scans of what were originally hard copy 
    > negatives,
    > transparencies or prints.
    
    Your subtraction of sky background, using a strip adjacent to the track, is 
    good.
    But in a wide angle shot, there are also effects of
    vignetting, and simple light reduction because
    1. the lens area, as seen from from objects near a corner, is reduced
    2. the light is spread out because it hits the film/sensor plane at an angle
    3. the distance from the lens to the sensor is larger, making the image 
    larger/faster.
    
    For a simple lens, all three factors may be near cos(angle), so raise this 
    to a power of three!
    So these contributions should not be subtracted, but divided!
    For a 135 film equivalent focal length of 35 mm I find 31.7 degrees to the 
    corner, and cos^3 = 0.615
    
    Also, satellite speed and distance can vary substantionally across the 
    image.
    
    /Björn 
    
    
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