Re: Bright Sat Photometry?

From: Cees Bassa (cgbsat@googlemail.com)
Date: Wed Sep 16 2009 - 10:53:58 UTC

  • Next message: Scott: "Satobs 16 Sept 2009"

    On Sun, Sep 13, 2009 at 12:47 AM, Marco Langbroek
    <marco.langbroek@wanadoo.nl> wrote:
    > One problem to tackle is that the satellites and stars don't have the same
    > angular velocity. I.e., you can't directly compare a star trail to a
    > satellite trail. Correcting for that is rather complex. Also, very bright
    > objects (like the ISS) tend to get saturated.
    
    In principle it is possible to determine the brightness (flux
    actually) from both trailed and non trailed objects by measuring
    counts in apertures around objects.
    
    For non-trailed objects like stars, you can measure the counts inside
    an circle and an annulus centered on the object. The counts in the
    circle would be the flux of the star and the sky, while the flux in
    the sky can be estimated from the counts in the aperture. For trailed
    objects a similar approach will work, but the apertures would be
    something like rotated rectangles.
    
    The results will be the flux of the object over the whole exposure,
    which can be compared to the flux of stars with known magnitudes. The
    flux is no longer dependent on angular velocity because you are
    comparing all the objects flux over the entire exposure.
    
    The problem will be, as Greg explained, that in order to make that
    comparison, the exposure should have been made through a filter in
    which stars have known magnitudes (see
    http://en.wikipedia.org/wiki/Photometric_system).
    
    If this is not the case, then accurate magnitudes cannot be determined.
    
    I am not sure if software exists to do this, the principles behind it
    are straightforward.
    
    Note that this only applies to digital images, either with digital
    cameras, or CCD cameras (which will be more accurate because no
    processing is done on them). Measuring magnitudes by eye is a whole
    other ball game.
    
    Regards,
        Cees
    
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