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 ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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