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Stokes Parameters These quantities are related to object ellipticities. Define the flux-weighted second moments of the object as:
Mxx = <x/r2> ,Myy = <y2/r2> , Mxy = <xy/r2>
In the case that the object's isophotes are self-similar ellipses, one can show that:
Q = Mxx-Myy = [(a-b)/(a+b)]×cos2φ
U = Mxy = [(a-b)/(a+b)]×sin2φ

where a and b are the semimajor and semiminor axes and φ is the position angle. Q and U are Q and U in the table PhotoObj and are referred to as "Stokes parameters." They can be used to reconstruct the axis ratio and position angle, measured relative to row and column of the CCDs. This is equivalent to the normal definition of position angle (east of north), for the scans on the equator. The performance of the Stokes parameters are not ideal at low signal-to-noise ratio, in which case the adaptive moments will be more useful.