Fitting PSF to an image to get total flux (Python)
You can't really get a PSF for an extended object.  The Point-Spread-Function is a parameter of your imaging system which describes the shape of the image of a putative point source.  

I don't have any knowledge of what your python package is intended to do, so I can't comment directly on the results.  I will point (sorry) out that the geometric centroid of the image is not generally the same as the energy(brightness) centroid of the image.

Further, the total flux is just the sum of all the pixels covered by the asteroid's image (less background noise and whatnot).  It's not clear why you need the PSF if you are just doing energy/power calculations.