MOPEX User's Guide - IRSA

MOPEX User’s Guide
in this case is more complicated than a simple rectangle, and is a combination of all the rectangles
for each detection in the blend (see Figure 8.23). Passive de-blending has been proven to be an
essential component of point source extraction.
In order to fit the data, a modified Simplex algorithm is employed (see §8.6.1 below). The
Simplex algorithm does not use derivatives of the functions involved in minimization. This is a
desirable feature since the transformation from local coordinates to global coordinates can be a
very complicated function of its arguments.
Two parameters, Dither Flux Fraction and Dither Pixel Fraction determine the initialization of
the Simplex algorithm. Each detection is split into three vertices. Their positions ri and fluxes fi
are initialized by randomly dithering the position R and flux F of the detection:
ri = R + rand * DitherPixelFraction
Fi = F * (1+ rand * DitherFluxFraction)
where rand is a random number uniformly distributed from -1 to 1.
Equation 8.25
The unsuccessful termination of the fitting for each point source is determined by the two
parameters Minimize Ftol and Max N Iteration; if successful, then the two parameters Max N
Success Iteration and Minimize Ftol Success govern the termination. If the number of iterations
exceeds Max N Iteration or the relative change in χ 2 becomes smaller than Minimize Ftol before
χ 2 / dof reaches Chi Threshold, then the fitting for this point source terminates and the point
source is assigned the corresponding failure status. If, on the other hand, the fitting is successful
and χ 2 / dof drops below Chi Threshold, the program will continue fitting in order to improve the
results until either the number of iterations after reaching Chi Threshold exceeds Max N Success
Iteration or the relative change in χ2 becomes smaller than Minimize Ftol Success. After that,
the point source is assigned the corresponding success status.
Basic Concepts in MOPEX 210
PRF fitting in MOPEX