inflection-point bessel-function intensity-curvature jacobian-matrix diffraction-limited-spots difference-of-gaussian scale-space-feature-detection resolution-limit low-snr fluorescent-speckle-microscopy point-spread-function-fitting objective-function-optimization generalized-sparrow-limit detection-success-rate change-of-sign-2nd-derivative airy-disk-modeling subresolution-cluster-fluorophores feature-fusion-due-to-noise computationally-inexpensive-detection