Code III: Regularized PSF

Even when there are numerous designated point sources, the empirical PSF derived with Code II will be noisy at low intensity levels. For the seeing-dominated PSFs of ground-based images, this must be expected in the outer halo that surrounds the approximately Gaussian core (King 1971). An obvious next step, therefore, is to regularize the PSF in order to damp out such noise fluctuations, and this can be achieved simultaneously with the estimation of , and if an extra entropic term is added to Eq. (12). Moreover, if this new term also has a floating default, its kernel can be chosen to optimize the regularization by taking advantage of prior information about the PSF. For example, apart from diffraction spikes, a ground-based PSF should be circularly-symmetric. It is useful then to take the floating default to be the azimuthally-averaged . The final then departs from circular symmetry only if the stellar images demand it - perhaps due to tracking errors - and such distortions are corrected in the restored image.

Thus far only a 1-D experimental version of Code III has been tested.