Image spread function model

The model system essentially consists of small fluorescent beads. A large variety of such beads is commercially available. Their size, surface properties, and fluorescence spectra can be chosen within wide limits. The beads are deposited on coated electron microscopic grids in the form of small drops. After drying, individual droplets are first visualized by confocal fluorescence microscopy and subsequently by standard transmission electron microscopy. Thus, the model system permits one to compare directly confocal with electron microscopic results. This is demonstrated in Fig. 1 (see color plate). Also, the size and average area density of the beads can be matched to those of the NPC. The model system is also well suited for optimizing imaging conditions and quantitating resolution in terms of the point spread function. 

The maximum entropy method was one of several techniques that were used to restore the Hubble images. However, all techniques were hampered by lack of complete knowledge of the point-spread function. Using a point-spread function that included errors (either because of noise in a measured point-spread function, or modeling imperfections in predicted point-spread function) further adds to the ill-posed nature of the restoration problem. Although the modulation transfer function tended not to be zero-valued (at least not as severely as in the radio interferometry case described below), this was of only small comfort when imaging weak objects, as the... 

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