Current Challenges

Many of the successes listed above are relatively forgiving targets for image reconstruction. In faint-galaxy work, we are limited more often by signal-to-noise ratio than by knowledge of the PSF or residual effects from the restoration. For galactic nuclei, the data quality is usually high enough that some noise amplification can be tolerated. Some of the most challenging restoration tasks are in the regime of high dynamic range, exemplified by QSO host galaxies, gravitational lenses, stellar ejecta, and even comets. In all these cases, reconstruction is vulnerable to small errors in the PSF, and the inevitable photon noise in low-signal regions is dominated by the Poisson noise from aberrated photons properly belonging to bright parts of the image. This problem is particularly severe for the kinds of problems just listed - where the primary interest is in faint extended structure close to very bright (usually unresolved) sources. Here, noise amplification can destroy the interesting signal, since the noise is set by statistics unrelated to the true local brightness. Nisenson (1994) has described in these proceedings some indirect analysis methods for gravitational lenses; early image-restoration work on the multiply imaged QSO PG 1115+080 (Groth et al. 1991; see also Kristian et al. 1993) amply illustrated how sensitive such results can be to both noise and PSF accuracy.

The issue of PSF accuracy has been addressed in several contributions. Analysis of existing imagery has already run into the limits of present calculations (with TinyTIM) and empirical libraries in several instances. Crowded-field photometry has reached a precision limited by the our ability to track changes in the PSF across the WF/PC field. Empirical libraries, though guaranteed to have the right optical mapping, are usually too sparsely sampled near the edges of the WF/PC chips, and break down for objects with extreme spectral shapes (including strong emission lines). This makes both wide field and high dynamic range restorations some of the most stringent tests for image restoration techniques, and just as important of our ability to properly compute the appropriate PSF.