Flashlamp pumped oscillators utilizing Nd:Cr:GSGG or Nd:YAG rods were stabilized against varying levels of thermal focusing by use of a Variable Radius Mirror (VRM). In its simplest form, the VRM consisted of a lens followed by a concave mirror. Separation of the two elements controlled the radius of curvature of the reflected phase front. Addition of a concave-convex variable-separation cylindrical lens pair, allowed astigmatism to be corrected. These distributed optical elements together with a computer controlled servo system formed an adaptive optic capable of correcting the varying thermal focusing and astigmatism encountered in a Nd:YAG confocal unstable resonator (0 - 30 W) and in Nd:Cr:GSGG stable (hemispherical or concave- convex) resonators so that high beam quality could be maintained over the entire operating range. By utilizing resonators designed to eliminate birefringence losses, high efficiency could also be maintained. The ability to eliminate thermally induced losses in GSGG allows operating power to be increased into the range where thermal fracture is a factor. We present some results on the effect of surface finish (fine grind, grooves, chemical etch strengthening) on fracture limit and high gain operation.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.