We have devised a method of quantifying the size and shape of the probe in a Gaussian-beam lithography system. The technique is robust, being insensitive to noise, but is sensitive to changes in the probe size of as little as +/-0.5 nm. We have determined that the probe shape of our system is indeed well fit by a Gaussian, with a best-focus full-width half-maximum of 6.5 +/- 1 nm. We are able readily to quantify the effects of astigmatism on the system. In addition. the approach we describe can be extended to deal with arbitrary point-spread functions. (C) 2004 American Vacuum Society.