We present a theoretical analysis of an electrostatic triode mirror combined with an einzel lens for the correction of spherical and chromatic aberration. We show that this device adaptively corrects spherical and chromatic aberration simultaneously and independently. Chromatic aberration can be compensated over a relative range of 鈭?8%to +100%, and spherical aberration over 卤100%range. We compare the analytic calculation with a numerical simulation and show that the two descriptions agree to within 5%in the relevant operating regime of the device.