... Ideal image formation can be modeled as the convolution of the true brightness distribution of the object with the telescope point spread function. ... Optical aberrations due to misalignments, thermal gradients, outgassing, manufacturing errors and other effects change the configuration of the optics, causing the phase of the wavefront propagated through the telescope to deviate from a perfect sphere. ... These effects are very apparent in images taken with the first set of HST instruments. ...