Ion beam figuring of small BK7 and Zerodur optics : thermal effects

optics these last years. One of the disadvantages of ion figuring is the high surface temperature of the workpiece. Temperature aspects have already been mentioned by some authors but are weakly detailed. Therefore thermal effects on BK7 and Zerodur, mainly surface figure distortions and surface temperature measurements, have been investigated here in more detail. Distortion of the optical figure due to high temperature and temperature gradients induced by ion figuring is described. High figure distortion has been observed on 10 cm diameter BK7 samples. Distortions can also be observed on Zerodur samples (although much weaker). Differences between BK7 and Zerodur distortions are discussed. In order to analyse the nonuniform heating of the workpiece during ion bombardment, the surface temperature has been monitored with an infrared camera. Furthermore, surface temperature measurements with respect to ion beam parameters have also been performed. All these measurements show the necessity to care about thermal aspects during ion beam figuring for some sensible materials like BK7, but even for low expansion material like Zerodur.