RELEVANT PARAMETERS FOR TIP-TILT SYSTEMS ON LARGE TELESCOPES

In order to provide criteria for the design of tip-tilt systems on lar ge telescopes, we derive analytic formulas for the aberrations of the tip-tilt corrected wave front as a function of the tracking algorithm and of the tracking frequency. We present a new definition for the req uired bandwidth of tip-tilt systems based upon the residual high-order aberrations, and we suggest a new, simple method to eliminate the inf luence of coma on the image centroid measurement. The effect of the sc ience camera pixel size on the image quality is investigated. A simple approximation is given for the power spectrum of the image motion, an d measured power spectra are compared to this approximation. We show t hat in order to measure the rms image motion properly, it is important to run the measurement over a sufficient period of time (> 10 min). I n a second paper (Glindemann et al. 1997, PASP, 109, 688-696) in this journal, a tip-tilt system (CHARM) is presented where the parameters d erived here are used to evaluate the system performance.