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Guiding errors

The parameterisation of guiding errors is largely a control engineering task the details of which are beyond the scope of this report. Briefly, equation (gif) is used with the mean speed U and turbulent intensity as provided by the flow measurements described in section 4.1.3. The wind spectrum is modeled by a Von Karman spectrum (gif) in which the experimental values of length scale are input. The wind tunnel tests ([Zago 89b] and [Alexandrou]) provided also the torque coefficients of the telescope for the different orientation and enclosure configurations. A series of simulations allowed to parameterize the response functions about both axes as a function of the factor .

Two distinct control cases were considered: for observations in the visible wavelength the active loop tilt control (field stabilization) by the secondary mirror (M2) was evaluated to be so effective with a bandwidth Hz that residual errors are negligible [Quattri 88]. At the time of writing this report the development of the M2 is still in progress and it is not possible to know if this analysis will be confirmed. It may be that the bandwidth of the tilt control loop will be in fact limited by the need to integrate out atmospheric image motion in the input signal.

For observations in the infrared wavelength it is planned to use the secondary mirror in the chopping mode. In this case the active loop tilt control cannot be used and the guiding accuracy relies only on the servo loops of the main drives of the telescope. Only results for this latter case are presented here below.

Lorenzo Zago,, Sun Feb 26 22:57:31 GMT+0100 1995