CHARM: A tip-tilt tertiary system for the Calar Alto 3.5-m telescope

We discuss a tip-tilt tertiary mirror system developed for the Calar Alto 3.5-m telescope in Spain, that corrects the rapid image motion caused by atmospheric turbulence. Using either a visible or an infrared tip-tilt sensor, the image motion is reduced typically to less than 0.03 '' rms (corresponding to 0.29 rad(2) mean square wavefront tilt error) at 2.2 mu m on a 3.5-m telescope. The system is equipped with a CCD camera to measure the image motion in the visible. Alternatively, using a novel technique for reading out a subarray of an infrared detector, the infrared science camera can be used to measure the slope of the wavefront by taking short exposure images on a very small subarray while simultaneously taking long exposure images on the rest of the array. This enables the system to be used even in obscured regions where only infrared stars are available. Astronomical results from several observing runs with this system at the Calar Alto 3.5-m telescope are presented.