  The output port of GEO 600 is dominated by unwanted high order modes (HOMs).
The current thermal actuation system, a ring heater behind one of the folding
mirrors, causes a significant amount of astigmatism, which produces HOMs. We
have built and installed an astigmatism correction system, based on heating
this folding mirror at the sides (laterally). With these side heaters and the
ring heater behind the mirror, it is possible to tune its radius of curvature
in the horizontal and the vertical degree of freedom. We use this system to
match the mirrors in the two arms of GEO 600 to each other, thereby reducing
the contrast defect. The use of the side heaters reduces the power of the HOMs
at the output of GEO 600 by approximately 37%.
