A broadband beam splitter coating with the reflecting range from 400 to 900 nm and the transmitting range from 920 to 2300 nm was developed. To avoid absorptive losses at low wavelengths, Ta<sub>2</sub>O<sub>5</sub> was chosen as the high index material. A new setup based on a Hartmann-Shack sensor was built to measure the wavefront error and its spectral dependence. An area of 120 mm diameter was measured. While in a standard coating, large resonant-like wavefront errors occur, an optimized coating with significantly reduced spectral wavefront error was designed and produced.