Optical measurements of ultra-sensitive far-infrared TES bolometers with FDM readout

The next generation of far-infrared space-based observatories will require large arrays of ultra-sensitive detectors, such as transition edge sensors (TES). We have constructed a new optical testbed in order to perform an end-to-end demonstration of a detector system comprising a 64-pixel TES bolometer array sensitive to the 34-60-micron wavelength range and a frequency domain multiplexing readout with baseband feedback. The testbed contains internal cold (3-35 K) and hot (up to -300 K) black-body calibration sources, a light pipe for external illumination, and a broad-band reference detector for characterizing the spectral content of the calibration sources, as well as reimaging optics for controlling the illumination of detector arrays and an external Fourier transform spectrometer for measuring their spectral response. We present our latest measurements of the optical performance of prototype arrays under multiplexed readout and discuss the implications for future space missions.