FIG. 5: Damping behavior of the copper-powder filter. The measurement of the solid line was performed with a network analyzer in a frequency range 70 kHz − 20 GHz. The low-frequency data are determined by applying an ac signal on one end of the filter and measuring the response signal on the other end with an oscilloscope. pass filters (cutoff frequency ν = 2 kHz) are installed behind the high-voltage amplifier to suppress noise of the used electronics. An additional low-pass filter with a cutoff at 4 kHz is installed consecutively to the preamplifier. The cryostat itself is hanging from a platform supported by three air cushions for damp- ing the mechanical noise. Typical cut-off frequency of the low-pass system is 10 Hz. For additional damping of mechanical distortions (vibrations of pumping lines etc.) the scan head unit is fixed in an incised copper tube. The tube acts like a tuning fork, with a sharp resonance and damping of all other frequencies. After assembling the instrument, first measurements were carried out at room tempera- ture and at ambient pressure to calibrate the scanning unit. First, the calibration for large scan areas are performed on a commercially available calibration device (’NanoGrid’, Schae- fer Technologie GmbH, Langen). The grid is a 2 mm × 2 mm sized polymer chip with crossed III. CALIBRATION AT ROOM TEMPERATURE 9