Dynamic MTF measurement

Abstract

MTF (Modulation Transfer Frequency) of a detector is a key parameter for imagers. When image is not moving on the detector, MTF can be measured by some methods (knife edge, slanted slit,…). But with LEO satellites, image is moving on the surface of the detector, and MTF has to be measured in the same way: that is what we call "dynamic MTF". CNES (French Space Agency) has built a specific bench in order to measure dynamic MTF of detectors (CCD and CMOS), especially with component working in TDI (Time delay and integration) mode. The method is based on a moving edge, synchronized with the movement of charges inside the TDI detector. The moving part is a rotating cube, allowing a very stable movement of the image on the surface of the detector The main difficulties were: -stability of the rotating speed -synchronization between cube speed and charge transfer inside the detectors -synchronization between cube position and data acquisition. Different methods have been tested for the displacement of the knife edge: -geometrical displacement -electrical shift of the charge transfer clocks. Static MTF has been performed before dynamic measurements, in order to fix a reference measurement, Then dynamic MTF bench has been set up. The results, for a TDI CCD show a very good precision. So this bench is validated, and the dynamic MTF value of the TDI CCD is confirmed.

Dynamic MTF estimate of the optical imager onboard Alsat-1B satellite

“…A wide range of works have been proposed in the literature for on orbit MTF assessment [2][3][4]. However, few of works have been devoted to Dynamic MTF measurement and estimation on ground prior to launch [5], [6].…”