Investigating the impact of ghost images on the accuracy of modulation transfer function measurement using knife-edge method

In this study, we investigated the impact of ghost images on the modulation transfer function (MTF) of a Korsch-type telescope using nonsequential ray-tracing simulations and the experimental measurements of the knife-edge method with a collimator and light source targets. Our findings showed that ghost images introduce a directional bias into the edge spread function depending on the field position, which affects the line spread function and MTF. Furthermore, our measurement results demonstrated that ghost images can significantly affect the MTF on the edge field of the green channel. The ghost-to-signal ratio in the multispectral (MS) green channel was approximately 2.5%, which is approximately 0.25% higher than that in the panchromatic channel. To estimate the impact of ghost images in the MS green channel, we performed a parametric analysis using a nonsequential ray-tracing simulation, exploring potential strategies, such as adjusting the window thickness, the distance between the detector and the window, the transmittance of the window surface, and the reflectance of the detector surface. By comparing the positions and intensities of the ghost images obtained from the simulations with those measured experimentally, we identified the simulation input parameters that best reproduced the measured results. Our study provides valuable insight into the importance of managing ghost images when designing and operating Korsch-type telescopes to achieve the optimal image quality.