Dun Liu
Chinese Academy of Sciences, China
Title: Effect of the number of primary lens level on the MTF of diffractive imaging system
Biography
Biography: Dun Liu
Abstract
For space telescopes with large apertures and light-weight, phase-type Fresnel lenses has been proposed to replace the primary mirror. However, the diffraction efficiency of 2-level phase-type Fresnel lenses fabricated by binary optics technology can only achieve 40.5% and 81% for 4-level. The non design orders diffractive light may affect the modulation transfer function (MTF) of diffractive imaging system. In this paper, the wave propagation method was used to simulate the propagation of diffractive light. By coherent superposition of finite diffractive waves, the point spread function (PSF) was calculated at several signal wavelength which evenly covers the spectral range of system. The Fourier transform of the PSF was MTF. The MTFs of an 80 mm diffractive imaging system were analyzed when the number of Fresnel primary lens' level was 2, 4 and 8. The MTF decreased at low frequency with 2-level Fresnel primary lens and the biggest decrease was 6.6%. The deviation from the design value is less than 0.5% when the level is 4 and 8. The results show that the effect of diffractive stray light on the MTF of the system decreases with the increase of the number of level. During the analysis, we found that only the incident light illuminating the primary lens’s central area can directly attach the image plane by non design diffractive orders. So, a hybrid-level Fresnel lens was put forward to reduce the effect of diffractive stray light. The MTF increased apparently after optimized and was close to the design value.