Abstract
Diffractive telescope technology is an innovation solution in construction of large light-weight space telescope. However, the nondesign orders of diffractive optical elements (DOEs) may affect the imaging performance as stray light. To study the stray light characteristics of a diffractive telescope, a prototype was developed and its stray light analysis model was established. The stray light characteristics including ghost, point source transmittance, and veiling glare index (VGI) were analyzed. During the star imaging test of the prototype, the ghost images appeared around the star image as the exposure time of the charge-coupled device improving, consistent with the simulation results. The test result of VGI was 67.11%, slightly higher than the calculated value 57.88%. The study shows that the same order diffraction of the diffractive primary lens and correcting DOE is the main factor that causes ghost images. The stray light sources outside the field of view can illuminate the image plane through nondesign orders diffraction of the primary lens and contributes to more than 90% of the stray light flux on the image plane. In summary, it is expected that these works will provide some guidance for optimizing the imaging performance of diffractive telescopes.
Highlights
As aperture increases, the mass of space telescope increases dramatically and the large mirror with high accuracy surface figure is difficult to fabricate
It is known that the diffractive optical elements (DOEs) have multiorder diffraction, but only the +1 order is used in the diffractive telescopes and the other orders may propagate to the image plane as stray light
The ghost images are not focused at the image plane, but the focal point of these ghost images are near the image plane, so we be careful to deal with that ghost images
Summary
The mass of space telescope increases dramatically and the large mirror with high accuracy surface figure is difficult to fabricate. Xiaotong Li et al studied the ghost images caused by DOEs in the high-power laser system.[4] Their method is based on the paraxial analysis. Several works studied the effects of the diffraction efficiency on the modulation transfer function in hybrid optical system with weakly powered diffractive lens.[5,6,7] For diffractive telescope systems, the prototypes in “Fresnel diffractive array imager[8,9] and “membrane optic imager real-time exploitation” only verified the imaging capability of the diffractive structures.[10] But, the stray light performance of these prototypes was not considered. The optical properties and bidirectional scatter distribution function (BSDF) were given to each surface and the stray light analysis model was established. The test of ghost images and the VGI were carried out
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