Optical testing of a membrane diffractive optic for space-based solar imaging

摘要

We describe imaging capabilities of a 0.2 m membrane diffractive primary (DOE) used as a key element in FalconSat-7, a space-based solar telescope. Its mission is to take an image of the Sun at the H-alpha wavelength (656nm) over a narrow bandwidth while in orbit. In this case the DOE is a photon sieve which consists of billions of tiny holes, with the focusing ability dependent on an underlying Fresnel zone geometry. Uniform radial expansion/contraction of the substrate due to temperature or relative humidity change will result in a shift in focal length without introducing errors in phase of the transmitted wavefront and without a decrease in efficiency. We will also show that while ideally the DOE surface should be held flat to within 5.25 microns, an opto-mechanical analysis showed that local deformations up to 32 microns are possible without significantly degrading the image quality.