Spatial coherence mapping of structured sources: a flexible instrument for solar studies.

We conceptually describe and design, to first order, an instrument to locally map the spatial coherence of extended and structured sources, such as fiber bundles or the Sun, considered as a mosaic of individual solar cells, which is our main motivation. Our solar coherence instrument (SCI) is an instrument for an afocal solar space telescope; the light from its exit pupil dynamically selects one individual solar cell at a time and performs a series of Young-like experiments with different baselines in order to measure the spectral degree of coherence and therefore the effective correlation length that can be assigned for that solar cell. SCI needs flexibility in terms of selective imaging and Young experiments, which is provided by two digital micromirror devices (DMDs), a technology currently under space qualification. SCI is a compact instrument based on retroreflections, and it generates all data required to image the source, to select the cells, and to implement sequentially a series of Young apertures on a re-imaged pupil. It was designed using the (already launched) Hinode solar optical telescope as a baseline, and the first estimation of the SNR, using commercial DMDs and array sensors, while measuring the modulation of Young interference fringes validates our first-order design.

Numerical simulations of spectral shifts in the far-field spectrum of light due to source correlations.

We discuss how to simulate numerically the far-field propagation of the spectrum of light by propagating the cross-spectral density for a planar light source with a given coherence model. To test our approach, we performed simulations for two source models: a Gaussian Schell-model source and a nonuniformly Gaussian-correlated source. We show that our algorithm correctly reproduces the theoretical solutions by Emil Wolf for planar Gaussian Schell-model source, namely, the spectral shifts of spectral lines due to source correlations. Our approach can be used for two-dimensional source models in which the spatial coherence components are frequency independent. It can also be used for nonhomogeneous extended sources of partial coherent light.