Coronagraph
The instrument uses Phase Mask coronagraphs. They reject the light of a central source by introducing phase shifts using a quadrant-design plate at the instrument input focal plane. These shifts cause the light from the source to interfere destructively at the detector array. Unlike conventional occulting Lyot coronagraphs, phase plates allow measurements to be obtained very close to the central object. Further from the central object, they provide performance similar to that of a conventional occulting coronagraph.
The 4 quadrant phase mask consists in dividing the Airy disk (image of a point source) in the center of the field into 4 domains, and of applying a phase difference of p to two of them, so that the image is eliminated by destructive interference. Simulations show that this concept of division in 4 quadrants is more effective and less sensitive to the disturbances of the atmosphere than the previous concepts of coronograph with modification of phase. It is under development and test at LESIA. Several phase masks were manufactured like those presented on Figure 1 and were tested in laboratory to evaluate their quality of realization and to characterize their performance.
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