We propose to vary the shape of the sub-apertures both in cases of aperture masking and for segmented space telescopes. Most sub-apertures assume the shape of circles, rectangles and hexagons, but it is possible to use other shapes such as ellipses. Compared to polygons, ellipses reduce diffraction spokes in the PSF. They can have different axis ratios, and more importantly be placed at different rotation angles. This allows their easier identification in the focal plane for tip-tilt measurement. In the pupil auto-correlation (uv) plane it also allows the unique separation of sub-aperture pairs for piston measurements. Optimisation of separation angles lead to rough values of 67, 84, 113 and 96 degrees, for a range of ellipticity values.
|