The evolved Laser Interferometry Space Antenna (eLISA) is the implementation of the original Laser Interferometry Space Antenna (LISA) concept that will be proposed for the European Space Agency’s (ESA) L.3 Cosmic Visions opportunity. The eLISA observatory uses lasers to range between pairs of freely falling test masses in adjacent, widely separated spacecraft. The measurement is made continuously, requiring simultaneous transmission and reception of a 1064-nm laser beam through an optical telescope.
An on-axis telescope design for space-based laser interferometry requires the reflected laser light (1064 nm) from the secondary mirror to the detector be suppressed by approximately ten orders of magnitude. A well-designed mask in the center of the secondary mirror can suppress the direct (Narcissus) reflection as well as reduce the formation of a bright spot (Poisson spot) on axis due to diffraction. The design of a generic circular mask and binary petal mask as a realization of a hyper-Gaussian function fails to suppress the laser light within the geometry of the problem (40-mm-diameter mirror and 2-mm radius of obscuration). This work utilizes a unique optimization technique to design a coronagraphic, partially transparent, circular mask that meets the design requirement.
The mask shape is intended for integration in the surface of the secondary mirror of the telescope. A unique optimization algorithm was used to design the shape of the mask as a circular, partially transparent mask where the “offset” to the opacity could be varied depending on the sensitivity and tolerances of the mirror. This design could be integrated into a curved or flat mirror. There are no sharp petal tips associated with this design, making fabrication easier.