Optical design and intensity interferometry simulations in support of the Kilometer Space Telescope

Very large telescopes will be needed for the future of space science, space laser-com, and other interstellar or interplanetary applications. However, as telescopes' diameters increase, their weight and cost increase dramatically. An approximation for ground-based observatories is that their cost increases to the power of 2.7 of its diameter [SAO, Special Report #385 (1979), p. 9]. Large space-based telescopes become limited by the rocket size and power. Multiple telescopes are also needed for very long baseline interferometry (VLBI), which further increases cost. A solution to overcome these issues is the use of inflatable telescopes. A thin mirror material clearly has much lower mass; however, controlling its surface error or wavefront can be a challenge. Intensity interferometry is an imaging method that has a much looser sensitivity to wavefront error (WFE), and thus is an ideal match for very large inflatable telescopes. A spherical inflatable mirror is the most practical; however, it suffers from spherical aberration. This paper presents new optical designs and simulations for intensity interferometry in support of large inflatable spherical telescopes. The optical system design includes a novel five-mirror off-axis, free-form, spherical aberration corrector. The system design shown is a 10 m diameter f/1 spherical primary mirror with 1.2 arcmin (0.02°) field of view (FOV). MATLAB simulations of intensity interferometry combine signal with noise and WFE. Visibility-based VLBI image simulations are shown based on various telescope arrays. Simulations show that with a 1 GHz detector, a 1 cm RMS WFE is tolerable. So the challenge of the optical design is then more about gathering and concentrating the light down to a reasonable size detector. Further simulations of signal combined with noise indicate that the signal rate must be about 100 times higher than the noise rate for an adequate intensity interferometry measurement. Visibility-based image simulations reveal that many telescopes (5-20 per axis) are needed in a 2D array out to the first visibility minimum to adequately resolve unknown features of a distant object.