Field-of-view Limitations of Phased Telescope Arrays

The optical performance of phased telescope arrays used for imaging applications is degraded by various design, manufacturing, and operational errors. However, even if diffraction-limited performance is achieved on-axis, there are both fundamental and practical limitations to off-axis performance which will severely limit the useful field-of-view of such systems. Two separate and distinct mechanisms that degrade off-axis performance are pupil mapping errors encountered in the beam combining process and the field curvature of the individual telescopes making up the array. The pupil mapping errors are operational errors which have been discussed in some detail elsewhere. Field curvature of traditional telescopes is a rather benign aberration which doesn't really degrade the image of a point source but rather displaces it axially by an amount proportional to the square of the field angle. However, in phased telescope arrays, the field curvature of the individual telescopes result in relative phase errors and relative tilt errors between the various subapertures which severely degrades the image quality of off-axis object points in addition to producing a traditional field curvature for the array. This paper deals specifically with the off-axis image degradation due to the field curvature of the independent telescopes and how it varies with the subaperture configuration and the telescope design parameters. Two very interesting results of these parametric studies are that the off-axis image quality of phased telescope arrays is a very strong function of subaperture size requiring the field curvature of the individual telescopes to be very well corrected for subaperture diameters greater than 1 meter and that the sensitivity of image quality to residual field curvature is highly dependent upon the subaperture configuration. Circular configurations of subapertures (located at constant radial positions) are far less sensitive to field curvature than other configurations. Quantative results of parametric studies will be presented.