A Ten-Meter Ground-Station Telescope for Deep-Space Optical Communications: A Preliminary Design

This article describes a telescope design for a 10-m optical ground station for deep-space communications. The design for a direct-detection optical communications telescope difiers dramatically from a telescope for imaging applications. In general, the requirements for optical manufacturing and tracking performance are much less stringent for direct detection of optical signals. The technical challenge is providing a design that will operate in the daytime/nighttime conditions required for a Deep Space Network tracking application. The design presented addresses these requirements. The design will provide higher performance at lower cost than existing designs. I. Introduction The primary difierences between a deep-space optical communications telescope and an imaging telescope are the wave-front error and the telescope fleld-of-view (FOV) requirements. To provide the highest resolution, an imaging telescope requires that all the energy received from the source be focused into a near-difiraction-limited image. This requires that the wave front across the aperture be a small fraction (<1/10th) of a wavelength. The wave-front error speciflcation requires nanometer-level manufacturing tolerances of the optical surfaces and positioning of the components. Small changes in temperature result in thermal-expansion-induced distortion of the optical surfaces and require that the mirrors be manufactured from expensive low-thermal-expansion glass. A telescope intended for direct detection of optical signals has a dramatically difierent wave-front error requirement. The energy at the detector does not need to be difiraction limited. It is assumed that the signal from the spacecraft appears as a point source that is blurred into a spot determined by the atmosphere. The only requirement is energy be focused into an angular spot comparable to the atmospheric blur. The surface speciflcations become geometrical. Instead of specifying a wave-front error in units of length, the surface error is expressed as a slope error in radians. The required surface flgure is easier to achieve and less expensive than for a difiraction-limited system.