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Current and future space-exploration endeavors will require new capabilities for large data transfer between Earth and other planets in the Solar system. Data communication with Earth from other planets will be completed through DSN (Deep Space Network) arrays on Earth and satellites around Earth. In an effort to develop advanced Space communication capabilities for large data transfer, NASA John H. Glenn Research Center at Lewis Field (GRC) is also investing a revolutionary concept, named iROC (integrated Radio and Optical Communication), featuring a space-communication terminal which tightly integrates a compact optical transmitter with a radio communication system. A particular design named TeleTenna (Telescope within (RF) Antenna) for future iROC flight-demonstration is being developed at GRC, in which a laser-transmission telescope is placed at the center of an RF antenna. The TeleTenna system capabilities to be demonstrated should include advanced pointing techniques for laser-transmission without a beacon over vast Space distances up to at least 2.0 AU (Astronomical Unit). The pointing-precision imposed on this TeleTenna design for beaconless optical communication should be achievable with an interferometric Star Tracker (iST) for celestial pointing calculation and the metrology for tracking the outgoing laser-beam.
The outer-diameter of the Primary-Mirror (PM) of the telescope (either of Cassegrain or Ritchey-Chrétien type) in the TeleTenna concept for data transmission from Mars to Earth is 0.25 meters, and the Secondary-Mirror (SM) outer-diameter is 0.025 meters. The laser-transmission tertiary optics behind the PM include the laser-fiber port, collimator lens, focus lens, quarter-wave plate, and a beam-splitter in that order; all aligned with the telescope axis.
The test-bed that the first author and GRC team setup back in 2017 for some preliminary studies on beaconless-pointing and optics alignment metrology for the TeleTenna concept, and some experimental results will be presented in this paper. The investigated metrology includes an optics alignment sensing metrology to image a beam reflected from a fiducial on the secondary mirror of the surrogate telescope onto a pixelated sensor (PixSen) behind the telescope. Additionally, the metrology includes sampling a portion of the laser beam and redirecting it onto the iST image plane. The objectives of this procedure are to determine angular change of a laser beam as it comes out of the surrogate telescope.
Among other findings, the work presented here shows that the alignment measurements performed at the edge of the Fine Steering Mirror (FSM) articulation range lead to nonlinearity in the relationship between the out-going beam direction registered on the iST and the fiducial reflected beam direction on an alignment sensor placed behind the telescope. For this reason, the adjustment of FSM angular position can realign only one of the beams with its respective camera but not both, and therefore an additional metrology instrument is required for high pointing precision. In the presented proof-of-concept metrology, this additional metrology component could be the piezo-controller of the FSM and/or an autocollimator that gives with accuracy the position of the FSM. These findings are relevant to the current development and design of the iROC system at GRC.
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