The primary mirror of very large submillimeter-wave telescopes will necessarily be segmented into many separate mirror panels. These panels must be continuously co-phased to keep the telescope wavefront error less than a small fraction of a wavelength, to ten microns RMS (root mean square) or less. This performance must be maintained continuously across the full aperture of the telescope, in all pointing conditions, and in a variable thermal environment.
A wavefront compensation segmented mirror sensing and control system, consisting of optical edge sensors, Wavefront Compensation Estimator/Controller Soft ware, and segment position actuators is proposed. Optical edge sensors are placed two per each segment-to-segment edge to continuously measure changes in segment state. Segment position actuators (three per segment) are used to move the panels. A computer control system uses the edge sensor measurements to estimate the state of all of the segments and to predict the wavefront error; segment actuator commands are computed that minimize the wavefront error.
Translational or rotational motions of one segment relative to the other cause lateral displacement of the light beam, which is measured by the imaging sensor. For high accuracy, the collimator uses a shaped mask, such as one or more slits, so that the light beam forms a pattern on the sensor that permits sensing accuracy of better than 0.1 micron in two axes: in the z or local surface normal direction, and in the y direction parallel to the mirror surface and perpendicular to the beam direction.
Using a coaligned pair of sensors, with the location of the detector and collimated light source interchanged, four degrees of freedom can be sensed: transverse x and y displacements, as well as two bending angles (pitch and yaw). In this approach, each optical edge sensor head has a collimator and an imager, placing one sensor head on each side of a segment gap, with two parallel light beams crossing the gap.
Two sets of optical edge sensors are used per segment-to-segment edge, separated by a finite distance along the segment edge, for four optical heads, each with an imager and a collimator. By orienting the beam direction of one edge sensor pair to be +45º away from the segment edge direction, and the other sensor pair to be oriented –45º away from the segment edge direction, all six degrees of freedom of relative motion between the segments can be measured with some redundancy.
The software resides in a computer that receives each of the optical edge sensor signals, as well as telescope pointing commands. It feeds back the edge sensor signals to keep the primary mirror figure within specification. It uses a feed-forward control to compensate for global effects such as decollimation of the primary and secondary mirrors due to gravity sag as the telescope pointing changes to track science objects.
Three segment position actuators will be provided per segment to enable controlled motions in the piston, tip, and tilt degrees of freedom. These actuators are driven by the software, providing the optical changes needed to keep the telescope phased.
A novel aspect of this design is the angled optical edge sensor configuration. By angling the light beam of each edge sensor pair at + and –45º, a full four degrees of freedom can be sensed at each segment edge by each sensor pair. This configuration results in full observability of the segment optical state, and is crucial in achieving the needed performance.
The software incorporates a structural/ optical model of the telescope in a least-squares or Kalman filter-based estimator/ controller, which processes the optical edge sensor signals in a low-bandwidth control loop. The estimator produces an estimate of the optical state of the mirror, and predicts the resulting wavefront error, balancing current against previous measurements in a least-squares optimization. The controller calculates the segment actuator commands that will minimize not the sensor signals, but the predicted wavefront error. This formulation allows the controller to compensate for the optical effects of motions (such as lateral sag of the segments) that are not directly actuated. The result is far better performance than could be achieved using a conventional sensor-nulling approach.
This work was done by David C. Redding, John Z. Lou, Andrew Kissil, Charles M. Bradford, David Woody, and Stephen Padin of Caltech for NASA’s Jet Propulsion Laboratory.
The software used in this innovation is available for commercial licensing. Please contact Daniel Broderick of the California Institute of Technology at
This Brief includes a Technical Support Package (TSP).
Wavefront Compensation Segmented Mirror Sensing and Control
(reference NPO-47964) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package for Wavefront Compensation Segmented Mirror Sensing and Control, prepared under the sponsorship of NASA. It focuses on advancements in telescope technology, particularly for the CCAT (Cerro Chajnantor Atacama Telescope) project, which features a 25-meter segmented mirror design. The primary goal is to enhance wavefront sensing (WFS) and control systems to improve the performance of large telescopes.
The CCAT telescope will utilize submillimeter-wave interferometry for wavefront sensing, allowing for direct measurement of optical errors using bright celestial objects as reference sources. This method is advantageous because it operates in the same mode as the telescope's scientific observations, although it presents challenges due to the limited number of bright objects in the submillimeter sky. Mars is identified as a suitable target for measurements, while other planets may introduce complications due to their resolved nature.
The document discusses the use of Zernike phase-contrast techniques to address phase aberrations in the optical system. It describes the optical layout, where light passes through a phase aberration before being focused to form a Point-Spread Function (PSF) on the image plane. The system's design aims to measure and correct wavefront errors effectively, ensuring high-quality imaging.
Additionally, the document outlines the technical specifications and operational principles of the wavefront compensation system, emphasizing the importance of accurate wavefront measurements for maintaining the telescope's performance. The integration of advanced optical elements and sensors is crucial for achieving the desired precision in wavefront control.
The Technical Support Package also serves as a resource for broader technological, scientific, and commercial applications stemming from NASA's research. It highlights the collaborative efforts within the Innovative Technology Assets Management at JPL (Jet Propulsion Laboratory) to promote the dissemination of knowledge and technology developed through aerospace-related projects.
In summary, this document encapsulates the innovative approaches being taken to enhance the capabilities of large segmented telescopes, particularly through advanced wavefront sensing and control techniques, which are essential for the future of astronomical observations and research.
