John Z. Lou

The CCAT 25m diameter submillimeter-wave telescope

CCAT will be a 25 m diameter telescope operating in the 2 to 0.2 mm wavelength range. It will be located at an altitude of 5600 m on Cerro Chajnantor in Northern Chile. The telescope will be equipped with wide-field, multi-color cameras for surveys and multi-object spectrometers for spectroscopic follow up. Several innovations have been developed to meet the <0.5 arcsec pointing error and 10 μm surface error requirements while keeping within the modest budget appropriate for radio telescopes.

Optical state estimation using wavefront data

The use of wavefront measurements to deduce the state of multiple optics in a telescope beam train - their misalignments and figure errors - can be confused by the fact that there are multiple potential sources for the same measured error. This talk applies Kalman filtering techniques as a tool for separating true telescope errors from artifactual testing errors in the alignment and testing of NASAs James Webb Space Telescope, a large segmented-aperture cryogenic telescope to be launched after 2010.

JWST on-orbit multi-field wavefront control with a Kalman filter

An effective multi-field wavefront control (WFC) approach is demonstrated for the James Webb Space Telescope (JWST) on-orbit optical telescope element (OTE) fine-phasing using wavefront measurements at the NIRCam pupil, and the optical and computational implications of this approach are discussed. The integration of a Kalman Filter as an optical state estimator into the JWST wavefront control process to further improve the robustness of the fine-phasing JWST OTE alignment will also be discussed. Through a comparison of WFC performances between the JWST on-orbit and ground-test optical system configurations, the connection (and a possible disconnection) between WFC and optical system alignment under these circumstances are analyzed. Our MACOS-based [2] computer simulation results will be presented and discussed.

Wavefront Controls for a Large Submillimeter-Wave Observatory

The 25-m aperture Cornell Caltech Atacama Telescope (CCAT) will have a primary mirror that is divided into 162 individual segments, each of which is equipped with 3 positioning actuators. This paper presents a mathematical description of the telescope, its actuators and sensors, and uses it to derive control laws for figure maintenance. A Kalman Filter-based Optical State Estimator is used to continuously estimate the aberrations of the telescope; these are used in a state-feedback controller to maintain image quality. This approach provides the means to correct for the optical effects of errors that occur in un-actuated degrees of freedom, such as lateral translations of the segments. The control laws are exercised in Monte Carlo and simulation analysis, to bound the closed-loop performance of the telescope and to conduct control design trades.

Modeling a large submillimeter-wave observatory

The 25 meter aperture Cornell Caltech Atacama Telescope (CCAT) will provide an enormous increase in sensitivity in the submillimeter bands compared to existing observatories, provided it can establish and maintain excellent image quality. To accomplish this at a very low cost, it is necessary to conduct accurate engineering trades, including the most effective segment and wavefront sensing and control approach, to determine the best method for continuously maintaining wavefront quality in the operational environment. We describe an integrated structural/optical/controls model that provides accurate performance prediction. We also detail the analysis methods used to quantify critical design trades.

Phase retrieval methods for wavefront sensing

Phase retrieval is an image-based wavefront sensing process, used to recover phase information from defocused stellar images. Phase retrieval has proven to be useful for diagnosis of optical aberrations in space telescopes, calibration of adaptive optics systems, and is intended for use in aligning and phasing the James Webb Space Telescope. This paper describes a robust and accurate phase retrieval algorithm for wavefront sensing, which has been successfully demonstrated on a variety of testbeds and telescopes. Key features, such as image preprocessing, diversity adaptation, and prior phase nulling, are described and compared to other methods. Results demonstrate high accuracy and high dynamic range wavefront sensing.

Performance sensitivity studies on the PIAA implementation of the High Contrast Imaging Testbed

We have investigated the dependence of the High Contrast Imaging Testbed (HCIT) Phase Induced Amplitude Apodization (PIAA) coronagraph system performance on the rigid-body perturbations of various optics. The structural design of the optical system as well as the parameters of various optical elements used in the analysis are drawn from those of the PIAA/HCIT system that have been and will be implemented, and the simulation takes into account the surface errors of various optics. In this paper, we report our findings when the input light is a narrowband beam.

Telescope multi-field wavefront control with a Kalman filter

An effective multi-field wavefront control (WFC) approach is demonstrated for an actuated, segmented space telescope using wavefront measurements at the exit pupil, and the optical and computational implications of this approach are discussed. The integration of a Kalman Filter as an optical state estimator into the wavefront control process to further improve the robustness of the optical alignment of the telescope will also be discussed. Through a comparison of WFC performances between on-orbit and ground-test optical system configurations, the connection (and a possible disconnection) between WFC and optical system alignment under these circumstances are analyzed. Our MACOS-based [2] computer simulation results will be presented and discussed.

Design for CCAT, a 25 m diameter telescope operating from 200 GHz to 1.5 THz

CCAT will be a 25 m diameter Ritchey-Chretien telescope operating in the 0.2–1.5 mm wavelength range. It will be located at an altitude of 5600 m on Cerro Chajnantor in northern Chile, near the ALMA site. CCAT will have an f/0.4 primary, with an active surface to compensate gravitational and thermal deformations. The primary will be supported by a carbon fiber reinforced plastic (CFRP) space frame truss on an elevation over azimuth mount made of steel. Cameras and spectrometers with up to 1 deg field of view (FoV) will be located at the two f/6 Nasmyth foci. CCAT will be inside an enclosure to reduce wavefront and pointing errors due to wind forces and thermal deformation due to solar illumination. The key performance challenges for CCAT are a half wavefront error (HWFE) <10 μm rms and pointing error <0.2.

Model-Based Wavefront Control for CCAT

The 25-m aperture CCAT submillimeter-wave telescope will have a primary mirror that is divided into 162 individual segments, each of which is provided with 3 positioning actuators. CCAT will be equipped with innovative Imaging Displacement Sensors (IDS) - inexpensive optical edge sensors - capable of accurately measuring all segment relative motions. These measurements are used in a Kalman-filter-based Optical State Estimator to estimate wavefront errors, permitting use of a minimum-wavefront controller without direct wavefront measurement. This controller corrects the optical impact of errors in 6 degrees of freedom per segment, including lateral translations of the segments, using only the 3 actuated degrees of freedom per segment. The edge sensors do not measure the global motions of the Primary and Secondary Mirrors. These are controlled using a gravity-sag look-up table. Predicted performance is illustrated by simulated response to errors such as gravity sag.