Alan L. Duncan

Imaging Fourier transform spectrometry with a Fizeau interferometer

Fourier Transform Spectrometers usually consist of a single telescope with the beam split into two paths prior to the focal plane. The beams form a Michelson interferometer with beam recombination occurring at the focal plane. The path length of one beam is varied in order to scan through the white light fringe packet while a series of images is collected. Fourier transforming each pixel of the image across the series results in a spectral data cube of the scene. We propose using a multiple telescope FIzeau Imager for collecting Fourier Transform Spectrometer data. The path lengths through one telescope are varied while a series of images is collected. The processing is similar to the standard IFTS with some modification due to the necessity of image restoration. We present preliminary results from a laboratory multiple telescope FIzeau Imaging system.

Closed-loop wave-front correction using phase diversity

Closed loop wave front correction of low order Zernike polynomials has been demonstrated using a phase diversity wavefront sensor. The Lockheed-Martin Advanced Technology Center phase diversity brassboard was used to demonstrate low bandwidth correction of aberrations consisting of the Zernike polynomials describing focus, coma and spherical. The method of Lofdahl-Scharmer is used to estimate and correct fixed aberrations in an optical system. The General Regression Neural Network method is used to estimate slowly varying aberrations in the same optical system. Closed loop experimental results from these tests are presented.

Experimental results from the Lockheed Phase Diversity Test Facility

Optical wavefront errors can be determined from focused-defocused image pairs. We have developed on optical test facility at Lockheed for the purpose of investigating real-time wavefront control and post- processing algorithms using Phase Diversity techniques. Experimental results indicate that it is possible to control and correct for at least 6 parameters in the wavefront, at a bandwidth of about 10 Hz.

Multiple-aperture telescope array with a high fill factor

Traditionally a telescope system consists of a large collecting element, usually called the primary, located at the entrance pupil and some smaller elements to relay or convey the light to an image plane. As telescope systems become larger and larger, in order to achieve higher resolution and collect more light, a point is reached where the size of the required elements exceeds the current state of the art in fabrication and support. For telescopes larger than this, the entrance pupil must either be divided into manageable segments, or the entrance pupil is divided into an array of separate telescopes. A multiple telescope array consists of afocal collector telescopes distributed in the entrance pupil, relay optics to bring the light to the center and control tilt and piston errors, and a focal combiner telescope to form the image. Sparse telescope arrays have been designed for various applications. This paper addresses the issues and design constraints leading to a multiple telescope array with a high fill factor.

Multiple instrument distributed aperture sensor (MIDAS) science payload concept

We describe the Multiple Instrument Distributed Aperture Sensor (MIDAS) concept, an innovative approach to future planetary science mission remote sensing that enables order of magnitude increased science return. MIDAS provides a large-aperture, wide-field, diffraction-limited telescope at a fraction of the cost, mass and volume of conventional space telescopes, by integrating advanced optical interferometry technologies. All telescope optical assemblies are integrated into MIDAS as the primary remote sensing science payload, thereby reducing the cost, resources, complexity, I&T and risks of a set of back-end science instruments (SIs) tailored to a specific mission. MIDAS interfaces to multiple science instruments, enabling sequential and concurrent functional modes, thereby expanding the potential planetary science return many fold. Passive imaging modes with MIDAS enable remote sensing at diffraction-limited resolution sequentially by each science instrument, or at lower resolution by multiple science instruments acting concurrently on the image, such as in different wavebands. Our MIDAS concept inherently provides nanometer-resolution hyperspectral passive imaging without the need for any moving parts in the science instruments. For planetary science missions, the MIDAS optical design provides high-resolution imaging for long dwell times at high altitudes, thereby enabling real-time, wide-area remote sensing of dynamic surface characteristics. In its active remote sensing modes, using an integrated solid-state laser source, MIDAS enables LIDAR, vibrometry, surface illumination, and various active or ablative spectroscopies. Our concept is scalable to apertures well over 10m, achieved by autonomous deployments or manned assembly in space. MIDAS is a proven candidate for future planetary science missions, enabled by our continued investments in focused MIDAS technology development areas. In this paper we present the opto-mechanical design for a 1.5m MIDAS point design, including its accommodation of back-end science instruments.

Phase diversity experiment to measure piston misalignment on the segmented primary mirror of the Keck II Telescope

We are developing a technique to measure segment misalignment of large telescopes based on wavefront estimation using phase-diverse images. We report the current results of an experiment to measure piston errors on the Keck II primary segmented mirror, through atmospheric turbulence, using phase-diverse phase retrieval. The segment piston errors are separated from the random turbulence by averaging phase estimates from many frames. Phase estimates from real data collected with segments intentionally moved in piston reproduce the observed speckle patterns well. However, average phase maps do not reveal the segment piston errors. Simulations show that the observed data were collected in a regime of turbulence where the current algorithm often fails, but would be expected to work very well when the adaptive optics system is operating. There is reason to believe that we can eventually make the algorithm work with these or similar data if apparent mismatches between the data and our current imaging model are removed.

Segmented Planar Imaging Detector for EO Reconnaissance

We propose an electro-optical (EO) imaging sensor concept that provides a low-mass, low-volume alternative to conventional system designs by using arrays of photonic integrated circuits. We will discuss design concepts and present basic experimental results.

Fast Phase Diversity Wavefront Sensing for Mirror Control

We show with simulation experiments that closed-loop phase- diversity can be used without numerical guard-bands for wavefront sensing of low-order wavefronts from extended objects using broad-band filters. This may allow real-time correction at high bandwidth for certain applications. We also present a proper maximum likelihood treatment of Shack- Hartmann data, which includes an imaging model to extract curvature information from the lenslet images. We demonstrate by simple simulations that this approach should allow higher-order wavefront information to be extracted than with traditional Shack-Hartmann wavefront sensing for a given number of lenslets.

EXCEDE technology development III: first vacuum tests

This paper is the third in the series on the technology development for the EXCEDE (EXoplanetary Circumstellar Environments and Disk Explorer) mission concept, which in 2011 was selected by NASAs Explorer program for technology development (Category III). EXCEDE is a 0.7m space telescope concept designed to achieve raw contrasts of 1e6 at an inner working angle of 1.2 l/D and 1e7 at 2 l/D and beyond. This will allow it to directly detect and spatially resolve low surface brightness circumstellar debris disks as well as image giant planets as close as in the habitable zones of their host stars. In addition to doing fundamental science on debris disks, EXCEDE will also serve as a technological and scientific precursor for any future exo-Earth imaging mission. EXCEDE uses a Starlight Suppression System (SSS) based on the PIAA coronagraph, enabling aggressive performance. Previously, we reported on the achievement of our first milestone (demonstration of EXCEDE IWA and contrast in monochromatic light) in air. In this presentation, we report on our continuing progress of developing the SSS for EXCEDE, and in particular (a) the reconfiguration of our system into a more flight-like layout, with an upstream deformable mirror and an inverse PIAA system, and (b) testing this system in a vacuum chamber, including IWA, contrast, and stability performance. Even though this technology development is primarily targeted towards EXCEDE, it is also germane to any exoplanet direct imaging space-based telescopes because of the many challenges common to different coronagraph architectures and mission requirements. This work was supported in part by the NASA Explorer program and Ames Research Center, University of Arizona, and Lockheed Martin SSC.

Multiple instrument distributed aperture sensor (MIDAS) evolved design concept

An innovative approach to future space telescopes that enables order of magnitude increased science return for astronomical, Earth-observing and planetary science missions is described. Our concept, called Multiple Instrument Distributed Aperture Sensor (MIDAS), provides a large-aperture, wide-field, diffraction-limited telescope at a fraction of the cost, mass and volume of conventional space telescopes. MIDAS integrates many optical interferometry advances as an evolution of over a decade of technology development in distributed aperture optical imaging systems. Nine collector telescopes are integrated into MIDAS as the primary remote sensing science payload, supporting a collection of six back-end science instruments tailored to a specific mission. By interfacing to multiple science instruments, enabling sequential and concurrent functional modes, we expand the potential science return of future space science missions many fold. Passive imaging modes with MIDAS enable remote sensing at diffraction-limited resolution sequentially by each science instrument, as well as in somewhat lower resolution by multiple science instruments acting concurrently on the image, such as in different wavebands. Our MIDAS concept inherently provides nanometer-resolution hyperspectral passive imaging without the need for any moving parts in the science instruments. For Earth-observing and planetary science missions, the MIDAS optical design provides high-resolution imaging at high altitudes for long dwell times, thereby enabling real-time, wide-area remote sensing of dynamic planetary surface characteristics. In its active remote sensing modes, using an integrated solid-state laser source, MIDAS enables surface illumination, active spectroscopy, LIDAR, vibrometery, and optical communications. Our concept is directly scalable to telescope synthetic apertures of 5m, limited by launch vehicle fairing diameter, and above 5m diameter achieved by means of autonomous deployments or manned assembly in space. MIDAS is a proven candidate for space flight missions, enabled by our continued investments in focused technology development areas.