In situ measurement of chromatic dispersion in an optical hypertelescope: a laboratory demonstration.

Optical interferometry is a technique capable of achieving better spatial resolution than the world's largest monolithic telescopes at a fraction of the cost. Most interferometer architectures split the imaging bandwidth into a number of channels in order to prevent image degradation due to a large spectral bandwidth. An optical hypertelescope permits a much broader spectral bandwidth on a single channel than a conventional interferometer. However, a broader spectral bandwidth becomes more sensitive to differential chromatic dispersion, and this dispersion must be measured and corrected in order to maintain a high signal-to-noise ratio. A prototype dispersion measurement system is presented that is capable of measuring chromatic dispersion in an eight aperture hypertelescope. The optical design, calibration, data acquisition, and dispersion measurement process are described in detail. This system is capable of measuring differential dispersion to better than λ/100R M S and is scalable to a system with an order of magnitude more apertures.

Broadband chromatic dispersion in fiber-coupled optical interferometry.

Chromatic dispersion is a well-known technical challenge in optical interferometry, and the issue is exacerbated when using optical fibers for beam transport. The important sources of chromatic dispersion in a fiber-coupled optical interferometer are investigated using a Mach-Zehnder interferometer operating between 975-1650 nm, with particular attention paid to various dispersive effects in fibers. The compensation of chromatic dispersion is also investigated, and a compensation strategy using bulk glass and fiber stretching is described. A notional dispersion budget is presented for a fiber-coupled interferometer operating in the near infrared, showing that dispersion can be compensated to the λ/20 RMS level over a nearly 700 nm wide bandpass.

Temporally averaged speckle noise in wavefront sensors for beam projection in weak turbulence.

Adaptive optics (AO) compensation for imaging or coherent illumination of a remote object relies on accurate sensing of atmospheric aberrations. When a coherent beacon is projected onto the object to enable wavefront sensing, the reflected reference wave will exhibit random variation in phase and amplitude characteristics of laser speckle. In a Shack-Hartmann wavefront sensor (SHWFS) measurement, speckle effects cause fluctuations in the intensity of focal spots and errors in the position of their centroids relative to those expected from purely atmospheric phase aberrations. The resulting error in wavefront measurements negatively impacts the quality of atmospheric phase conjugation. This paper characterizes the effect of reflected laser speckle on the accuracy of SHWFS measurements for ground-to-space beam projection systems in weak turbulence conditions. We show via simulation that the speckle-induced error in centroiding depends on the ratio between beacon diameter and the diffraction-limited resolution of the lenslet and confirm these results with experimental data. We provide experimental validation that averaging of SHWFS lenslet spot intensities over speckle realizations converges to the incoherent intensity as expected. We further show that the effects of shot noise and speckle noise add in quadrature, simplifying noise analysis. Finally, we characterize the effect of temporal averaging under typical conditions of target motion and integration time. This work provides a straightforward set of relations that can help investigators more accurately estimate the required integration time for wavefront sensing in the presence of laser speckle.

Design and fabrication of prototype piezoelectric adjustable X-ray mirrors.

Lynx, a next generation X-ray observatory concept currently under study, requires lightweight, high spatial resolution X-ray mirrors. Here we detail the development and fabrication of one of the candidate technologies for Lynx, piezoelectric adjustable X-ray optics. These X-ray mirrors are thin glass shell mirrors with Cr/Ir X-ray reflective coatings on the mirror side and piezoelectric thin film actuators on the actuator side. Magnetron sputtering was used to deposit metal electrodes and metal-oxide piezoelectric layers. The piezoelectric (Pb0.995(Zr0.52Ti0.48)0.99Nb0.01O3) was divided into 112 independent piezoelectric actuators, with 100% yield achieved. We discuss the fabrication procedure, residual thermal stresses and tuning of the Cr/Ir coating stress for the purposes of stress balancing.

X-ray verification of an optically aligned off-plane grating module.

Off-plane x-ray reflection gratings are theoretically capable of achieving high resolution and high diffraction efficiencies over the soft x-ray bandpass, making them an ideal technology to implement on upcoming x-ray spectroscopy missions. To achieve high effective area, these gratings must be aligned into grating modules. X-ray testing was performed on an aligned grating module to assess the current optical alignment methods. Results indicate that the grating module achieved the desired alignment for an upcoming x-ray spectroscopy suborbital rocket payload with modest effective area and resolving power. These tests have also outlined a pathway towards achieving the stricter alignment tolerances of future x-ray spectrometer payloads, which require improvements in alignment metrology, grating fabrication, and testing techniques.