Optics Bench Research Articles

AIROPA IV: Validating point spread function reconstruction on various science cases

We present an analysis of six independent on-sky datasets taken with the Keck-II/NIRC2 instrument. Using the off-axis point spread function (PSF) reconstruction software AIROPA, we extract stellar astrometry, photometry, and other fitting metrics to characterize the performance of this package. We test the effectiveness of AIROPA to reconstruct the PSF across the field of view in varying atmospheric conditions, number and location of PSF reference stars, stellar crowding, and telescope position angle (PA). We compare the astrometric precision and fitting residuals between a static PSF model and a spatially varying PSF model that incorporates instrumental aberrations and atmospheric turbulence during exposures. Most of the fitting residuals we measure show little to no improvement in the variable-PSF mode over the single-PSF mode. For one of the data sets, we find photometric performance is significantly improved (by ∼10 × ) by measuring the trend seen in photometry as a function of off-axis location. For nearly all other metrics we find comparable astrometric and photometric precision across both PSF modes, with a ∼13 % smaller astrometric uncertainty in variable-PSF mode in the best case. We largely confirm that the spatially variable PSF does not significantly improve the astrometric and other PSF fitting residuals over the static PSF for on-sky observations. We attribute this to unaccounted instrumental aberrations that are not characterized through afternoon adaptive optics (AO) bench calibrations.

Optics Bench on a Graphene Flake

Optics Bench on a Graphene Flake

Coronagraphic phase diversity: performance study and laboratory demonstration

The final performance of current and future instruments dedicated to exoplanet detection and characterization (such as SPHERE on the European Very Large Telescope, GPI on Gemini North, or future instruments on Extremely Large Telescopes) is limited by uncorrected quasi-static aberrations. These aberrations create long-lived speckles in the scientific image plane, which can easily be mistaken for planets. Common adaptive optics systems require dedicated components to perform wave-front analysis. The ultimate wave-front measurement performance is thus limited by the unavoidable differential aberrations between the wavefront sensor and the scientific camera. To reach the level of detectivity required by high-contrast imaging, these differential aberrations must be estimated and compensated for. In this paper, we characterize and experimentally validate a wave-front sensing method that relies on focal-plane data. Our method, called COFFEE (for COronagraphic Focal-plane wave-Front Estimation for Exoplanet detection), is based on a Bayesian approach, and it consists in an extension of phase diversity to high-contrast imaging. It estimates the differential aberrations using only two focal-plane coronagraphic images recorded from the scientific camera itself. In this paper, we first present a thorough characterization of COFFEE's performance by means of numerical simulations. This characterization is then compared with an experimental validation of COFFEE using an in-house adaptive optics bench and an apodized Roddier & Roddier phase mask coronagraph. An excellent match between experimental results and the theoretical study is found. Lastly, we present a preliminary validation of COFFEE's ability to compensate for the aberrations upstream of a coronagraph.

Design of a highly stable and uniform thermal test facility for MGRS development

We have designed combined passive and active thermal control system to achieve sub microkelvin temperature stability and uniformity over an optics bench size enclosure, which has an analogous structure to the LISA spacecraft. For the passive control, we have constructed a new thermal enclosure that has a multilayer structure with alternative conducting and insulating layers, which enables the temperature uniformity and ease the burden of the active control. The thermal enclosure becomes an important test facility for Modular Gravitational Reference Sensor (MGRS) development. For the active control, we have developed a model predictive control (MPC) algorithm, which will regulate temperature variations of the proof-mass (PM) down to sub-microkelvin over the LISA science band. The LISA mission requires extremely tight temperature control, which is as low as 30 μK/ over 0.1 mHz to 1 Hz. Both temporal stability and spatial uniformity in temperature must be achieved. Optical path length variations on optical bench must be kept below 40 pm/ over 0.1 mHz to 1 Hz. Temperature gradient across the proof mass housing also must be controlled to reduce differential thermal pressure. Thermal disturbances due to, for example, solar radiation and heat generation from electronics, are expected to be significant disturbance source to the LISA sensitivity requirements. The MGRS will alleviate the thermal requirement due to its wider gap between the proof-mass and the housing wall. However, a thermally stable and uniform environment is highly desirable to achieve more precise science measurement for future space science missions.

Refined laboratory simulations of dark-speckle coronagraphy

Dark-speckle coronagraphy with large telescopes is expected to image extrasolar planets. We present new results about the dark-speckle coronagraphic camera, obtained in the laboratory with a simulated atmosphere and an adaptive optics bench. In good seeing conditions, and with an accurate subtraction of a reference frame, we show that our instrument allows high-resolution, high-contrast imaging, more effectively than classical long exposure, for magnitude differences larger than 5 at visible wavelength. With the present capability of the adaptive optics system used, on a 1.5 m telescope pupil, we should expect to detect stellar companions 10 4 fainter than the primary star. Instrument concepts and processing techniques are discussed.

Anomalous electroconvection in weakly conducting liquids

According to the classical treatments given in standard textbooks, the presence of an electric field in an insulating fluid should favor natural convection. The effect of an electric field on thermal convection was reinvestigated with an experimental setup consisting essentially of a thin platinum wire stretched in the axis of a metal ring. Both electrodes were immersed in a cell containing the liquid under investigation. The wire was heated by Joule effect, and a dc voltage up to 5 kV was applied between the wire and the ring. The cell, fitted with optical windows, was placed in the beam of a striograph (Schlieren optics bench) by which the heat front rising above the wire was observed. The heat exchange between the wire and the liquid could also be monitored by estimating the steady temperature of the wire, through the measurement of its resistance. With these techniques, it was found that the electric field may actually impede thermal convection if the liquid contains a sufficient amount of ionizable impurities. The interpretation is based on the space charge which appears wherever a gradient of temperature—hence of conductivity—and an electric field simultaneously exist. A mathematical analysis, made possible by a few simplifying assumptions, shows that an electrostatic attractive force opposes the upward gravitational pull acting on the heated liquid, and satisfactorily predicts the stable position of the heat front above the wire. Other manifestations of this space charge are briefly described.