Robert F. Smythe

Keck Interferometer Nuller Data Reduction and On-Sky Performance

We describe the Keck Interferometer nuller theory of operation, data reduction, and on-sky performance, particularly as it applies to the nuller exozodiacal dust key science program that was carried out between 2008 February and 2009 January. We review the nuller implementation, including the detailed phasor processing involved in implementing the null-peak mode used for science data and the sequencing used for science observing. We then describe the Level 1 reduction to convert the instrument telemetry streams to raw null leakages, and the Level 2 reduction to provide calibrated null leakages. The Level 1 reduction uses conservative, primarily linear processing, implemented consistently for science and calibrator stars. The Level 2 processing is more flexible, and uses diameters for the calibrator stars measured contemporaneously with the interferometer’s K-band cophasing system in order to provide the requisite accuracy. Using the key science data set of 462 total scans, we assess the instrument performance for sensitivity and systematic error. At 2.0 Jy we achieve a photometrically-limited null leakage uncertainty of 0.25% rms per 10 minutes of integration time in our broadband channel. From analysis of the Level 2 reductions, we estimate a systematic noise floor for bright stars of ~0.2% rms null leakage uncertainty per observing cluster in the broadband channel. A similar analysis is performed for the narrowband channels. We also provide additional information needed for science reduction, including details on the instrument beam pattern and the basic astrophysical response of the system, and references to the data reduction and modeling tools.

The Keck Interferometer

The Keck Interferometer (KI) combined the two 10 m W. M. Keck Observatory telescopes on Mauna Kea, Hawaii, as a long-baseline near- and mid-infrared interferometer. Funded by NASA, it operated from 2001 until 2012. KI used adaptive optics on the two Keck telescopes to correct the individual wavefronts, as well as active fringe tracking in all modes for path-length control, including the implementation of cophasing to provide long coherent integration times. KI implemented high sensitivity fringe-visibility measurements at H (1.6 μm), K (2.2 μm), and L (3.8 μm) bands, and nulling measurements at N band (10 μm), which were used to address a broad range of science topics. Supporting these capabilities was an extensive interferometer infrastructure and unique instrumentation, including some additional functionality added as part of the NSF-funded ASTRA program. This paper provides an overview of the instrument architecture and some of the key design and implementation decisions, as well as a description of all of the key elements and their configuration at the end of the project. The objective is to provide a view of KI as an integrated system, and to provide adequate technical detail to assess the implementation. Included is a discussion of the operational aspects of the system, as well as of the achieved system performance. Finally, details on V^2 calibration in the presence of detector nonlinearities as applied in the data pipeline are provided.

The Keck Interferometer Nuller: System Architecture and Laboratory Performance

The first high-dynamic-range interferometric mode planned to come on line at the Keck Observatory is mid-infrared nulling. This observational mode, which is based on the cancellation of the on-axis starlight arriving at the twin Keck telescopes, will be used to examine nearby stellar systems for the presence of circumstellar exozodiacal emission. This paper describes the system level layout of the Keck Interferometer Nuller (KIN), as well as the final performance levels demonstrated in the laboratory integration and test phase at the Jet Propulsion Laboratory prior to shipment of the nuller hardware to the Keck Observatory in mid-June 2004. On-sky testing and observation with the mid-infrared nuller are slated to begin in August 2004.

The Keck Interferometer Nuller (KIN): configuration, measurement approach, and first results

The Keck Interferometer Nuller (KIN) will be used to examine nearby stellar systems for the presence of circumstellar exozodiacal emission. A successful pre-ship review was held for the KIN in June 2004, after which the KIN was shipped to the Keck Observatory. The integration of the KINs many sub-systems on the summit of Mauna Kea, and initial on-sky testing of the system, has occupied the better part of the past year. This paper describes the KIN system-level configuration, from both the hardware and control points of view, as well as the current state of integration of the system and the measurement approach to be used. During the most recent on-sky engineering runs in May and July 2005, all of the sub-systems necessary to measure a narrowband null were installed and operational, and the full nulling measurement cycle was carried out on a star for the first time.

An introduction to the Guide-2 telescope testbed for the SIM Planet Quest Light mission

The Space Interferometry Mission Light (SIM-Lite) is a new mission concept to perform a micro-arcsecond narrow-angle astrometry to search approximately 50 nearby stars for Earth-like planets, and to perform a global astrometry with an accuracy of six micro-arcsecond position and parallax measurements. The SIM-Lite consists of two Michelson interferometers and one telescope. The main six-meter baseline science interferometer observes a target star and a set of reference stars. The four-meter baseline interferometer (guide-1) monitors the attitude of the instrument in the direction of a target star. A Guide-2 telescope (G2T) tracks a bright star to monitor the attitude of the instrument in the other two orthogonal directions. To demonstrate the concept of the G2T, we have developed a testbed using brassboard optics built for the SIM project. The G2T testbed consists of a 35 cm siderostat, a beam compressor, and a fast steering mirror (FSM) in closed loop with a CCD based pointing sensor. A heterodyne laser angle metrology system is used to monitor angular positions of the FSM with required accuracy of 20 micro-arcsecond during SIM-Lite narrow-angle observation time. We present the concept of the testbed architecture and preliminary test results of the angular metrology (aMet) system.

Final Laboratory Integration and Test of the Keck Interferometer Nuller

Mid-infrared (8-13μm) nulling is a key observing mode planned for the NASA-funded Keck Interferometer at the Keck Observatory on the summit of Mauna Kea in Hawaii. By destructively interfering and thereby canceling the on-axis light from nearby stars, this observing mode will enable the characterization of the faint emission from exo-zodiacal dust surrounding these stellar systems. We report here the null leakage error budget and pre-ship results obtained in the laboratory after integration of the nulling beam combiner with its mid-infrared camera and key components of the Keck Interferometer. The mid-infrared nuller utilizes a dual-polarization, modified Mach-Zehnder (MMZ) beam combiner in conjunction with an atmospheric dispersion corrector to achieve broadband achromatic nulling.

Far-infrared interferometry technology development: a progress report

We report on the progress in developing cryogenic delay lines and integrated optics components. These are some of the critical components needed to enable far-IR direct-detection interferometers. To achieve background-limited performance in the 40 to 400 μm region, th einterferometer optics and delay lines must be cooeld to near liquid Helium temperatures. Our cryogenic delay line designs incorporate a number of novel features and has been operated at liquid nitrogen temperatures. Our integrated optics effort has focued on producing single-mode spatial filters and beam combiners.

Efficient thermoelectric cooling of concentrated heat loads

An efficiency improvement of 87% is demonstrated in cooling of concentrated heat loads when using thermoelectric coolers (TECs) constructed with thermally conductive printed circuit boards (TCPCBs) as compared to traditional ceramic-based TECs. Laser diodes and infrared detectors must be actively cooled but are smaller than typical TECs. As a result, heat spreading must occur between the optical component and the semiconductor pellets near the edge of the TEC. Typically, TECs based on aluminum nitride circuit boards are chosen and in some cases an AlN plate is added between the optical component and the TEC. To address this, TECs have been developed that replace the ceramic circuit boards with laminated TCPCBs containing a thick copper backing. The copper backing improves heat spreading within the TEC. A study was conducted to quantify differences in coefficient of performance (COP, heat pumped divided by electrical power consumed) when cooling concentrated heat loads. A heat source 3 mm wide was cooled by TECs ~12 mm wide, comparing ceramic-based and TCPCB-based TECs of otherwise identical design. With a fixed hot side temperature and heat load, each TEC was powered to achieve a desired temperature at the heat source. Ceramic-based and TCPCB-based TECs exhibited COPs of 0.235 and 0.440 respectively, an 87% improvement. Further improvements are achievable: adding a thick copper plate between the heat source and the TEC resulted in a COP of ~0.59 for both TEC types.

Progress on SIM-Lite brassboard interferometer integration and test

Main brassboard Michelson interferometer components have been recently developed for the future flight phase implementations of SIM Lite mission. These brassboard components include two fine steering mirrors, pathlength modulation and cyclic averaging optics and astrometric beam combiner assembly. Field-independent performance tests will be performed in a vacuum chamber using two siderostats in retro-reflecting positions and a white light stimulus. The brightness and color dependence of the angle and fringe tracking performance will be measured. The performance of filtering algorithms will be tested in a simulated spacecraft attitude control system perturbation. To demonstrate capability of a dim star observation, the angle and fringe tracking CCD sensors are cooled to -110 C using a cold diode heat pipe system. The new feed-forward control (angle and path-length) algorithms for the dim star observation will be tested as well. In this paper, we will report the recent progress toward the integration and performance tests of the brassboard interferometer.

Terrestrial Planet Finder cryogenic delay line development

Delay lines provide the pathlength compensation that makes the measurement of interference fringes possible. When used for nulling interferometry, the delay line must control pathlengths so that the null is stable and controlled throughout the measurement. We report on a low noise, low disturbance, high bandwidth optical delay line capable of meeting the TPF interferometer optical path length control requirements at cryogenic temperatures.