The Keck Interferometer

Abstract

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.

First Faint Dual-Field Off-Axis Observations in Optical Long Baseline Interferometry

“…The implementation of the DFPR instrument on the Keck Interferometer is an evolution of the Self-Phase-Referencing instrument described in Woillez et al (2012), and relies on many aspects of the infrastructure developed for NASA's Keck Interferometer project (Colavita et al 2013). Therefore, the details given in this section focus on the additions specific to the dual-field aspect and its application to faint object observations: the dual-field subsystem (Section 2.1), which separates the bright reference from the faint science object, stabilizing them on the fast fringe tracker and focal instrument, respectively; the internal optical path control (Section 2.2), which actively compensates the instrumental optical path difference (OPD) fluctuations for each of the bright and faint objects; and finally the phase-referencing architecture (Section 2.3), which is responsible for measuring the residual OPD fluctuations of instrumental and atmospheric origins of the bright reference object and applying an appropriate correction to the faint science object.…”

“…The dual star modules (DSMs; Figure 1) are at the interface between the Keck AO systems (Keck AO; Wizinowich et al 2000Wizinowich et al , 2006 and the interferometer coudé trains (Colavita et al 2013). Their purpose is to select two independent fields at each telescope within the Keck AO field of view and deliver them via the primary and secondary beam trains to the interferometric basement where OPD compensation and beam combination are performed.…”

“…These acquisition operations are fully automated with a combination of the AO acquisition widget (Le , and the interferometer sequencer (Colavita et al 2013). In addition, the sequencer is capable of permuting which of the Sci and Ref targets is sent into the primary and secondary beam trains.…”

“…The detection electronics for the infrared system is an exact replica of the 633 nm metrology systems except for the detector cards modified to operate at 1319 nm (more details on the detection chain can be found in Colavita et al 2013). It is capable of measuring optical path evolution with an accuracy of 2.5 nm to frequencies up to 4 kHz.…”

“…The ASTRA longitudinal metrology follows the same control pattern as the cloudé metrology, presented in Colavita et al (2013), where the piston measurements are high-pass filtered before correction by the FDLs. This implementation makes the system more tolerant to metrology breaks, but the internal OPD fluctuations on time scales longer than the filter time constant of τ = 2 s are not corrected.…”