Water vapor is the dominant source of randomly-changing atmospheric dispersion on timescales of seconds to minutes in the near- and mid-infrared. The dispersion changes are sufficient to limit the performance of the Keck Nuller unless steps are taken to measure and compensate for them. Here we present the first measurements of water vapor differential column fluctuations with the mid-infrared Keck Nuller and its near-infrared fringe tracker, taken in October 2005, and discuss theoretical and practical aspects of our dispersion feedforward implementation. The data show much larger fluctuations than were seen in median Mauna Kea conditions measured at radio wavelengths, and probably account for the generally poor performance of the Nuller during the observing run. The measurements in the two bands show strong correlations, indicating that the planned feedforward of the near-infrared value to stabilize the dispersion in the mid-infrared will substantially reduce the residual dispersion fluctuations seen by the Nuller.
The Keck Interferometer Nuller (KIN) is now largely in place at the Keck Observatory, and functionalities and
performance are increasing with time. The main goal of the KIN is to examine nearby stars for the presence of exozodiacal
emission, but other sources of circumstellar emission, such as disks around young stars, and hot exoplanets are
also potential targets. To observe with the KIN in nulling mode, knowledge of the intrinsic source spectrum is essential,
because of the wide variety of wavelengths involved in the various control loops - the AO system operates at visible
wavelengths, the pointing loops use the J-band, the high-speed fringe tracker operates in the K-band, and the nulling
observations take place in the N-band. Thus, brightness constraints apply at all of these wavelengths. In addition, source
structure plays a role at both K-band and N-band, through the visibility. In this talk, the operation of the KIN is first
briefly described, and then the sensitivity and performance of the KIN is summarized, with the aim of presenting an
overview of the parameter space accessible to the nuller. Finally, some of the initial observations obtained with the KIN
The Keck Interferometer links the two 10m Keck Telescopes located atop Mauna Kea in Hawaii. It was the first 10m
class, fully AO equipped interferometer to enter operation. Further, it is the first large interferometer to implement a
nuller, whereby the on axis light from a bright point source (e.g. a star) can be removed interferometrically, allowing
study of light from nearby, low contrast sources (e.g. exo-zodiacal dust).
This paper describes the control system we have implemented to enable operation of the Keck interferometer nuller. We
give a general overview of the control system, plus details of how control differs from the already implemented and
operational, standard visibility science mode of the interferometer. The nuller is challenging in its requirements for
control because of the necessary control precision and the complexity of the number of points of control. We have
implemented some novel control methods to meet these requirements and we describe those here.
The Keck Interferometer Nuller is designed to detect faint off-axis mid-infrared light a few tens to a few hundreds of milliarcseconds from a bright central star. The starlight is suppressed by destructive combination along the long (85 m) baseline, which produces a fringe spacing of 25 mas at a wavelength of 10 μm, with the central null crossing the position of the star. The strong, variable mid-infrared background is subtracted using interferometric phase chopping along the short (5 m) baseline. This paper presents an overview of the observing and data reduction strategies used to produce a calibrated measurement of the off-axis light. During the observations, the instrument cycles rapidly through several calibration and measurement steps, in order to monitor and stabilize the phases of the fringes produced by the various baselines, and to derive the fringe intensity at the constructive peak and destructive null along the long baseline. The data analysis involves removing biases and coherently demodulating the short-baseline fringe with the long-baseline fringe tuned to alternate between constructive and destructive phases, combining the results of many measurements to improve the sensitivity, and estimating the part of the null leakage signal which is associated with the finite angular size of the central star. Comparison of the results of null measurements on science target and calibrator stars permits the instrumental leakage - the "system null leakage" - to be removed and the off-axis light to be measured.
Proc. SPIE. 6274, Advanced Software and Control for Astronomy
KEYWORDS: Signal to noise ratio, Mirrors, Detection and tracking algorithms, Interferometers, Control systems, Servomechanisms, Data corrections, K band, Nulling interferometry, Secondary tip-tilt mirrors
The real-time control system for the Keck Interferometer Nuller provides the N-band fringe tracking capabilities of the instrument, as well as correcting for atmospheric dispersion in the system. There are three closed-loop servos for controlling the N-band path, as well as two K-band servos which provide open-loop control. A system of synchronized "gates" allows all N-band fringe trackers to operate simultaneously, making it possible to interleave servo corrections with data collection. Several methods of improving servo performance and maintenance of control schemes are discussed.
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.
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 KIN's 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.
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.