Keck Interferometer Nuller Data Reduction and On-Sky Performance

Abstract

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.

Keck Interferometer nuller instrument performance

“…The microsequence uses null-peak mode to measure the phase of the long-baseline fringe; this implementation directly servos to minimize the leakage on each baseline, rather than indirectly through servoing to a calibrated phase offset (as we do during acquisition). 10,11 It is similar in philosophy to Ref. 20, and as an additional benefit, provides a better SNR than a traditional implementation.…”

“…Thus the long baseline nullers are used to accommodate the dynamic range between the bright star and the faint exozodiacal dust. Said differently, the same SNR would accrue were we to measure visibility V instead of null leakage L, and these two quantities are simply related 10,13 for typical cases as L = (1-V)/(1+V). But practically, when calibration and systematic effects are considered, a leakage of, say, 0.01, can be determined more accurately from a direct measurement of the leakage itself than from the deviation from unity of a direct measurement of a visibility with nominal value 0.99.…”

“…10. In principle, the processing is simple: the null leakage is just the ratio of the fringe amplitude on the short baseline with both long baseline nullers at null, to the fringe amplitude with them at peak.…”

Recent science highlights from the Keck Interferometer

“…The detection of low level emission around a bright point source is a problem of dynamic range rather than pure sensitivity and the KI nuller was specifically designed to address this challenge. The nuller hardware is described in Colavita et al (2008) 1 and the performance and data pipeline in Colavita et al (2009) 2 and Colavita et al (this volume).…”

Keck Interferometer Nuller science highlights

“…The main performance driver for the KIN is dynamic range, as required to detect faint exo-zodi structures around nearby main sequence stars, and the instrument was correspondingly designed to provide very accurate null (or equivalently visibility) measurements in the mid-infrared. By the end of the commissioning phase (2004)(2005)(2006)(2007), the measured null accuracy was established to be ~0.2% (1σ) per night for bright targets and for all wavelengths ranging from 8 to 11 microns (Colavita et al 2009 2 ). The KIN's other key instrumental parameters are (i) its spatial resolution (defined as the nuller's 50% transmission point) which goes from 10 mas at 8 microns to about 15 mas at 13 microns, (ii) its spectral resolution of about 20:1 (providing 10 spectral channels across the N band), (iii) its sensitivity of 1.5 Jy at 10 microns, and finally (iv) its effective field of view, which is restricted to about 0.4" x 0.1" (in radius) around the central nulled target.…”