ABSTRACT
The comblike spectrum of a white light-illuminated Fabry-Pérot etalon can serve as a cost-effective and stable reference for precise Doppler measurements. Understanding the stability of these devices across their broad (hundreds of nanometers) spectral bandwidths is essential to realizing their full potential as Doppler calibrators. However, published descriptions remain limited to small bandwidths or short time spans. We present an ~6 month broadband stability monitoring campaign of the Fabry-Pérot etalon system deployed with the near-infrared Habitable Zone Planet Finder (HPF) spectrograph. We monitor the wavelengths of each of ~3500 resonant modes measured in HPF spectra of this Fabry-Pérot etalon (free spectral range = 30 GHz, bandwidth = 820-1280 nm), leveraging the accuracy and precision of an electro-optic frequency comb reference. These results reveal chromatic structure in the Fabry-Pérot mode locations and their evolution with time. We measure an average drift on the order of 2 cm s-1 day-1, with local departures up to ±5 cm s-1 day-1. We discuss these behaviors in the context of the Fabry-Pérot etalon mirror dispersion and other optical properties of the system and the implications for the use of similar systems for precise Doppler measurements. Our results show that this system supports the wavelength calibration of HPF at the â²10 cm s-1 level over a night and the â²30 cm s-1 level over ~10 days. Our results also highlight the need for long-term and spectrally resolved study of similar systems that will be deployed to support Doppler measurement precision approaching ~10 cm s-1.
ABSTRACT
An optical etalon illuminated by a white light source provides a broadband comb-like spectrum that can be employed as a calibration source for astronomical spectrographs in radial velocity (RV) surveys for extrasolar planets. For this application the frequency stability of the etalon is critical, as its transmission spectrum is susceptible to frequency fluctuations due to changes in cavity temperature, optical power and input polarization. In this paper we present a laser frequency comb measurement technique to characterize the frequency stability of a custom-designed fiber Fabry-Pérot interferometer (FFP). Simultaneously probing the stability of two etalon resonance modes, we assess both the absolute stability of the etalon and the long-term stability of the cavity dispersion. We measure mode positions with MHz precision, which corresponds to splitting the FFP resonances by a part in 500 and to RV precision of ≈ 1 m s-1. We address limiting systematic effects, including the presence of parasitic etalons, that need to be overcome to push the metrology of this system to the equivalent RV precision of 10 cm s-1. Our results demonstrate a means to characterize environmentally-driven perturbations of etalon resonance modes across broad spectral bandwidths, as well as motivate the benefits and challenges of FFPs as spectrograph calibrators.
ABSTRACT
BACKGROUND: Cognitive impairment has been found to be a predictor of adverse medical outcomes, including nonadherence, recurrent medical crises resulting in early readmissions, and death. OBJECTIVE: The Mini-Cog has been proposed for bedside/clinic cognitive testing. Its validity as a measure of central nervous system (CNS) impairment has never been tested against measures of CNS-medical history, CNS scans, selected laboratory findings, observed in-hospital nondelirious memory impairment, or collateral history from family. METHODS: We observed Mini-Cog performance in 107 post/nondelirious medical intensive care unit patients and tested its association with age, CNS-medical history, CNS scans, selected laboratory findings, and behavioral history (in-hospital observation of memory problems and collateral history from family or significant others). RESULTS: The overall Mini-Cog covaried with age, various measures of CNS impairment, abnormal laboratory findings, and measures of preadmission "forgetfulness" per family and by in-hospital staff observation. Unique variance in predicting overall Mini-Cog scores included age, positive CNS scan, and behavioral history. Of 91 patients found to be "alert and oriented × 3," 76% were impaired in immediate memory, short-term memory, or clock drawing. CONCLUSIONS: The Mini-Cog appears to be a brief, yet valid, measure of CNS dysfunction that significantly enhances sensitivity of evaluation at the bedside. Failure to evaluate patients with a formal examination like the Mini-Cog appears to miss up to 76% of patients with moderate cognitive impairment.
