Instrument Simulator and Data Reduction Pipeline for the iLocater Spectrograph

Bechter, Eric B.; Bechter, Andrew; Crepp, Justin R.; Crass, Jonathan; King, David L.

doi:10.1088/1538-3873/aaf278

Cited by 10 publications

(14 citation statements)

References 43 publications

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“…Characterization of birefringence in SMF feeds and mitigating the impact on RV is an on-going area of research (Bechter et al in prep) With a core size of only ≈ 5-10 µm, 1 injecting starlight from a telescope directly into SMF's is non-trivial, requiring exquisite beam control Jovanovic et al 2014). Developing an efficient SMF injection system thus represents a critical step in realizing the potential of a diffraction-limited spectrograph capable of extremely precise Doppler RV measurements (Bechter et al 2015;Blake et al 2015;Bechter et al 2016Bechter et al , 2019.…”

Section: Introductionmentioning

confidence: 99%

Characterization of Single-mode Fiber Coupling at the Large Binocular Telescope

Bechter

Crass

Tesch

et al. 2019

PASP

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Optimizing on-sky single-mode fiber (SMF) injection is an essential part of developing precise Doppler spectrometers and new astrophotonics technologies. We installed and tested a prototype SMF injection system at the Large Binocular Telescope (LBT) in April 2016. The fiber injection unit was built as part of the de-risking process for a new instrument named iLocater that will use adaptive optics (AO) to feed a high resolution, near-infrared spectrograph. In this paper we report Y-band SMF coupling measurements for bright, M-type stars. We compare theoretical expectations for delivered Strehl ratio and SMF coupling to experimental results, and evaluate fundamental effects that limit injection efficiency. We find the pupil geometry of the telescope itself limits fiber coupling to a maximum efficiency of ρ tel ≈ 0.78. Further analysis shows the individual impact of AO correction, tip-tilt residuals, and static (non-common-path) aberrations contribute coupling coefficients of ρ Strehl ≈ 0.33, ρ tip/tilt ≈ 0.84, and ρ ncpa ≈ 0.8 respectively. Combined, these effects resulted in an average Y-band SMF efficiency of 0.18 for all observations. Finally, we investigate the impact of fiber coupling on radial velocity (RV) precision as a function of stellar apparent magnitude.

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Section: Introductionmentioning

confidence: 99%

Characterization of Single-mode Fiber Coupling at the Large Binocular Telescope

Bechter

Crass

Tesch

et al. 2019

PASP

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“…Numerical simulations used to characterize the impact of optical aberrations draw from a foundation of code developed to model the iLocater spectrograph. 11 Used previously to construct an instrument RV error budget, 12 and most recently to quantify uncertainties expected from NIR hybrid array HxRG detectors, 13 this code basis has been augmented to incorporate the effects of wavefront phase errors.…”

Section: Methodsmentioning

confidence: 99%

“…iLocater's data reduction pipeline is used to locate and extract spectral orders, apply a wavelength solution, and cross-correlate extracted spectra. 11 A custom built binary mask based on M0V stars is used to derive wavelength shifts and compute a RV shift. A simulation free of aberrations is recorded as a benchmark.…”

Section: Methodsmentioning

confidence: 99%

“…A common algorithm used for recovering Doppler velocities from stellar spectra is masked crosscorrelation. 18 We implement our own cross-correlation method, described in a previous publication 11 to quantify the effect of aberrations on cross-correlation algorithms. Similar to the procedure in §2.2.1, the effect of 15 common aberrations (the first 16 Zernikes not including piston) are simulated individually and applied to the stellar spectrum.…”

Section: Absorption Line Shiftsmentioning

confidence: 99%

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Studying the Impact of Optical Aberrations on Diffraction-Limited Radial Velocity Instruments

Bechter¹,

Bechter²,

Crepp³

et al. 2021

Preprint

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Spectrographs nominally contain a degree of quasi-static optical aberrations resulting from the quality of manufactured component surfaces, imperfect alignment, design residuals, thermal effects, and other other associated phenomena involved in the design and construction process. Aberrations that change over time can mimic the line centroid motion of a Doppler shift, introducing radial velocity (RV) uncertainty that increases time-series variability. Even when instrument drifts are tracked using a precise wavelength calibration source, barycentric motion of the Earth leads to a wavelength shift of stellar light causing a translation of the spectrum across the focal plane array by many pixels. The wavelength shift allows absorption lines to experience different optical propagation paths and aberrations over observing epochs. We use physical optics propagation simulations to study the impact of aberrations on precise Doppler measurements made by diffraction-limited, high-resolution spectrographs. Using the optical model of the iLocater spectrograph, we quantify the uncertainties that cross-correlation techniques introduce in the presence of aberrations and barycentric RV shifts. We find that aberrations which shift the point-spread-function (PSF) photo-center in the dispersion direction, in particular primary horizontal coma and trefoil, are the most concerning. To maintain aberration-induced RV errors less than 10 cm/s, phase errors for these particular aberrations must be held well below 0.05 waves at the instrument operating wavelength. Our simulations further show that wavelength calibration only partially compensates for instrumental drifts, owing to a behavioural difference between how crosscorrelation techniques handle aberrations between starlight versus calibration light. Identifying subtle physical effects that influence RV errors will help ensure that diffraction-limited planet-finding spectrographs are able to reach their full scientific potential.

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“…Our simulations consist of a combination of previously developed simulation code for the iLocater spectrograph with a Python-based HxRG detector noise generator. 13,14 Section 2 describes the general methodology behind our simulations and a description of how detector noise is translated into RV uncertainties. Section 3 provides theory and practical models implemented for each noise source.…”

Section: Introductionmentioning

confidence: 99%

Assessing the suitability of H4RG near-infrared detectors for precise Doppler radial velocity measurements

Bechter

Crepp

et al. 2019

J. Astron. Telesc. Instrum. Syst.

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At wavelengths longwards of the sensitivity of silicon, hybrid structured mercury-cadmium-telluride (HgCdTe) detectors show promise to enable extremely precise radial velocity (RV) measurements of late-type stars. The most advanced near infrared (NIR) detector commercially available is the HAWAII series (HxRG) of NIR detectors. While the quantum efficiency of such devices has been shown to be ≈ 90%, the noise characteristics of these devices, and how they relate to RV measurements, have yet to be quantified. We characterize the various noise sources generated by H4RG arrays using numerical simulations. We present recent results using our end-to-end spectrograph simulator in combination with the HxRG Noise Generator, which emulates the effects of read noise, parameterized by white noise, correlated and uncorrelated pink (1/f ) noise, alternating column noise, and picture frame noise. The effects of nonlinear pixel response, dark current, persistence, and interpixel capacitance (IPC) on RV precision are also considered. Our results have implications for RV error budgets and instrument noise floors that can be achieved with NIR Doppler spectrographs that utilize this kind of detector.

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Instrument Simulator and Data Reduction Pipeline for the iLocater Spectrograph

Cited by 10 publications

References 43 publications

Characterization of Single-mode Fiber Coupling at the Large Binocular Telescope

Characterization of Single-mode Fiber Coupling at the Large Binocular Telescope

Studying the Impact of Optical Aberrations on Diffraction-Limited Radial Velocity Instruments

Assessing the suitability of H4RG near-infrared detectors for precise Doppler radial velocity measurements

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