Final design and on-sky testing of the iLocater SX acquisition camera: broad-band single-mode fibre coupling

Crass, Jonathan; Bechter, Andrew; Sands, Brian; King, David L.; Ketterer, Ryan; Engstrom, Matthew; Hamper, Randall; Kopon, Derek; Smous, James; Crepp, Justin R.; Montoya, Manny; Durney, Olivier; Cavalieri, David; Reynolds, R.; Michael, VanSickle,; Onuma, Eleanya; Thomes, J.A.; Mullin, Scott A.; Shelton, Chris; Wallace, J. Kent; Bechter, Eric B.; Vaz, Amali; Power, J.; Rahmer, Gustavo

doi:10.1093/mnras/staa3355

Cited by 17 publications

(16 citation statements)

References 39 publications

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“…A single-mode fiber (SMF) is the ideal transport vehicle owing to the fact it has a field-of-view (FOV) that can be matched to the 1 λ/D width of the point-spread function (PSF), enabling efficient coupling for the planet light and suppression of unwanted starlight. Its small core size only allows light to be guided in a single mode providing spatial filtering, which further suppresses starlight speckles from coupling in and more importantly delivers an ultra stable output beam profile, highly desired by many instruments (Schwab et al 2012;Woillez et al 2003;Crass et al 2020). In addition, its narrow FOV reduces the amount of sky background injected into the spectrograph, which can be several orders of magnitude greater in the case of a seeing-limited spectrograph.…”

Section: Introductionmentioning

confidence: 99%

Enhancing Direct Exoplanet Spectroscopy with Apodizing and Beam Shaping Optics

Calvin¹,

Jovanović²,

Ruane

et al. 2021

PASP

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Direct exoplanet spectroscopy aims to measure the spectrum of an exoplanet while simultaneously minimizing the light collected from its host star. Isolating the planet light from the starlight improves the signal-to-noise ratio (S/ N) per spectral channel when noise due to the star dominates, which may enable new studies of the exoplanet atmosphere with unprecedented detail at high spectral resolution (>30,000). However, the optimal instrument design depends on the flux level from the planet and star compared to the noise due to other sources, such as detector noise and thermal background. Here we present the design, fabrication, and laboratory demonstration of specially-designed optics to improve the S/N in two potential regimes in direct exoplanet spectroscopy with adaptive optics instruments. The first is a pair of beam-shaping lenses that increase the planet signal by improving the coupling efficiency into a single-mode fiber at the known position of the planet. The second is a grayscale apodizer that reduces the diffracted starlight for planets at small angular separations from their host star. The former especially increases S/N when dominated by detector noise or thermal background, while the latter helps reduce stellar noise. We show good agreement between the theoretical and experimental point spread functions in each case and predict the exposure time reduction (∼33%) that each set of optics provides in simulated observations of 51 Eridani b using the Keck Planet Imager and Characterizer instrument at W. M. Keck Observatory.

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Section: Introductionmentioning

confidence: 99%

Enhancing Direct Exoplanet Spectroscopy with Apodizing and Beam Shaping Optics

Calvin¹,

Jovanović²,

Ruane

et al. 2021

PASP

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show abstract

“…We tested the MLR-TT on-sky in November 2019 at the LBT, using the left (SX) mirror of the telescope [15]. During the run the Large Binocular Telescope Interferometer adaptive optics (LBTI-AO) system was using the SOUL upgrade, which is designed to produce an SR of up to 78% in I-band [36] under optimal conditions.…”

Section: Resultsmentioning

confidence: 99%

“…Fiber throughput is determined by measuring the output flux from each fiber with the bare MMF serving as an incident flux reference. Output flux is measured with a FemtoWatt receiver [15]. The fiber bundle holding the six sensing MMFs is routed to a separate opto-electric enclosure, housing the read-out optics and electronics.…”

Section: G On-sky Integrationmentioning

confidence: 99%

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On-sky results for the integrated microlens ring tip-tilt sensor

et al. 2021

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We present the first on-sky results of the micro-lens ring tip-tilt (MLR-TT) sensor. This sensor utilizes a 3D printed micro-lens ring feeding six multi-mode fibers to sense misaligned light, allowing centroid reconstruction. A tip-tilt mirror allows the beam to be corrected, increasing the amount of light coupled into a centrally positioned single-mode (science) fiber. The sensor was tested with the iLocater acquisition camera at the Large Binocular Telescope in November 2019. The limit on the maximum achieved root mean square reconstruction accuracy was found to be 0.19 λ/D in both tip and tilt, of which approximately 50% of the power originates at frequencies below 10 Hz. We show the reconstruction accuracy is highly dependent on the estimated Strehl ratio and simulations support the assumption that residual adaptive optics aberrations are the main limit to the reconstruction accuracy. We conclude that this sensor is ideally suited to remove post-adaptive optics non-common path tip tilt residuals. We discuss the next steps for the concept development, including optimizations of the lens and fiber, tuning of the correction algorithm and selection of optimal science cases.

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“…iLocater's acquisition camera steers and couples each independent beam from the LBT's binocular primary mirrors into SMFs. 14 The spectrograph is then illuminated by these SMFs which have a spatial output profile that can be closely approximated by a circular Gaussian function.…”

Section: Psf Generationmentioning

confidence: 99%

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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Final design and on-sky testing of the iLocater SX acquisition camera: broad-band single-mode fibre coupling

Cited by 17 publications

References 39 publications

Enhancing Direct Exoplanet Spectroscopy with Apodizing and Beam Shaping Optics

Enhancing Direct Exoplanet Spectroscopy with Apodizing and Beam Shaping Optics

On-sky results for the integrated microlens ring tip-tilt sensor

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

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