The SCALES instrument is a high-contrast imager and integral field spectrograph that operates in the infrared region and is intended to be utilized behind the W.M. Keck Observatory's adaptive optics system. The SCALES integral field spectrograph operates over a broad wavelength range from 2.0 to 5.0 µm. The instrument includes a microlens array-based integral field spectrograph that, when combined with a lenslet to slicer reformatter referred to as "slenslit," allows for low (R = 35 - 250) and moderate (R = 2000 - 6500) spectral resolution spectroscopy. We have done extensive end-to-end modeling of the SCALES optical path using both geometric optics and physical optics. This analysis has been vital in predicting both spectral format and optical performance. We have also combined the predicted geometric point spread function (PSF) given a complete end-to-end system including the SCALES lenslet array IFU, with modeled diffraction effects to understand the crosstalk between the spectra. The PSF modeling is being integrated with the SCALES instrument simulator to provide realistic data products that are being used to develop the SCALES data pipeline.
The Slicer Combined with an Array of Lenslets for Exoplanet Spectroscopy (SCALES) instrument is a lenslet-based integral field spectrograph that will operate at 2 to 5 microns, imaging and characterizing colder (and thus older) planets than current high-contrast instruments. Its spatial resolution for distant science targets and/or close-in disks and companions could be improved via interferometric techniques such as sparse aperture masking. We introduce a nascent Python package, NRM-artist, that we use to design several SCALES masks to be non-redundant and to have uniform coverage in Fourier space. We generate high-fidelity mock SCALES data using the scalessim package for SCALES’ low spectral resolution modes across its 2 to 5 micron bandpass. We include realistic noise from astrophysical and instrument sources, including Keck adaptive optics and Poisson noise. We inject planet and disk signals into the mock datasets and subsequently recover them to test the performance of SCALES sparse aperture masking and to determine the sensitivity of various mask designs to different science signals.
SCALES (Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy) is a 2 micron to 5 micron high-contrast lenslet-based Integral Field Spectrograph (IFS) designed to characterize exoplanets and their atmospheres. The SCALES medium-spectral-resolution mode uses a lenslet subarray with a 0.34 x 0.36 arcsecond field of view which allows for exoplanet characterization at increased spectral resolution. We explore the sensitivity limitations of this mode by simulating planet detections in the presence of realistic noise sources. We use the SCALES simulator scalessim to generate high-fidelity mock observations of planets that include speckle noise from their host stars, as well as other atmospheric and instrumental noise effects. We employ both angular and reference differential imaging as methods of disentangling speckle noise from the injected planet signals. These simulations allow us to assess the feasibility of speckle deconvolution for SCALES medium resolution data, and to test whether one approach outperforms another based on planet angular separations and contrasts.
The Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES) is an under-construction thermal infrared high-contrast integral field spectrograph that will be located at the W. M. Keck Observatory. SCALES will detect and characterize planets that are currently inaccessible to detailed study by operating at thermal (2 μm to 5 μm) wavelengths and leveraging integral-field spectroscopy to readily distinguish exoplanet radiation from residual starlight. SCALES’ wavelength coverage and medium-spectral-resolution (R ∼ 4,000) modes will also enable investigations of planet accretion processes. We explore the scientific requirements of additional custom gratings and filters for incorporation into SCALES that will optimally probe tracers of accretion in forming planets. We use ray-traced hydrogen emission line profiles (i.e., Brγ, Brα) and the SCALES end-to-end simulator, scalessim, to generate grids of high-fidelity mock datasets of accreting planetary systems with varying characteristics (e.g., Teff, planet mass, planet radius, mass accretion rate). In this proceeding, we describe potential specialized modes that best differentiate accretion properties and geometries from the simulated observations.
The Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES) is a 2 μm to 5 μm, high-contrast Integral Field Spectrograph (IFS) currently being built for Keck Observatory. With both low (R ≲ 250) and medium (R approximately 3500 to 7000) spectral resolution IFS modes, SCALES will detect and characterize significantly colder exoplanets than those accessible with near-infrared (approximately 1 μm to 2 μm) high-contrast spectrographs. This will lead to new progress in exoplanet atmospheric studies, including detailed characterization of benchmark systems that will advance the state of the art of atmospheric modeling. SCALES’ unique modes, while designed specifically for direct exoplanet characterization, will enable a broader range of novel (exo)planetary observations as well as galactic and extragalactic studies. Here we present the science cases that drive the design of SCALES. We describe an end-to-end instrument simulator that we use to track requirements and show simulations of expected science yields for each driving science case. We conclude with a discussion of preparations for early science when the instrument sees first light in approximately 2025.
A next-generation instrument named, Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy (SCALES), is being planned for the W. M. Keck Observatory. SCALES will have an integral field spectrograph (IFS) and a diffraction-limited imaging channel to discover and spectrally characterize the directly imaged exoplanets. Operating at thermal infrared wavelengths (1-5 μm, and a goal of 0.6-5 μm), the imaging channel of the SCALES is designed to cover a 12′′ × 12′′ field of view with low distortions and high throughput. Apart from expanding the mid-infrared science cases and providing a potential upgrade/alternative for the NIRC2, the H2RG detector of the imaging channel can take high-resolution images of the pupil to aid the alignment process. Further, the imaging camera would also assist in small field acquisition for the IFS arm. In this work, we present the optomechanical design of the imager and evaluate its capabilities and performances.
We present the design of SCALES (Slicer Combined with Array of Lenslets for Exoplanet Spectroscopy) a new 2-5 micron coronagraphic integral field spectrograph under construction for Keck Observatory. SCALES enables low-resolution (R∼50) spectroscopy, as well as medium-resolution (R∼4,000) spectroscopy with the goal of discovering and characterizing cold exoplanets that are brightest in the thermal infrared. Additionally, SCALES has a 12x12” field-of-view imager that will be used for general adaptive optics science at Keck. We present SCALES’s specifications, its science case, its overall design, and simulations of its expected performance. Additionally, we present progress on procuring, fabricating and testing long lead-time components.
Since the start of science operations in 1993, the twin 10-meter W. M. Keck Observatory (WMKO) telescopes have continued to maximize their scientific impact and to produce transformative discoveries that keep the observing community on the frontiers of astronomical research. Upgraded capabilities and new instrumentation are provided though collaborative partnerships with Caltech, the University of California, and the University of Hawaii instrument development teams, as well as industry and other organizations. This paper summarizes the performance of recently commissioned infrastructure projects, technology upgrades, and new additions to the suite of observatory instrumentation. We also provide a status of projects currently in design or development phases and, since we keep our eye on the future, summarize projects in exploratory phases that originate from our 2022 strategic plan developed in collaboration with our science community to adapt and respond to evolving science needs.
Precision Doppler spectroscopy serves as an important tool for Radial Velocity (RV) measurements by observing Doppler shift in the stellar spectrum, which are used for various applications. Passively stabilized Fabry-Perot (FP) etalon based wavelength calibration is one of the techniques used for Doppler spectroscopy. The FP is kept in a pressure and temperature-stabilized environment for it to produce equispaced transmission lines. Since the FP is stable and the line shape is invariant across wavelength pass band, they can be used to determine the spectrograph’s instrumental artifacts and hence analyze spectrograph performance. Knowledge of instrument effects also helps in better prediction of the wavelength calibration model for the spectrograph. We have tested a passively stabilized FP on Vainu Bappu Telescope (VBT) Echelle spectrograph and Hanle Echelle spectrograph (HESP) and observed field curvature and distortion in both. We are analyzing the artifacts introduced and correcting for the same using image processing methods to compensate for the same in wavelength calibration model developed for the FP-based calibrator.
The Echelle spectrograph operating at Vainu Bappu Telescope, India, is a general purpose instrument used for many high-resolution spectroscopic observations. A concerted effort is being made to expand the scientific capability of the instrument in emerging areas of observational astronomy. We aim at evaluating the feasibility of the spectrograph to carry out precision radial velocity (RV) measurements. In the current design, major factors limiting the RV precision of the spectrograph arise from the movable grating and slit, optical aberrations, positional uncertainty associated with optomechanical mounts, and environmental and thermal instabilities in the spectrograph room. RV instabilities due to temperature and pressure variations in the environment are estimated to vary between 120 and 400 ms − 1, respectively. The positional uncertainty of the grating in the spectrograph could induce a spectral shift of ∼1.4 km s − 1 across the Echelle orders. A Zemax model is used to overcome the uncertainty in the zero-positioning and lack of repeatability of the moving components. We propose to obtain the Th-Ar lamp observations and using the Zemax model as the reference, predict the drifts in the positions of the optical components. The perturbations of the optical components from the nominal position are corrected at the beginning of the observational run. After a good match is obtained between the model and the observations, we propose to use a Zemax model to improve the wavelength calibration solution. We could match the observations and model within ±1 pixels accuracy after the model parameters are perturbed in a real-time setup of the spectrograph. We present the estimation of the perturbations of optical components and the effect on the RV obtained.
The Echelle spectrograph operating at Vainu Bappu Telescope is a general purpose instrument designed for high-resolution spectroscopy. It is being considered for precision Doppler measurements without altering the existing design and basic usage. However, the design level limitations and environmental perturbations are a major source of instability and systematic errors. As a result, a small Doppler signal in the stellar spectra is completely swamped by the large and uncontrolled instrumental drift. We discuss some of the remedial measures we took to improve the radial velocity performance of the spectrograph. We show that an autoguider assembly has greatly reduced the mechanical jitter of the star image at the fiber input, making the illumination of the spectrograph slit at the other end stable. We have also installed an iodine absorption cell to track and eliminate the instrumental drifts to facilitate precision radial velocity observations. Furthermore, we have developed a generic algorithm that uses iodine exposures to extract the stellar radial velocities without the need for the complex forward modeling. Our algorithm is not accurate to the level of traditional iodine technique. However, it is convenient to use on a low-cost general-purpose spectrograph targeting a moderate radial velocity (RV) precision at a few 10 to 100 ms − 1 level. Finally, we have demonstrated the usefulness of our approach by measuring the RV signal of a well-known short-period, planet-hosting star.
Precision in the Radial Velocity (RV) measurements depends upon the efficiency of the technique to remove instrumental artifacts from stellar measurements. Iodine absorption cell technique is being implemented for high precision studies with the Echelle spectrograph operating at Vainu Bappu Telescope (VBT), Kavalur, India. Since the star spectrum is convolved with the PSF of the spectrograph, the asymmetries in the PSF are imposed on the stellar spectral lines. The fiber fed Echelle spectrograph is a general purpose instrument, designed for high resolution (R = 60,000) spectroscopic observations. The asymmetries in the Point Spread Function (PSF) arise due to the off-axis launching of the stellar beam into the collimator and vignetting across the field. Apart from this, due to usage constraints, the grating of the spectrograph is a movable component. The impact on the Doppler shift calculations due to the movable components in the spectrograph is to be estimated. For upgrading the spectrograph for precision studies, the component level sensitivity for RV is to be analyzed. Thus, instrument design asymmetries and component induced PSF variations are analyzed to estimate the limitations of the spectrograph for precision studies. We have developed Zemax based optical design of the spectrograph to estimate the PSF variations and design limitations on the RV studies. Here, we present a model developed in Zemax and a preliminary analysis on RV sensitivity and the PSF asymmetries of the spectrograph. These instrument variations are to be taken as input during RV data reduction for precision measurements.
Precision Doppler spectroscopy serves as an important tool for Radial Velocity (RV) observations of stars. High precision spectroscopy is bound by two major challenges, first being the instrument instability which is mainly caused by temperature and pressure variations and second, the limitations imposed by traditional wavelength calibration methods. In this work we report our progress on the development of a passively stabilized Fabry-Perot (FP) calibrator. We have designed and built an air-spaced etalon with 30 GHz free spectral range for accurately tracking the short-term drift of our high resolution (R = 60,000) Echelle spectrograph on Himalayan Chandra Telescope (HCT), Hanle. Instrument is built using off-the-shelf components, with the required temperature and pressure stability being achieved in initial test runs. For transporting light in and out of the vacuum system without incurring losses at fiber interconnects, we have used a simple way to insert a FC/APC connectorized fiber into the flange. We also present the results of transmission spectra of the FP taken with high resolution Fourier Transform Spectrometer.
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