The Infrared Doppler (IRD) instrument is a fiber-fed high-resolution NIR spectrometer for the Subaru telescope covering the Y,J,H-bands simultaneously with a maximum spectral resolution of 70,000. The main purpose of IRD is a search for Earth-mass planets around nearby M-dwarfs by precise radial velocity measurements, as well as a spectroscopic characterization of exoplanet atmospheres. We report the current status of the instrument, which is undergoing commissioning at the Subaru Telescope, and the first light observation successfully done in August 2017. The general description of the instrument will be given including spectrometer optics, fiber injection system, cryogenic system, scrambler, and laser frequency comb. A large strategic survey mainly focused on late-type M-dwarfs is planned to start from 2019.
The InfraRed Doppler (IRD) instrument is a high-dispersion spectrograph that is available on Subaru Telescope to explore extrasolar planets via infrared radial velocity (RV) observations. The Subaru/IRD is especially useful in the search of a low-mass planet around cool M-type dwarfs for which infrared RV observations are essential. We report our early performance tests for IRD. IRD’s two H2RG detectors have been evaluated with our detector readout technique, ensuring that their readout noise is made sufficiently smaller than the stellar photon noise expected in our planned survey. We have also tested the instrumental stability of RV measurements from the laboratory data obtained with the IRD’s calibration systems including a laser frequency comb (LFC). Among our tested three types of velocity stability, the stability of comb spectra obtained with a multi-mode fiber (MMF) relative to that with another MMF is measured to be ∼1 m s−1. We also infer from these tests that stellar RV measurements with an MMF can be calibrated with a short-term stability of 2 m s−1 or better by the simultaneously-observed reference spectra of LFC. Furthermore, we report preliminary on-sky RV measurements calibrated with a Thorium-Argon hollow-cathode lamp for RV-stable stars (τ Ceti and Barnard's star) and a planet-host (51 Pegasi). These preliminary RV measurements help the further performance test of IRD that will be performed by the on-sky observations with LFC.
We report the results of fiber mode scrambler experiments for the Infra-Red Doppler instrument (IRD) on the Subaru 8.2-m telescope. IRD is a near infrared, high-precision radial velocity (RV) instrument to search for exoplanets around M dwarfs. It is a fiber-fed, high-resolution (R~70000) spectrograph with an Echelle grating and a state-of-the art laser frequency comb. Expected precision of RV measurements is 1m/s. To achieve 1m/s accuracy, we must reduce modal noise, which is intensity instability of light at the end of multimode fibers. Modal noise is caused by interference of finite number of propagating modes of light. This noise can cause false RV signals, which reduce the accuracy of RV measurements. A mode scrambler is a mechanism to reduce modal noise. However, the best mode scrambler system at near infrared wavelengths is still unknown. Thus, we tested many kinds of mode scramblers, various length fibers, a double scrambler, and octagonal fibers, as static scramblers. We also tested dynamic scramblers, which make output uniform by moving optical fibers dynamically. We report the effects of these mode scramblers.