Abstract
The Keck Planet Imager and Characterizer (KPIC) is an instrument at the Keck II telescope that enables high-resolution spectroscopy of directly imaged exoplanets and substellar companions. KPIC uses single-mode fibers to couple the adaptive optics system to Keck's near-infrared spectrometer (NIRSPEC). However, KPIC's sensitivity at small separations is limited by the leakage of stellar light into the fiber. Speckle nulling uses a deformable mirror (DM) to destructively interfere starlight with itself, a technique typically used to reduce stellar signal on a focal-plane imaging detector. We present the first on-sky demonstration of speckle nulling through an optical fiber with KPIC, using NIRSPEC to collect exposures that measure speckle phase for quasi-real-time wavefront control while also serving as science data. We repeat iterations of measurement and correction, each using at least five exposures (four with DM probes to determine phase and one unprobed exposure to measure the intensity) and taking about 6 min when using 59.0 s exposures, including NIRSPEC overheads. We show a decrease in the on-sky leaked starlight by a factor of 2.6 to 2.8 in the targeted spectral order, at a spatial separation of 2.0 λ / D in K-band. This corresponds to an estimated factor of 2.6 to 2.8 decrease in the required exposure time to reach a given signal-to-noise ratio, relative to conventional KPIC observations. The performance of speckle nulling is limited by instability in the speckle phase: when the loop is opened, the null-depth degrades by a factor of 2 on the timescale of a single phase measurement, which would limit the suppression that can be achieved. Future work includes exploring gradient-descent methods, which may be faster and thereby able to achieve deeper nulls. In the meantime, the speckle nulling algorithm demonstrated in this work can be used to decrease stellar leakage and improve the signal-to-noise of science observations.
| Original language | English (US) |
|---|---|
| Article number | 035001 |
| Journal | Journal of Astronomical Telescopes, Instruments, and Systems |
| Volume | 9 |
| Issue number | 3 |
| DOIs | |
| State | Published - Jul 1 2023 |
Funding
Y.X. acknowledges support from the National Science Foundation Graduate Research Fellowship (Grant No. 1122374). J.X. and D.E. acknowledge support from the Keck Visiting Scholars Program. D.E. is also supported by a NASA FINESST fellowship (Grant No. 80NSSC19K1423). Funding for KPIC was provided by the California Institute of Technology, the Jet Propulsion Laboratory, the Heising-Simons Foundation (Grant Nos. 2015-129, 2017-318, and 2019-1312), the Simons Foundation (through the Caltech Center for Comparative Planetary Evolution), and NSF (Grant No. AST-1611623). W. M. Keck Observatory access was supported by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (Grant No. 80NM0018D0004). The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This research made use of Astropy; NumPy; SciPy; and Matplotlib. The authors have no relevant financial interests in the manuscript and no other potential conflicts of interest to disclose.
Keywords
- adaptive optics
- exoplanets
- single-mode fiber
- speckle nulling
- spectroscopy
- wavefront control
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Control and Systems Engineering
- Instrumentation
- Astronomy and Astrophysics
- Mechanical Engineering
- Space and Planetary Science