DARKNESS: A Microwave Kinetic Inductance Detector Integral Field Spectrograph for High-Contrast Astronomy
The paper by Meeker et al. introduces DARKNESS, a pioneering high-contrast astronomical instrument that leverages the novel capabilities of Microwave Kinetic Inductance Detectors (MKIDs) for exoplanet imaging. DARKNESS represents a significant advancement in the field of high-contrast imaging by addressing speckle noise barriers which have historically limited the detection of faint celestial objects from ground-based observatories. This paper explores the design, implementation, and preliminary on-sky performance results, emphasizing both MKID technology and its associated impact on adaptive optics and coronagraphic techniques.
MKID Technology and Instrument Design
DARKNESS employs MKIDs, low temperature detectors that efficiently count photons and perform low-resolution spectroscopy in real-time. MKIDs capitalize on the kinetic inductance effect observed in superconductors to detect individual photons with high temporal resolution, offering a photon energy measurement accuracy within a few percent. The instrument hosts an MKID array with a 10,000 pixel format optimized for the near-infrared bandwidth of 0.8 to 1.4 µm. DARKNESS's MKIDs are built from platinum silicide (PtSi) on sapphire, demonstrating improved fabrication uniformity and enhanced photometric stability compared to titanium nitride used in earlier iterations.
Initial laboratory verification and commissioning results indicate promising capabilities of DARKNESS in handling atmospheric aberrations and achieving diffraction-limited performance. The measured spectral resolution ranges from 5 to 7 across its operational bandwidth, comparable to standard near-infrared photometric filters. Absolute instrument throughput measurement verified substantial quantum efficiency with an overall 80% throughput, albeit constrained by MKID quantum efficiency rather than optical path losses. The on-sky performance featured successful observations of binary star systems and demonstrated provisional speckle statistics techniques at short exposure times, providing insights into potential improvements in speckle suppression methodologies.
Implications and Future Directions
The authors speculate that the use of MKIDs in DARKNESS offers exciting prospects for enhancing real-time speckle correction in high-contrast imaging applications. Currently limited by readout speeds, ongoing efforts aim to improve temporal speckle nulling through planned upgrades to accompanying adaptive optics systems. Furthermore, DARKNESS's portable design facilitates its deployment in varied astronomical environments, including its integration with SCExAO at Subaru and planned operation with MagAO-X in the Southern Hemisphere.
Conclusion
The development of DARKNESS embodies a significant stride towards refining ground-based high-contrast imaging practices by employing cutting-edge detector technology. This MKID-based instrument not only sets a precedent for the utilization of superconducting detector technology in other astronomical endeavors, but also fosters potential pathways for achieving photon-noise limited contrasts in future exoplanet surveys. The implications extend beyond immediate practical applications, suggesting transformative effects on the theoretical understanding and methodologies in imaging distant planetary systems.