# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 3 MKID Applications in Observational Astronomy
### Observational Astronomy: Near-IR, Optical and UV
ARCONS is one of the earliest MKID instruments used for astronomical observations and was designed to demonstrate what MKIDs can offer for astronomy. It has been used, for example, to study transiting binaries [93] and to demonstrate optical imaging spectroscopy with MKIDs [91]. ARCONS has now been replaced by a similar instrument with a much bigger pixel number, DARKNESS [2, 94]. DARKNESS, the DARK-speckle Near-infrared Energy-resolving Superconducting Spectrophotometer [2, 94] is a second generation MKID camera for 800-1400 nm photons, using a 10,000 pixels array of MKIDs made from PtSi. DARKNESS has a spectral resolution \(R=\lambda/\Delta\lambda\) of 5-7 and is designed to be used either as a general MKID camera or as a dedicated instrument for the high contrast imaging of exoplanets. The direct observation of exoplanets in high contrast with a coronagraph to block out the host star's light is one of the most impressive applications for optical to near-IR MKIDs in astronomy at the moment. If a coronagraph is used to block the light from a host star to allow to observe its planets, turbulences in the air columns above the telescope cause perturbations, so called speckles. To reach maximum contrast between star and exoplanet and to be able to observe planets nearer to their hosts, these speckles have to be eliminated as best as possible by a feedback system to a deformable mirror. As speckles are chromatic and move fast the quick readout and intrinsic energy resolution of MKIDs result in a much better feedback loop compared to standard CCD cameras. Besides, the fact that MKIDs have no dark counts allows additional statistical post processing procedures that in combination with their time and energy resolution advantages should allow MKIDs to increase achievable contrast ratios with coronagraphs by up to two orders of magnitude [2]. As this increase in contrast could be sufficient to enable the upcoming 30 m class of optical telescopes to observe rocky planets in the habitable zone of nearby stars the interest in optimizing coronagraphic contrast ratio is immense. DARKNESS is optimized to work behind the PALM-3000 extreme adaptive optics system and the Stellar Double Coronagraph at the Hale Telescope at Palomar Observatory and has been used as a technology testbed for high contrast imaging with MKIDs. It is planned [95] to also use DARKNESS with the MagAO-X system on the 6.5 m Magellan Clay telescope in Las Campanas, Chile. Apart from DARKNESS there are currently two further MKID cameras being developed for coronagraphy. The Picture-C MKID camera [96] is similar to DARKNESS with a 10.000 pixels MKID array covering 540-660 nm. It is intended to be used with the second flight of the Picture-C (Planetary Imaging Concept Testbed Using a Recoverable Experiment--Coronagraph) project [97], a high altitude balloon with a coronagraph behind a 0.6 m telescope. Picture-C is optimized to image debris disks, exozodiacal light and young hot-Jupiters around nearby stars and is used as a testbed for future Earth-like exoplanet imagers. The currently largest MKID array in use is MEC, the MKID Exoplanet Camera [3, 98]. MEC is an integral field spectrograph with a detector array of 20,440 MKIDs made from PtSi, is sensitive between 800 nm and 1400 nm and reaches a spectral resolution of 5-7. MEC has been integrated with the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument [99] at the Subaru 8 m telescope on Mauna Kea, Hawaii in 2019 and is expected to significantly improve star to planet contrast ratios. MEC is planned to be made available for open use observations in the near future. The inherent energy resolution MKIDs offer can obviously be useful for integral field spectrographs (IFS). An IFS is basically a camera where every picture element provides its own spectrum. With current Si-based detectors this can be achieved by using several filters or stretching the light of every object of interest into a line spectrum across the detector with diffractive optics.