# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 3 MKID Applications in Observational Astronomy
### Observational Astronomy: Near-IR, Optical and UV  
Starting at around 1.5 \(\upmu\)m wavelength [86,87] the amount of Cooper pairs broken by even a single photon gets big enough to generate sufficiently large signals to allow MKIDs to detect and count single photons. This energy limit can depend, among others, on the detector's geometry, the thickness of the superconducting layer or the pixel size and work is going on to push it further into the mid-IR [64]. For photons at or above this photon counting limit, MKIDs offer four main advantages over competing detector technologies for observational astronomy:  
* No dark counts: Every photon detected by an MKID produces its own, individual signal. The only event that would produce signal spikes in the absence of photons (a dark measurement) is the impact of two or more lower energy photons (from e.g., IR black body radiation) on the detector within a time frame below its time resolution of a few \(\upmu\)s. As these are statistically very unlikely MKIDs have virtually no dark counts. The photon flux measured by MKIDs is therefore precise especially in low light scenarios as it is only limited by statistical fluctuations of the optical system's quantum efficiency. * Energy resolution: The signal produced by a single photon absorbed by an MKID depends on the number of Cooper pairs broken by the photon's energy. The more Cooper pairs are broken the bigger the signal gets. A statistical effect called the Fano limit restricts the achievable precision, but the photon's energy can still be reconstructed from its signal height with up to medium resolution. This allows MKID arrays to measure every detected photon's energy and therefore to offer a low to medium resolution spectrum for every MKID pixel in the detector array, without requiring refractive optics. * High time resolution: As every detected photon produces its own signal pulse, and as every MKID in a detector array is monitored around once every \(\upmu\)s it is possible to determine every photon's arrival time with a precision between one and a few \(\upmu\)s. (Design considerations of MKID detectors, like for example very narrow resonances, can result in detector reaction times that are slower than the monitoring frequency, but rarely more than a few \(\upmu\)s.) This extreme time resolution allows MKIDs to be used to observe fast, time varying events. * Scalability: Other low temperature detector technologies also offer photon counting without dark counts, energy resolution, or high time resolution. But modern astronomical instrumentation often requires pixel numbers in the millions or beyond. The unique advantage of MKIDs is their great multiplexability, and with it their capability to scale up to large detector arrays, even in the mega-pixel range. This allows MKIDs to overcome the severe limitation in pixel numbers that other superconducting detectors suffer from and makes it possible to utilize the significant low temperature detector advantages in modern astronomical instruments. Millisecond pulsars are one of the fastest phenomena observable in astronomy, and some do emit in the optical. The Crab Pulsar for example pulsates with a period of about 33 ms, mainly in radio but also at shorter wavelengths. These time scales are difficult to resolve with standard CCD or CMOS cameras but pose no challenge to an MKID instrument that is read out much faster. The ARray Camera for Optical to Near-IR Spectrophotometry, ARCONS, is an MKID instrument that was used to observe pulsars [88, 89, 90], among them the Crab Pulsar, and demonstrated in collaboration with the Green Bank Observatory a correlation between optical pulses and so called giant radio pulses [89]. ARCONS is a 2024 pixels camera with MKIDs made from sub-stochiometric TiN\({}_{\rm x}\), it is sensitive between 350 nm and 1100 nm and capable to reach an energy resolution of up to 10% [91, 92]. It was commissioned in 2012 and used with the Hale Telescope at Palomar Observatory and the Shane Telescope at Lick Observatory.