# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 3 MKID Applications in Observational Astronomy
### Observational Astronomy: Near-IR, Optical and UV
The filter approach has a low energy resolution equal to the number of filters and is very slow. Projecting multiple line spectra next to each other onto a CCD array is used very successfully but requires complex optics and has its limits when too many objects are present as lines start to overlap or in the observation of extended sources. Even though optical to near-IR MKIDs can so far only reach a low to medium energy resolution they still allow to significantly enhance the performance of IFSs in dedicated situations. Giga-Z [100, 101, 102] is a proposed MKID wide-field multi-object spectrograph for the 350-1350 nm range and promises to significantly improve LSST follow-up observations if deployed behind a 4 m class telescope and if an MKID array of 100,000 pixels can be used. (LSST, the Large Synoptic Survey Telescope, recently renamed the Vera C. Rubin Observatory, is a 8.4 m telescope under construction on Cerro Pachon, Chile. It will monitor the night sky constantly and is expected to detect an enormous amount of transient astronomical events like e.g., supernovae.) A more recently proposed IFS is KRAKENS, the Keck Radiometer Array using KID ENergy Sensors [103, 104, 102]. Krakens is planned to be a facility class IFS for the Keck-1 telescope on Mauna Kea with a 30,600 MKID pixels array, a 42.5\" x 45\" field of view and a sensitivity range between 380 nm and 1350 nm. In combination with a desired improvement of MKID spectral resolution to R = 20 this would enable a broad range of novel observational opportunities, and a proposed update to 57,600 pixels and a 1\" x 1\" field of view would further enhance these. It is clear that an MKID IFS needs to have a large number of pixels to be able to compete with existing technologies and that it would profit significantly from further progress in MKID single pixel energy resolution. A further possibility to utilize the capability of MKIDs to resolve the energy of individual detected photon is KIDSpec [105, 106, 102], an MKID instrument currently in development at Durham University. KIDSpec, the Kinetic Inductance Detector Spectrograph is a highly efficient, medium spectral resolution, wide band-pass astronomical spectrograph. The idea behind KIDSpec is to use a linear MKID array as photo sensitive element and at the same time as order resolver for a low line density echelle grating. This way the MKID detectors replace the cross-disperser in a classical echelle spectrograph, greatly simplifying the optical layout, reducing optical losses from a second grating and allowing longer slits. KIDSpec is expected to achieve a spectral resolution R = 4000-10,000 in a 400-1500 nm band and to have better sensitivity for low intensity lines as MKIDs have no dark counts and allow better sky background and cosmic ray subtraction. Further proposals to utilize photon counting MKIDs in astronomical instrumentation include a general push towards better miniaturized instruments [107] and a Gaia type satellite to map the Milky Way in the near-IR [108] (Gaia is a space observatory by the European Space Agency. Its mission is to measure the precise position, luminosity and motion of over one billion stars in the optical wavelength range). This would give better results near the galactic center and some spiral arms compared to GAIA as IR is blocked much less by dust compared to optical light. MKIDs are one option for this kind of observation and are attractive as their fast readout allows for easier and faster all-sky mapping. There are also development efforts going on to adapt the MKID detection principle in order to be able to detect soft and hard X-Rays and medium to high energy particles [109, 110, 111, 112]. As at higher energies, absorbing the particle or photon to be registered in the detector starts to get challenging this requires use of so called Thermal Kinetic Inductance Detectors (TKIDs). A TKID operates as micro-calorimeter as it has a dedicated particle absorber and the temperature rise of that absorber results in Cooper pair breaking in the TKID's inductor.