# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 3 MKID Applications in Observational Astronomy
### Observational Astronomy: Infrared to Microwaves
#### 3.1.2 MKID Mid-IR to Microwave Imagers  
To build an astronomical camera for the mid-IR to microwave range multiple MKID resonators have to be arranged as pixels in a detector array located in the focal plane of a telescope. Different methods are used to focus and couple the incoming light onto the MKID resonators themselves where it can break Cooper pairs. The incoming energy flux can then be measured via the QP density of each individual MKID pixel and an image can be reconstructed. One of the first projects to use MKIDs for scientific instrumentation has been the photometric imaging camera MUSIC (MUltiwavelength Sub-millimeter Inductance Camera) and its precursor and test bed DemoCam, both intended for the Caltech Submillimeter Observatory (CSO) on Mauna Kea, Hawaii. DemoCam [24; 25] could detect two bands at 240 and 350 GHz (1.2 and 0.9 mm) with 16 pixels. Each pixel had a slot antenna to couple incoming light to a microstrip-line, which then split, passed through band defining on-chip, superconducting bandpass filters and then coupled to two Al/Nb MKIDs per pixel. DemoCam was used for tests and to gather general know-how for MUSIC but was also used for first astronomical observations [25; 26]. MUSIC [27; 28; 29; 30; 31] was intended to reach 576 individual pixels sensitive to 4 different colors at 150, 230, 290, and 350 GHz (2.0, 1.3, 1.0, and 0.86 mm). MUSIC used the same broadband, superconducting phased-array slot-dipole antennas coupled to Al/Nb MKIDs as DemoCam, just with more bandpass filters and 4 MKIDs per pixel for a total detector count of 2304. MUSIC was used for several scientific observation runs during its commissioning phase [29; 32] but was decommissioned after the Caltech Submillimeter Observatory was closed. That also put the further development of MAKO [33] on hold, a 500 pixel MKID sub-mm camera prototype for the CSO to study scaling up to larger arrays using TiN based LEKIDs. A very successful application of MKIDs as mm wavelength camera is the NIKA and NIKA2 collaboration. The New IRAM KIDs Array (NIKA) [34; 35; 36; 37] is a 400 pixels cryogenic camera for astronomical observations with the 30 m IRAM (Institute for Millimetric Radio Astronomy) telescope on Pico Veleta, Spain. NIKA originally could observe in 2 frequency bands: 125-175 GHz (2.4-1.7 mm) with a 144 pixels array and 200-280 GHz (1.5-1.1 mm) with a larger 256 pixels array. The band selection is achieved with a cold dichroic and the Al MKIDs use fractal Hilbert-curve shaped inductors as receiving antenna for incoming mm wavelength light. NIKA has been operating successfully since 2013 and has been updated with its successor NIKA-2 in 2015. NIKA-2 [37; 38; 39; 40] covers two similar bands centered around 150 GHz (2.0 mm) and 260 GHz (1.15 mm) and offers a 1040 pixels array for 150 GHz and two arrays with 1200 pixels each for both polarization directions of the 260 GHz band, for a total of 3440 MKIDs. NIKA-2 is openly accessible for users of the IRAM telescope since 2017 and together with NIKA has already produced significant scientific output like [40; 41; 42; 43] to list just a few examples. The Large Millimeter Telescope (LMT) Alfonso Serrano, Sierra Negra, Mexico, is the world's largest single aperture telescope for mm wavelengths, and two projects involving MKID cameras are ongoing at the LMT. MUSCAT, the Mexico UK Sub-mm Camera for AsTronomy [44; 45] is a collaborative effort to install a 1600 pixels MKID camera at the LMT with a sensitivity band centered around 270 GHz (1.1 mm). MUSCAT uses aluminum MKID pixels that are insensitive to the incoming light's polarization and utilizes a meandering inductor optimized to absorb at the 1.1 mm wavelength. ToITEC [46; 47] is the second MKID camera project for the LMT. It is intended as millimeter-wave imaging polarimeter with three bands around 1.1, 1.4, and 2.0 mm (280, 220, and 150 GHz), selected by dichroic filters.