# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 3 MKID Applications in Observational Astronomy
### Observational Astronomy: Infrared to Microwaves
#### 3.1.1 MKID on-Chip Spectrometers  
The DEep Spectroscopic HIgh-redshift MApper (DESHIMA) [8; 9; 10] uses this working principle. DESHIMA is a single pixel microwave spectrometer sensitive between 332 and 377 GHz (903-795 \(\upmu\)m). It uses an on-chip microwave antenna on which incoming light is focused on in order to couple incoming photons into a superconducting feedline made from NbTiN. This feedline is coupled to 49 individual MKIDs, each with its own planar superconducting passband filter between MKID and feedline. This way only frequencies corresponding to the passband filter can travel from the feedline to the MKID. As a part of these MKIDs is fabricated from Al, which has a smaller gap \(\Delta\) then NbTiN, photons that travel along the feedline without dissipation still break Cooper pairs in the Al and thus produce a signal in the MKID proportional to the photon flux within the filter's bandwidth. DESHIMA with its 49 frequency channels reaches a spectral resolution _f_/\(\Delta\)_f_ of 380. As the single pixel of the DESHIMA spectrometer only covers a few cm\({}^{2}\) on the chip these could be combined into a compact, multi-pixel spectral imager in the future [8]. DESHIMA has already been used for first demonstration observations with the 10 m diameter ASTE (Atacama Submillimeter Telescope Experiment) telescope in Pampa La Bora, Chile [8; 9]. The design of an on-chip filter-bank spectrometer utilizing MKIDs is also pursued by SuperSpec [11; 12; 13]. SuperSpec is targeting the 195-310 GHz (1.54-0.97 mm) frequency range and is aiming for up to 500 channels to achieve a spectral resolution R of 700 [12]. Development is ongoing and prototypes with 50 channels and an R of 275 in the 255-278 GHz (1.18-1.07 mm) range have been demonstrated [11]. SuperSpec uses a Nb feedline and MKIDs made from sub-stoichiometric TiN\({}_{\rm x}\) and is planned to be used soon with the Large Millimeter Telescope (LMT) [PERSON], Sierra Negra, Mexico. It is also proposed to be used as the detector element in X-Spec [14], a multi-object spectrometer for the proposed Cerro Chajnantor Atacama Telescope (CCAT), Chile. Targeting a frequency range of 103-114.7 GHz (2.9-2.6 mm) is the CAMbridge Emission Line Surveyor (CAMELS) [15; 16; 17], another on-chip filter-bank spectrometer using a NbN feedline and MKIDs with a sensitive part made from \(\beta\)-phase Ta [17]. CAMELS intends to utilize 512 filter channels to reach a spectral resolution R of 3000 and is planned to operate at the 12 m Greenland Telescope. A further MKID instrument, WSPEC [18] intends to cover the 135-175 GHz (2.2-1.7 mm) and 190-250 GHz (1.6-1.2 mm) bands with R = 200. WSPEC is not using on-chip superconducting filter banks but a normal-metal, rectangular waveguide splitting into 54 branches each with a precisely machined filter structure and each terminated with an MKID made from Al. Micro-Spec (u-Spec) [19; 20; 21; 22] is an MKIDs based project designed to measure spectra in the 0.5-1.0 mm (300-600 GHz) range using a different principle. Quite simplified, light from the telescope is focused onto an antenna structure that couples the photons into a Nb feedline. This feedline is split up into up to 256 arms [20], each with its own phase delay structure, realized by microstrip transmission lines of varying lengths. These 256 feedlines end in feed horns on one side of an on-chip, 2-dimensional parallel-plate waveguide region. They emit light into that region where it forms an interference pattern on the other side. Where the interference pattern is formed up to 345 receiving feed horns are positioned in an arc and couple the light into a further set of feedlines. Each is connected to a bank of order-sorting filters and its own Al or Mo\({}_{2}\)N MKID [22; 23] that are read out in order to measure the diffraction pattern and extract the spectrum from it. A spectral resolution between 512 [20] and 1200 [21] is expected. Micro-Spec is designed to be a highly miniaturized diffraction grating spectrometer that would fit on a 10 cm\({}^{2}\) Si chip.