Page:Epjconf mmUniverse2021 00017.pdf/3

EPJ Web of Conferences 257, 00017 (2022) mm Universe @ NIKA2 of the phase and amplitude modulation alternative techniques, not suitable for our utilization). The typical 1/f atmospheric knee is expected to be at ∼1 Hz [9], in the heritage of the New IRAM KID Array 2 (NIKA2) [10] at the Institut de Radioastronomie Millimétrique (IRAM) telescope in Pico Veleta (Spain). This, coupled with the requirement on the spectroscopic resolution, leads to the acquisition of hundreds to thousands of samples on time scales of 0.25 seconds. To achieve this performance and ensure a large field of view (FoV), we need large arrays of fast detectors and readout as is the case for the KIDs. To meet these requirements KISS integrates two full interferograms at 3.72 Hz (0.27 s). The presence of a double interferogram is due to the adopted sampling technique of the interferogram (see [11] for a detailed discussion): we double-sample the interferogram by measuring slightly above the zero optical path difference (ZPD) point that corresponds to the null interference. The first and second interferograms are the result of the forward and backward mirror motion, respectively, as shown in Fig. 1.



Figure 1. KISS data block recorded at 3.72 Hz: frequency signal as a function of the sample number. There are a total of 1024 data samples: the first 128 are dedicated to the modulation and the remaining ones are to the forward and backward interferogram acquisition. The signal is the result of the median value subtraction (i.e., by scaling to 0 Hz the median value), which represents the photometry, i.e., the signal integrated over the electromagnetic bandwidth.

In addition, we have introduced an electrical modulation before each interferogram recording, as shown in Fig. 1. This readout modulation is adopted to improve the sky signal reconstruction accuracy for types of instruments for which a fast sampling frequency is required, both to remove atmospheric fluctuations and to perform full spectroscopic measurements on each sampled sky position. Kinetic inductance detectors are superconducting resonators that are read out by injecting a reference signal in the circuit and recording the output one. The physical quantity that can be directly translated into an astrophysical signal is the resonance frequency, which is different for each single KID. The optical load changes this electrical parameter and by recording it we retrieve the astrophysical signal. The raw data from the KID read-out system is the complex signal (I, Q) from which we can compute the phase signal (φ = arctan 2(I/Q)). Before each on-sky scan, the single KID is optimized (tuned) for the background at the observation time. The electrical modulation introduced before each interferogram sampling allow us to follow the background evolution, reducing Rh