Page:Epjconf mmUniverse2021 00017.pdf/4

the downsides from the evolved optical load that introduces imperfection on the initial tuning approximation. In addition, it allows us to convert the raw $\phi$ to the resonance frequency: for a detailed discussion see [12].

This whole method is translated into cubes of data composed of the x-y positions on the sky plus the electromagnetic information: we retrieve a whole interferogram per sky position. Starting from the raw interferogram data presented above we go to maps of the sky as follows: 1) using the modulation we calibrate the signal in frequency shift, 2) the data corresponding to the forward and backward interferograms are separated; 3) the interferograms are cleaned from low-frequency drifts and the ZPD is obtained, by regridding the interferograms on the OPD and identifying the maximum or minimum signal; 4) the interferograms in the time domain are projected into maps using all detectors, and 5) the final spectral maps are obtained from the Fourier transform of the interferograms accounting for the ZPD. The result is a set of N maps at the N bins of the frequency bandwidth.

In this paper, we present uncalibrated signals, which are to be intended as resonance frequency ones, as previously mentioned.

3 On-sky results

In this section, we show early results in uncalibrated signals of Jupiter and the calibrated flux of Venus. In previous works, we have already demonstrated the possibility of exploiting an FTS as a pure photometer. In particular, observing a bright and wide source like the Moon [13] which acted as a quick-look demonstrator for our challenging instrument, and calibrating Venus with Jupiter observations [12].

We exploited Jupiter to calibrate our detector response in an astrophysical signal and obtain the first scientific result, as shown in Fig. 2. We measure the brightness temperature of Venus at a frequency that has not been measured before. We obtain a brightness temperature KISS of $T_{b,{\text{Venus}}}^{\text{KISS}}=338\pm 27{\text{K}}$ at 150 GHz (the central frequency of the 120–180 GHz KISS band and the incertitude represents the 1-sigma level of confidence). We also show the “Bellotti” model [14] as a solid blue line for the Venus brightness temperature and its extrapolation of a simple power-law as a dashed line at mm wavelengths. In solid cyan line, we represent the best-fit power law. We observe that the Bellotti model underestimates the power law at high frequency, for more details see [12].

The next challenge is to obtain the spectral mapping of the two planets. The coupling between KISS and the Q-U-I JOint TEnerife (QUIJOTE) telescope results in 7 arcmin (at the central electromagnetic frequency of 150 GHz) full width at half maximum (FWHM). Venus and Jupiter have respectively $338\pm 27{\text{K}}$ and $174.1\pm 0.2{\text{K}}$ [15] in thermodynamic temperatures, and 66 arcsec and 50 arcsec maximum intrinsic apparent diameters when they are at the closest approach in the orbit. We are, therefore, affected by a strong dilution factor, which is defined geometrically as $D=\Omega /2\pi \sigma ^{2}$, where $\Omega$ is the source solid angle and $\sigma$ is the sigma of the single-pixel $\sigma =FWHM/2{\sqrt {2{\text{log}}(2)}}$. In conclusion, the planet signal is diluted within the beam and it results in few kelvins at the different apparent diameters, in the KISS case. This was a major issue for the early observations because we experienced the absence of bright and large-enough primary calibrators to accurately constrain the pointing model at the beginning of the commissioning phase. Despite these difficulties, we managed to detect and systematically observe these two cited planets.

After the first spectral mapping with the Moon presented in [3], we introduce here the results with spectral mapping capability with point sources with KISS which can be considered as the proper first light of the experiment. Figure 3 shows the spectral mapping of Jupiter, projecting the spectra at the given sky position per electromagnetic bin. We observe how,