An On-Sky Atmospheric Calibration of SPT-SLIM

TL;DR

This work develops an on-sky atmospheric-loading calibration for the SPT-SLIM MKID-based line intensity mapper. By combining on-site atmospheric tipper opacities with am_spectra simulations and an optical sidelobe term, the authors fit a detector response model $f(P) = A P^{1/2} + f0$ to multiple atmospheric loading observations and convert detector frequency shifts to sky brightness via $P_{RJ} = \eta_{opt} k_B T_b G_{KID}$. Applying the calibration to four moon observations yields a mean lunar temperature of $T_{moon} = 285 \pm 31$ K, in agreement with the phase-dependent lunar spectrum reported in the literature, thereby validating the spectral sensitivity of SPT-SLIM on-sky. The approach provides a practical framework for calibrating MKID-based instruments under varying atmospheric loading and can be extended to other sky maps and sources.

Abstract

We present the methodology and results of the on-sky responsivity calibration of the South Pole Telescope Shirokoff Line Intensity Mapper (SPT-SLIM). SPT-SLIM is a pathfinder line intensity mapping experiment utilizing the on-chip spectrometer technology, and was first deployed during the 2024-2025 Austral Summer season on the South Pole Telescope. During the two-week on-sky operation of SPT-SLIM, we performed periodic measurements of the detector response as a function of the telescope elevation angle. Combining these data with atmospheric opacity measurements from an on-site atmospheric tipping radiometer, simulated South Pole atmospheric spectra, and measured detector spectral responses, we construct estimates for the responsivity of SPT-SLIM detectors to sky loading. We then use this model to calibrate observations of the moon taken by SPT-SLIM, cross-checking the result against the known brightness temperature of the Moon as a function of its phase.

Figures (7)

• Figure 1: Best fit detector bandpasses for one of the five operational SPT-SLIM filterbanks. This set of bandpasses was measured in the SPT-SLIM cryostat at the South Pole using a Fourier Transform Spectrometer (FTS) Liu_20Pan_19. Each position-space interferogram generated from the FTS was fit with a decaying cosine function, which was then Fourier transformed back into a frequency-space Lorentzian peak. Median atmospheric transmission for the South Pole summer is overplotted in gray.
• Figure 2: Upper: 850 GHz atmospheric tipper opacity, South Pole ambient temperature, and the resulting PWV (from Equation \ref{['eq:radford']}) for the fifteen atmospheric response observations. Lower: Atmospheric spectra generated by AM for each observation. Note that only the PWV was changed for each observation, using scale factors obtained from Equation \ref{['eq:scale_pwv']}.
• Figure 3: An optical diagram of SPT-SLIM. Light from the SPT primary mirror (M1) intersects the SPT-SLIM secondary mirror (M2), and is redirected via the tertiary mirror (M3) into the SPT-SLIM cryostat. However, a portion of the main beam intersects M3. This creates a sidelobe oriented 90 degrees offset in elevation from the main lobe.
• Figure 4: Model fits for a detector at $\nu_0 = 154 \, \rm{GHz}$. The scattered points are measured values of MKID frequency as a function of telescope elevation, with each color corresponding to a different atmospheric response observation. Dotted lines are fits to the data of the model described in Section \ref{['sec:model']}.
• Figure 5: Responsivities for one wafer of SPT-SLIM detectors as a function of detector peak bandpass frequency when loaded at brightness temperatures of 10K, 100K, and 300K.