QUIJOTE-TFGI polarization calibration -- Ground characterization and on-sky validation with Tau A and the Moon

Abstract

Our objective is to characterize the QUIJOTE Thirty and Forty GHz instrument (TFGI), calibrate it with a reference calibration signal on the ground, compare our results with on-sky calibration based on bright sources, and study the stability of the calibration parameters over time. First, from the ground, we fit the data using a reference calibration signal (a diode) introduced to resolve degeneracies among the various instrument angles. Finally, we utilize on-sky observations of Tau A and the Moon to validate the results. By creating calibration datasets obtained with the reference diode, we evaluate the data quality and quantify phase switch errors to account for the fine polarization response. We also utilize Tau A and Moon observations to calibrate the system's response and stability over time. In addition, we calculate the refraction index of the Moon to be $n_{Moon}$ = 1.209 $\pm$ 0.007 (stat) $\pm$ 0.005 (sys) at 31 GHz under smooth-surface assumption. The results from fitting the instrument phase-switch error angle align with 0 deg at 2$σ$ precision, indicating that no further correction is required within a few percent precision. The calibrations with astrophysical sources (Tau A and the Moon) yield consistent results that constrain the polarization angle and responsivity. The polarization efficiency aligns well with ground measurements and the Tau A characterization, whereas the Moon-based calibration is more affected by systematics. We find hints of responsivity variations over time, although the relative responsivity between channels is found to remain stable. In the future, we conclude that installing a live calibrator will enhance performance by continuously monitoring responsivity and, in turn, improving the mitigation of systematic effects.

Figures (14)

• Figure 1: Block diagrams of the receivers (one detector). Up: Thirty GHz Instrument. Bottom: Forty GHz Instrument. The diagram is taken from TFGIreceivers_artal.
• Figure 2: Photo of the MFCI mounted on the calibration alignment plate. The diode used introduces the known signal into the TFGI detectors. The fixed plate is placed on top of the focal plane and defines the different detector locations where the diode can be positioned. The rotary motor allows the modification of the calibration diode's orientation ($\gamma$) in 22.5 deg steps to study different linear polarization inputs. A detailed description is presented in mateo_phd.
• Figure 3: Example of a full data block, $\sim$8 ms. The left part of the voltage output shows the diode ON, while the right part shows the diode OFF. Each color identifies a channel output. The signal exhibits saturation, reaching a value of 10 V, in some cases.
• Figure 4: The histogram of the slopes that quantify the signal stability in a data acquisition of 1 minute. Each count is defined as a function of the slope obtained from linear fitting. The slope values span a range $\sim$$-350$--100 nV/sample, indicating a lack of significant stability issues within 1 minute of integration. A sample is recorded in 250 $\mu$s.
• Figure 5: Schematic of the polarization angle measurement of the telescope. The axes represent the ground tilt (Horizontal) and the deviation from Zenith (Vertical).