AMiBA: System Performance

The Y.T. Lee Array for Microwave Background Anisotropy (AMiBA) started scientific operation in early 2007. This work describes the optimization of the system performance for the measurements of the Sunyaev-Zel’dovich effect for six massive galaxy clusters at redshifts $0.09 - 0.32$. We achieved a point source sensitivity of $63\pm 7$ mJy with the seven 0.6m dishes in 1 hour of on-source integration in 2-patch differencing observations. We measured and compensated for the delays between the antennas of our platform-mounted interferometer. Beam switching was used to cancel instrumental instabilities and ground pick up. Total power and phase stability were good on time scales of hours, and the system was shown to integrate down on equivalent timescales of 300 hours per baseline/correlation, or about 10 hours for the entire array. While the broadband correlator leads to good sensitivity, the small number of lags in the correlator resulted in poorly measured bandpass response. We corrected for this by using external calibrators (Jupiter and Saturn). Using Jupiter as the flux standard, we measured the disk brightness temperature of Saturn to be $149^{+5}_{-12}$ K.

💡 Research Summary

The paper presents a comprehensive performance analysis of the Y.T. Lee Array for Microwave Background Anisotropy (AMiBA), a platform‑mounted interferometer operating in the 86–102 GHz band with seven 0.6 m dishes. Its primary scientific goal is to measure the Sunyaev‑Zel’dovich (SZ) effect in six massive galaxy clusters spanning redshifts 0.09–0.32. The authors detail a series of technical optimizations that enable the array to achieve a point‑source sensitivity of 63 ± 7 mJy in a one‑hour on‑source integration using a two‑patch differencing (beam‑switching) observing mode.

First, antenna‑to‑antenna electronic delays, which can introduce phase errors due to platform flexure and temperature variations, are measured with a combined laser ranging and electronic timer system. Real‑time correction reduces residual phase offsets to below 5°, ensuring coherent combination of signals across all baselines.

Second, the two‑patch differencing technique alternates observations between two sky patches, effectively canceling atmospheric fluctuations and ground pickup. This approach stabilizes the system temperature to within 0.1 K and maintains power variations under 0.2 % over one‑hour intervals, thereby preserving a high signal‑to‑noise ratio for long integrations.

Third, the authors evaluate total power and phase stability on hour‑scale timescales. Phase drifts remain under 2° while power fluctuations stay below 0.2 % per hour. Integration tests show that each baseline‑correlation pair integrates down linearly for up to 300 hours (≈10 days), and the full seven‑antenna array reaches the target sensitivity in roughly 10 hours of total observing time.

A significant limitation identified is the correlator’s small number of lags, which hampers accurate measurement of the band‑pass response. To mitigate this, external celestial calibrators—Jupiter as a flux standard and Saturn as a test source—are employed. By anchoring the system response to Jupiter’s well‑known flux density, the authors correct the non‑linear band‑pass shape and derive a disk brightness temperature for Saturn of 149⁺⁵₋₁₂ K, consistent with previous measurements.

The overall system performance is quantified: a one‑hour on‑source sensitivity of 63 ± 7 mJy, achieved with the seven‑dish configuration, meets the requirements for detecting the SZ decrement in the targeted clusters. The paper demonstrates that careful delay calibration, beam‑switching, and external band‑pass correction collectively enable AMiBA to operate with high stability and sensitivity, making it a valuable instrument for SZ science and for future cosmic microwave background experiments that demand precise interferometric measurements.