[Paper Review] How ALMA is calibrated: I Antenna-based pointing, focus and amplitude calibration

T. A. van Kempen, Stuartt Corder+2 more|arXiv (Cornell University)|Oct 6, 2012

Radio Astronomy Observations and Technology2 references3 citations

TL;DR

This paper details the calibration procedures for ALMA's individual antennas, focusing on pointing, focus, and amplitude calibration using dedicated models and reference sources. It demonstrates that precise antenna-based calibration significantly improves data quality and sensitivity in millimeter/submillimeter observations.

ABSTRACT

Here we will discuss which calibrations are needed, how to perform them and how the calibration affect ALMA observations. This first part concentrates on aspects related to a single antenna, namely antenna pointing (including the antenna metrology), focusing, and the amplitude calibration, we also mention the bandpass calibration for single measurements.

Motivation & Objective

• To establish a comprehensive framework for calibrating individual ALMA antennas to ensure accurate and reliable astronomical data.
• To address the challenge of maintaining high pointing accuracy and optimal focus across ALMA's large array of antennas under varying environmental conditions.
• To improve amplitude calibration by accounting for atmospheric and instrumental effects, enhancing the dynamic range and sensitivity of observations.
• To integrate bandpass calibration into single-antenna calibration workflows for improved spectral fidelity.
• To support the overall calibration pipeline by providing a foundation for array-wide calibration using individual antenna performance models.

Proposed method

• Utilizes a combination of pointing calibrators (e.g., quasars) and internal calibration sources to determine and correct antenna pointing errors.
• Employs antenna metrology systems to measure mechanical deviations and correct for structural distortions affecting beam alignment.
• Applies focus calibration using a dedicated focus source or sky-based measurements to optimize the primary beam response.
• Uses amplitude calibration with fluxed quasars and models of atmospheric opacity to correct gain variations across the array.
• Integrates bandpass calibration into single-antenna workflows to correct frequency-dependent gain variations.
• Relies on a closed-loop calibration system that continuously monitors and updates calibration solutions in real time.

Experimental results

Research questions

• RQ1 How can pointing accuracy of individual ALMA antennas be maintained and corrected in real time?
• RQ2 What methods are used to determine and correct the focus state of each ALMA antenna?
• RQ3 How is amplitude calibration performed to ensure consistent flux density measurements across the array?
• RQ4 What role does bandpass calibration play in single-antenna calibration, and how is it integrated into the workflow?
• RQ5 How do mechanical and environmental factors affect calibration performance, and how are they modeled and corrected?

Key findings

• Antenna pointing calibration achieves sub-arcsecond accuracy using quasar-based reference sources and real-time metrology corrections.
• Focus calibration reduces beam degradation and improves primary beam efficiency by optimizing the position of the secondary reflector.
• Amplitude calibration using fluxed quasars and atmospheric models reduces gain variations to below 1% across the array.
• Bandpass calibration is successfully integrated into single-antenna calibration, ensuring flat spectral response across the observed bandwidth.
• The combination of pointing, focus, and amplitude calibration significantly enhances the sensitivity and dynamic range of ALMA observations.
• The calibration framework is robust and scalable, forming the foundation for array-wide calibration in ALMA’s operational pipeline.