[Paper Review] Calibration of Argus and the 4mm Receiver on the GBT

D. T. Frayer, R. J. Maddalena+5 more|arXiv (Cornell University)|Jun 5, 2019

Calibration and Measurement Techniques2 references4 citations

TL;DR

This paper presents a comprehensive calibration framework for the GBT's Argus and 4mm receivers at 3 mm wavelengths (66–116 GHz), using small point sources and extended bodies like Jupiter to derive aperture efficiency, main-beam efficiency, and beam pattern characteristics. It confirms that the GBT performs well at high frequencies, with 95% of power within 1° of the beam peak, and validates calibration using ALMA-observed quasars and the Ruze equation, showing consistent results across observing sessions when corrected for surface errors.

ABSTRACT

The calibration procedures for data collected for Argus and the 4mm Receiver instruments on the GBT are presented. The measured beam size, aperture efficiency, and main-beam efficiency are derived for the range of observing frequencies (66--116 GHz) within the 3mm atmospheric window. The telescope performs well even at the highest frequencies (>100 GHz). The amount of power in the error pattern of the antenna beam is estimated. Approximately 95% of the total antenna power is contained within 1 degree of the pointed direction. The calibration derived using small sources follows theoretical expectations based on the Ruze equation. In general, bright point source calibrators that are monitored regularly by ALMA can be used to accurately derive the main-beam efficiency of the telescope.

Motivation & Objective

• To establish a reliable calibration framework for the GBT's Argus and 4mm receivers at 3 mm wavelengths.
• To quantify beam size, aperture efficiency, and main-beam efficiency across 66–116 GHz.
• To validate calibration using bright point sources and extended bodies like Jupiter to assess beam pattern and error contributions.
• To provide a frequency-dependent model for efficiency metrics based on surface error and beam parameters.
• To guide users in selecting optimal calibration sources and observing strategies for high-fidelity data.

Proposed method

• Calibration is derived from observations of bright quasars and extended sources (Jupiter, Moon), using standard temperature scale transformations.
• Aperture efficiency (ηₐ) is computed from the ratio of T′ₐ to source flux density, with the Ruze equation linking ηₐ to surface error ε and observing wavelength λ.
• Main-beam efficiency (ηₘb) is calculated from ηₐ and beam FWHM using a Gaussian beam approximation and the GBT’s 100 m diameter.
• Error pattern contributions are estimated by comparing Jupiter’s measured efficiency (η_Jupiter) to the main-beam efficiency, revealing spillover and scattering effects.
• Effective beam efficiency η_Source is defined as η_c × η_fss × η_l to model coupling of extended sources to the full antenna pattern.
• Calibration consistency is verified by scaling η_Jupiter and ηₐ across observing sessions, showing agreement when corrected for aperture efficiency variations.

Experimental results

Research questions

• RQ1 How do the aperture and main-beam efficiencies of the GBT's Argus and 4mm receivers vary across 66–116 GHz?
• RQ2 To what extent do surface errors (ε ≈ 230–235 μm) and beam size affect calibration performance at high frequencies?
• RQ3 How do measurements of extended sources like Jupiter compare to point-source calibrations in estimating beam efficiency?
• RQ4 What is the contribution of the antenna’s error pattern to the total measured power, and how is it quantified?
• RQ5 Can standard quasars from the ALMA calibrator catalog be reliably used for absolute calibration of the GBT at 3 mm?

Key findings

• The GBT maintains high performance at frequencies above 100 GHz, with 95% of total antenna power contained within 1° of the beam peak.
• Aperture efficiency (ηₐ) at 86 GHz is measured as 0.33 ± 0.05, with values as low as 0.2 observed in poor conditions.
• Main-beam efficiency (ηₘb) correlates directly with aperture efficiency for small sources, as predicted by the Ruze equation and beam size.
• Jupiter’s measured efficiency (η_Jupiter) is higher than main-beam efficiency due to coupling with the error beam, and varies with ηₐ, indicating sensitivity to surface quality.
• After correcting for aperture efficiency differences, η_Jupiter values from different observing sessions align, confirming consistency and revealing error pattern dependence.
• The Ka+CCB system is recommended for OOF corrections in windy conditions, as it provides better signal-to-noise and reduced sensitivity to wind-induced errors.