Pointing calibration of GroundBIRD telescope using Moon observation data

TL;DR

This paper presents a novel method for calibrating the pointing of the GroundBIRD telescope using Moon observations, achieving high accuracy suitable for precise CMB measurements.

Contribution

The study introduces a new calibration technique utilizing Moon data, accounting for tilt, encoder, collimation offsets, and brightness nonuniformity, improving pointing accuracy.

Findings

Achieved a pointing accuracy of 3.3 degrees.

Method competes with past planet-based calibration methods.

Systematic errors from Moon brightness nonuniformity evaluated.

Abstract

Understanding telescope pointing (i.e., line of sight) is important for observing the cosmic microwave background (CMB) and astronomical objects. The Moon is a candidate astronomical source for pointing calibration. Although the visible size of the Moon ($\ang{;30}$) is larger than that of the planets, we can frequently observe the Moon once a month with a high signal-to-noise ratio. We developed a method for performing pointing calibration using observational data from the Moon. We considered the tilts of the telescope axes as well as the encoder and collimation offsets for pointing calibration. In addition, we evaluated the effects of the nonuniformity of the brightness temperature of the Moon, which is a dominant systematic error. As a result, we successfully achieved a pointing accuracy of $\ang{;3.3}$. This is one order of magnitude smaller than an angular resolution of $\ang{;36}$. This level of accuracy competes with past achievements in other ground-based CMB experiments using observational data from the planets.