Development and first year of results from the heliometer of Observatorio Nacional

TL;DR

This paper introduces a new solar heliometer at Observatorio Nacional that achieves high-precision measurements of the solar disk's diameter and shape, minimizing thermal and mechanical influences, and presents its first-year observational results related to solar activity fluctuations.

Contribution

The paper details the development of a state-of-the-art Solar Heliometer with materials and design improvements for enhanced stability and measurement accuracy, and reports initial observational results.

Findings

Detected variations in solar diameter during late 2011 solar activity upheaval

Achieved high-precision measurements with minimized thermal dependence

Demonstrated the heliometer's effectiveness for solar variability studies

Abstract

Recent research on global climate changes points to three distinct sources of climate disturbance: anthropogenic; natural changes in the oceans and atmosphere; and irregularities in the solar cycles. One of the most direct ways to survey an exogenous component of the climatic variability is through the measurement of variations in the diameter and shape of the solar disk. At Observatorio Nacional/MCTI, Rio de Janeiro, after several years of diameter observation using a CCD Solar Astrolabe, these measurements are now performed by a state-of-the-art Solar Heliometer. The heliometric method is one of the most successful techniques to measure small variations of angles. Its principle has been used for the latest space borne astrometric missions, aiming to milli-arcsecond precision. The success of this method relies in the fact that it minimizes the dependence of angular measurements to the thermal and mechanical stability of the instrument. However in the classic heliometer the objective is split into two halves to which is applied a linear displacement along the cut, thus still leaving room for a residual dependence with the focus, due to non-concentricity of the beams of the two images. The focus variation, as well as the effects brought by the large temperature variations during solar observations, was tackled in the Solar Heliometer by having all optical elements and their niches made on CCZ, and the telescope tube on carbon fiber, both materials of negligible thermal coefficient. Additionally, the measures are made perpendicular to the separation direction and the plate scale can be known at every time from the solar motion itself. We present the results from the first year of measurements, in special exploring the upheaval of solar activity on late 2011.