A new approach for the heliometric optics

TL;DR

This paper introduces a novel heliometric optical system using a split ceramic mirror that minimizes chromatic and refractive aberrations, enabling precise solar diameter measurements across all heliolatitudes.

Contribution

It presents a new mirrored heliometer design that overcomes chromatic and thermal issues of previous models, including the development of an annular version with concentric mirrors.

Findings

The Rio heliometer produces images free of chromatisms and distortions.

The design allows accurate solar measurements at all heliolatitudes.

The observatory's location enables zenithal observations with minimal atmospheric refraction.

Abstract

The heliometer of Fraunhofer in Koenigsberg (1824) is a refractor in which the lens is split into two halves to which is applied a linear displacement along the cut. Later in 1890s a variation of the heliometer has been realized in Goettingen using a beam splitting wedge: these methods were both subjected to chromatic and refractive aberrations; the second configuration being much less affected by thermal fluctuations. The mirrored version of the heliometer conceived at the Observatorio Nacional of Rio de Janeiro overcome these problems: the two halves of the vitrified ceramic mirror split at a fixed heliometric angle produce the two images of the Sun exempt of chromatisms and distortions. The heliometer of Rio is a telescope which can rotate around its axis, to measure the solar diameter at all heliolatitudes. A further development of that heliometer, now under construction, is the annular heliometer, in which the mirrors are concentric, with symmetrical Point Spread Functions. Moreover the location of the Observatory of Rio de Janeiro allows zenithal observations, with no atmospheric refraction at all heliolatitudes, in December and January.