First year results and challenges from the Heliometer at Obsevatorio Nacional, Rio de Janeiro

Andrei Alexandre Humberto, oat1@on.br, (1) ON/MCTI; (2) OATo/INAF; (3) SYRTE/OP; (4) OV/UFRJ Italy
D'Avila Victor Amorim, victor@on.br, ON/MCTI
Reis Neto Eugenio, eug.reis@gmail.com, MAST/MCTI
Penna Jucira Lousada, jucira@on.br, ON/MCTI
Boscardin Sergio Calderari, sergio.boscardin@on.br, ON/MCTI
Coletti Alissandro, sandroac34@gmail.com, Azeheb/FACC
Oliveira Luiz Carlos, loveira2@bol.com.br, NGC51
Sigismondi Costantino, costantino.sigismondi@gmail.com, Sapienza University of Rome

Abstract
The contrast between the solar activity from the previous solar cycle to the current one offers the opportunity to examine large scale phenomena and how they behaved along these two cycles. The apparent solar diameter was monitored at Observatorio Nacional/MCTI, Rio de Janeiro, during cycle 23 and up to 2008 using the CCD Solar Astrolabe, and presently this is done using the 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, arising from 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 observation, was tackled in the Solar Heliometer by having all optical elements and their niches made on CCZ, and the telescope tube on carbon filter, both materials of negligible thermal coefficients. 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.