Solar astrometry with Rio Astrolabe and Heliometer
Costantino Sigismondi, Sergio Calderari Boscardin, Alexandre Humberto Andrei, Eugenio Reis-Neto, Jucira Lousada Penna, Victor Amorim D'Avila
aa r X i v : . [ a s t r o - ph . I M ] N ov To appear in “LARIM XIV Latin American Regional IAU Meeting (2013)”
RevMexAA(SC)
SOLAR ASTROMETRY WITH RIO ASTROLABE & HELIOMETER
C. Sigismondi , S. C. Boscardin , A. H. Andrei , E. Reis-Neto , J. L. Penna , V. A. D’ ´Avila Abstract: Monitoring the micro-variations ofthe solar diameter helps to better under-stand local and secular trends of solar activ-ity and Earth climate. The instant measure-ments with the Reflecting Heliometer of Ob-servat´orio Nacional in Rio de Janeiro (RHRJ)have minimized optical and thermal distor-tion, statistically reducing air turbulence ef-fects down to 0.01 arcsec. Contrarily to satel-lites RHRJ has unlimited lifetime, and itbridges and extends the measures made withdrift-scan timings across altitude circles with0.1 arcsec rms with Astrolabes. The Astro-labe in Rio (ARJ) operated from 1998 to 2009to measure the solar diameter and the de-tected variations have statistical significance.Heliometer:
The 11 cm parabolic mirror ofRHRJ is splitted on its half, forming an appropriatefixed angle β . On the focal plane two images of theSun are formed and the distance d between the limbsis linked to the solar diameter D ⊙ by the equation d = β − D ⊙ . A neutral mylar filter of transmittance10 − screens the Sun. The images are free from chro-matic aberrations and thermal focal variations areminimized by the mirrors made in CCZ HS and thetube in carbon fiber, both with linear thermal ex-pansion α ≤ × − / o C . RHRJ performs ≥ Astrolabes:
They have been the standardinstruments for solar astrometry since the 1970s. A10 cm horizontal telescope (ARJ is refracting likeall Danjon astrolabes, DORAYSOL is reflecting) re-ceives the solar light from a isosceles prism, withits base in vertical position. The upper face of theprism reflects the light onto the objective, and thelower one onto a mercury mirror which defines theideal horizontal surface and then sends the light tothe objective. The instrument remains fixed during Observat´orio Nacional & MAST, Rio de Janeiro, Brazil the transit (drift) of the Sun through the altitudecircle defined by the prism angle, and the contacttimes of direct & reflected images are extrapolatedfrom the video. The prism angle of ARJ can bechanged manually by regulating a spring, in orderto enlarge the number of measurements to max 30per day. For solar astrometry reference both ARJand RHRJ have effective openings of ∼ × sub-arcsecond wedge producing a super-imposed secondary image systematically shifted ofthe same wedge angle. Hence all Astrolabes, SolarDisk Sextant and Picard data can be plotted to thesame reference value, even if the original averages areseparated by ≥ σ one from another: these shifts arecompatible with the tiny wedge angles, measurableso accurately only with the heliometer[2]. Conclusions:
The fluctuations of solar di-ameter during the last four decades, as measuredwith Astrolabes and ARJ [figure], have to be consid-ered statistically significant, as the eclipses data andthe planetary transits data confirm. Picard satelliteobservations of metrologic quality are covering thelast 3 years, coincident with an unusual low solaractivity similar with the Dalton minimum of 1810.The micro variations of the solar diameter claimedby some authors in this last decade can depend onsolar activity. The measurements of solar diameterfrom ground as RHRJ and Picard-Sol extend satel-lite measurements to decades, needed to fully under-stand the secular variations of the solar activity wellknown as responsible of major climate changes.REFERENCES [1] Andrei, A. H., et al. 2013, arxiv1306.3204[2] Sigismondi, C., 2013, arxiv1307.0548C.S. acknowledges the CNPq PV1 grant 2012-2013[1] Andrei, A. H., et al. 2013, arxiv1306.3204[2] Sigismondi, C., 2013, arxiv1307.0548C.S. acknowledges the CNPq PV1 grant 2012-2013