Heliometer

Abstract: AN instrument has been devised for the very precise measurement of small objects, such as thin wires, blood cells or the like. The principle involved, which is a century and a half old, is that of the heliometer. The image is split into two complete images which may be sheared across each other by operation of a suitable control. The two images of the object are set edge to edge, an adjustment which is capable of great accuracy, and the amount of shear involved is read off from a suitable scale. The shear is then reversed, crossing the two images over and setting them edge to edge once more. The difference between the two readings of shear is proportional to the diameter of the object and can be calibrated by means of a ‘stage micrometer’.

Abstract: Here the preliminary results of the calibration of the heliometric angle of the Heliometer in Rio de Janeiro are presented. They have been made in 2013 with a reference surveying rod, equipped with two metal spheres acting as artificial star, when illuminated by the Sun, and with the method of drift-scan timing.

Abstract: 1. In investigating the influence of personal equation in connection with double star measurements, the actual values of distance and position angle must be known, and hence the only possible method is to use an artificial star. The method adopted in the Students’ Observatory at University College, in order to obtain a known separation and direction, is to use a weak heliometer lens in front of the equatorial and a spark between cadmium electrodes some distance away. The direction of the line joining the two “star” images and the distance between them can be measured by a divided circle and micrometer screw respectively, while the intensity of the stars can be altered by using various stops. It was found that when a moderately small stop was used, the diffraction images became quite perceptible, and hence, before any further progress could be made in the determination of personal equation, it was necessary to investigate these diffraction images. 2. On consulting the literature of the subject, I found that the earliest papers dealing with the diffraction figure of the heliometer are by Bessel. He observed ( a ) that two opposite brushes of light emerged from the central disc perpendicularly to the line of separation; ( b ) that when the halves of the lens were separated, the disc was lengthened in the direction perpendicular to the line of separation; and ( c ) that the brushes of light were of equal length when the halves were united; but that, when the halves were separated, the brush of light on the same side of the image as the half-lens forming it was longer than the one on the opposite side; and he attributed the lengthening of the disc formed by the half-lens to the fact that the lens was corrected for spherical aberration for the whole lens, and not for the two halves separately.

Abstract: Out of the three quantities that characterize the state of an isolated gaseous body: pressure, temperature and volume the radius is the only one directly measurable for the Sun, what is specially true in the optical window and for ground base measurements The Heliometer of Observatorio Nacional, in Rio de Janeiro, measures the distance between two opposite limbs of the Sun in the same field of view, through the reflection on a 10 cm parabolic mirror split on its half and forming an appropriate angle This configuration is free from optical aberrations and focal variations along the measurement direction The mirrors are made on CCZ vitro-ceramic and the telescope structure is of carbon steel, resulting that there is no flexion or temperature deformation The instrument is compact, and can perform hundreds of measurements per duty day, around all heliolatitudes It attains an accuracy on the solar radius of 001 arcsec, becoming the ideal instrument to monitor from the ground the solar diameter, and to bridge satellites and astrolabes historical series of data We discuss the first years of regular observation, with emphasis on the instrumental calibrations and on the statistic study of the derived time series, attitude series, and solar geometry series On basis of these series we obtain how well the Heliometer and Solar Astrolabe results are matched