Bessels heliometer

While Struve traveled and recovered from the loss of his brother and two children, his sympathetic friend Bessel enjoyed some productive years. Beginning in 1821, he had begun to compile a catalog of the tens of thousands of stars brighter than ninth magnitude, lying between declination —15° and +45°. He had continued to pursue sources of error in observing star positions, and had refined his studies of the rate of Earth's precession. As a foundation for his own and others' future work on parallax and proper motion, he had established a reference system for the positions of stars. All this work culminated in 1830 with the appearance of Tabulae Regiomontanae (the Latin name translates as King's mountain, or Konigsberg in German). The Tabulae includes a listing of the positions of 38 exceptionally well-studied stars, which serve as fixed points in the stellar reference system, and rules for systematically treating data on stellar positions. Generations of astronomers would come to rely on this work of Bessel in making comparisons for parallax or proper motion determinations, or in accurately measuring the positions of the Sun, Moon, and planets.

In 1829, Bessel finally laid his hands on a new instrument that he had ordered from Fraunhofer almost a decade earlier. Bessel had helped Fraunhofer design the 6-inch telescope, of a type known as a heliometer. Utzschneider's workshop completed the heliometer after Fraunhofer's untimely death from tuberculosis.

Figure 5.6 The heliometer type telescope. Originally conceived as an instrument to measure the width of the disk of the Sun (helios in Greek), the heliometer was used by Bessel, in place of a wire micrometer, to measure the separation of stars. The two halves of the objective lens are slid apart until the image of star A lies directly under the image of star B. The amount of displacement of the two halves of the lens gives the angular separation of the stars. (Credit: Layne Lundstrom.)

Figure 5.6 The heliometer type telescope. Originally conceived as an instrument to measure the width of the disk of the Sun (helios in Greek), the heliometer was used by Bessel, in place of a wire micrometer, to measure the separation of stars. The two halves of the objective lens are slid apart until the image of star A lies directly under the image of star B. The amount of displacement of the two halves of the lens gives the angular separation of the stars. (Credit: Layne Lundstrom.)

This instrument would allow Bessel to implement his own plan to measure parallax, which, unlike Struve's plan, did not call for using double stars.

The heliometer first emerged in the mid-1700s as a device to measure the width of the disk of the Sun. (Hence its name — helios is Greek for the Sun.) It consists of a round lens split along a diameter. The two semicircles can slide past each other along the cut (figure 5.6). Each half of the lens produces its own image, so the heliometer is like a double-duty telescope. The astronomer who wanted to measure the width of the solar diameter slid the two halves of the lens apart until the top and bottom images of the Sun moved apart. When the left edge of the solar image on the bottom lined up with the right edge of the solar image on top, he read off a scale the linear displacement of the two halves of the lens. This reflected the width of the solar disk.

Bessel eventually thought of another use for the heliometer. Given stars A and B in the field of view, he could turn a screw to slide the two halves of the lens apart until the lower image of A and B moved by an amount equal to the stars' separa-tion—that is, until A in the lower image lay under B in the upper image, or vice versa. This would allow him to measure, with unprecedented accuracy, the distance between two stars separated by up to 2°, or 7200 arcseconds. This range is far greater than that available with wire micrometers. He would not be limited to measuring the parallax of one star with respect to its companion in an optical pair; he could measure the apparent displacement of a star, or star system, with respect to scattered ''field stars'' in the same part of the sky. Astronomers still use his principle today. Making measurements with respect to a number of field stars limits the effects of any small parallactic shifts in the field stars themselves.

Bessel did not immediately apply the heliometer to the study of 61 Cygni, but renewed his attack on the parallax of that system in 1834. Even then, Struve was still occupied with new duties and problems. The race for parallax took a long time to heat up.

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