History and Description of Mr Tebbutt's Observatory Windsor, New South Wales Classic Reprint

Excerpt from History and Description of Mr. Tebbutt's Observatory: Windsor, New South Wales

Right ascension. The exact perpendicularity of these threads to the declination threads, as well as the mutual parallelism of these latter threads themselves, has been carefully verified. The micrometer is furnished with four eyepieces having powers ranging from 90 to 325 diameters. The illuminating lamp is suspended in a crutch affixed to the Side of the telescope tube, and is provided with the proper arrangement for insuring its constant verticality and for colouring the light. The illuminating arrangement has always given complete satisfaction, but there being no means of exhibiting bright threads on a dark field, all faint objects were until the close of 1881 observed by means of the ring-micrometers previously referred to. It is, how ever, well known to astronomers that it is only under cer tain conditions which cannot always be secured, that differential measures with this form of micrometer yield satisfactory results. Since 1881, the use of the ring, ex cept in one particular instance, has been abandoned, and for the observation of faint objects a square bar-micrometer has been substituted. This micrometer, which was made by the Messrs. Cooke, was received at the close of April, 1882. It is Similar to that designed by Graham, and described on page vi of the Introduction to Cooper's Catalogue of Ecliptic Stars. At the request of the author, however, threads were stretched between the Opposite angles of the square to serve for ready and accurate ori entation of the micrometer. The lengths of the diagonals reckoning from the intersections of the middle lines of the bars is, according to numerous careful observations of pairs of well determined stars, equal to It was found that when the adopted declination thread was made coinci dent with a parallel of declination there were errors of difference of right ascension amounting to several tenths of a second of time in the case of stars whose difference of declination was considerable. A thorough investigation of the errors was made and a table formed for facilitating the calculation of the corrections of differential right ascension. No appreciable error is experienced in the determinations of differential declination. It is found that observations with this form of micrometer are more convenient and te liable than those with the ordinary ring. All the extra meridian observations involving measurement, from 1873 down to 1886, have been made with the equatorial just des.

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