ON THE STUDY OF DOUBLE STARS BY AMATEUR 



OBSERVERS. W 



Bv G. F. CHAMr.ERS, F.R.A.S. 



PRACTICAL HINTS ON OBSERVING DOUBLE STARS.* 



Dimensions of telescopes in inches of aperture 

 must not be regarded as conclusive as to the 

 possibilities or impossibilities of di\"iding close 

 Double Stars. There are several factors of import- 

 ance involved in the matter : ci^.. the (jualitx' of 

 the object-glass ; the steadiness of the mounting : 

 the power of the observer's eye : its training to detect 

 small }>()ints of light : his skill and experience 

 generallw How important such points as these are 

 is well illustrated by a remark b\- Burnham, made 

 many many years ago, when he presented one of his 

 early catalogues of new Double Stars to the Royal 

 Astronomical Societ\'.+ Speaking of some measures 

 which had been placed at his disposal bN' Baron 

 Dembowski he sa\-s : — " F"e\v obser\-ers ^vould be 

 able to measure such stars, and those habitualh- 

 observed by him. with an aperture of onI\- 7 inches. 

 It is another illustration of the truth that much more 

 depends upon the observer than the si/e of the 

 instrument. I ma}- add that nothing has been more 

 gratifying and flattering t<> me than to find m\' 

 measures of a difficult pair with INA inches aperture 

 agreeing closely with the measures of Baron 

 Dembowski with 7 inches." 



Another illustration of the proof of lunidiani's 

 general statement here is to be found in the Double 

 Star work done many years ago micrometrically by 

 Dawes, and in another aspect (the literar\- one) by 

 Webb. The form of micrometer best adapted for 

 general use is that known as the Bifilar micrometer 

 mounted in conjunction with a position circle. For 

 the benefit of beginners in the use of such an 

 instrument it is suggested that after bringing a 

 Double Star into the held of view the milled head 

 which moves the position circle be turned until the 

 position wires are approximately parallel to an 

 imaginar\- line joining the stars. The stars are then 

 by means of the slow motion on the telescope to be 

 brought between tlie position wires, and then a final 

 adjustment of the wires to parallelism with the 

 aforesaid imaginar\- line is to be made. The 

 distance wires are then to be approximateh- adjusted 

 to the distance of the stars, and then by means of 

 the slow motion of the telescope the stars are to be 

 brought on these w ires at a place clear of the position 

 wires, and the final rectification made. It will be 

 found most convenient to make these measures thus 

 at places on the wires where thev do not cross the 



position wires. Two readings, one of position and 

 one of distance, are then to be taken and entered on 

 a printed blank form of which a specimen will be 

 given presentl\-. It is advisable to take at least 

 four measures of each sort, and treat the mean of 

 the four as the result to be placed on record as the 

 measures of the star observed. 



In measuring Double Stars it is important that the 

 observer should so hold his head that the line joining 

 his e\'es should either be coincident in position with 

 the line joining the stars or directh- at right angles 

 to it. Carrying out this suggestion mav sometimes 

 involve a strain im the muscles of the neck which 

 must be guarded against, and this will best be done 

 by using a diagonal reflecting jirism on the eve-end 

 of the micrometer. The angle of position required 

 is that obtained by measuring round the circum- 

 ference of a circle (with the principal star of the pair 

 in the centre) from the N. point round to 360°, 

 backwards contrarv to the motion of the hands of a 

 clock. The reading of the micrometer w'ires should 

 be ami .)(j() when the wires are \'ertical. There 

 is no test of the verticalitv of the wires which can be 

 quickh' applied, and when the instrument is attached 

 to the telescope for use the position of the zero will 

 be found to be slightly different from night to night. 

 It is not worth while, therefore, to attempt to have the 

 zero in the correct place. And after tiie micrometer 

 is attached to the telescope preparatory to starting 

 work, or during the observations, the angular error of 

 the zero point out of place is obtained in the following 

 manner and applied as a correction to the mean 

 reading of the (losition angle of the star. The wires 

 are brought apjiroximatelv to point East and West 

 and a star of an^• sort is brought into the field. The 

 slow R.A. motion of the Equatorial is then used, and 

 hv trial the star is caused to tra\'el along one of the 

 position wires, the observer adjusting the position of 

 the w ires until the motion is along the wire. This 

 done the wires then lie along a parallel of declination 

 and are therefore displaced 90"^ from the N. point. 

 Subtract 90' from this reading and we have the 

 zero error. On the position circle there are usually 

 two verniers ISO'-" apart, so the observer has a choice 

 of readings 180° apart ; and, moreover, the \\ires will 

 be in the same position on two stars really differing 

 in angle h\ 180" so that it may be necessary to add 

 180 ' to the resulting measure. Whether or not this 



III the |)repaiati()ii of this seclion I h;i\e been iii;iteriallv helped li>- Mr (i. M. SeabroUe, F.K.A.S. 



! Mcliiiiirfi h'.A.S.. Mil. xli\', page 147. 



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