ON ADJUSTING AN EQUATORIAL BY MEANS OF 



CIRCUMPOLARS. 



Bv E. AKDRON HITTOX. M.A. 



Thk a\'erage amateur astronomer usualK' suffers from 

 the complaint of a hoh' discontent. He gets first 

 of all a three-inch instrument on a tripod. He 

 finds that he cannot conveniently use a power of 

 more than one hundred and twenty, and so procures 

 an equatorial stand and a couple of clasps, and then, 

 by the aid of the usual text books, sets about its, or 

 rather, their, adjustment. It must be confessed that 

 the result is not always satisfactory, and in man\- 

 cases a low power eyepiece is first used as a finder, 

 a higher power being afterwards substituted. The 

 object has then disappeared from the field before the 

 new focus is found, and has to be "fished" for by 

 the Right .\scension screw. Many of the objects in 

 Webb cannot be seen at all with a low power, or at 

 least cannot be distinguished when the field is 

 crowded with stars, and thus much time is lost and 

 many objects too. Xo amateur ought to be satisfied 

 until he can infallibly pick up an\- object desired in 

 a field of fifty to the inch of aperture. 



The want of a sidereal watch can be easily supplied 

 by obtaining a cheap Waterbury at the cost of half 

 a guinea. Its adjustment to sidereal time. b\" aid of 

 the " Sidereal time at Noon " given in Whitaker, is 

 the work of a few days onlv, especially where a good 

 clock is at hand. The first stroke of the hour from 

 a public clock w ill usually be the best to adjust b\-, 

 and if the time by the watch is put down on paper, 

 the rate is eas\' to determine, but patience should 

 be exercised, and the exact three minutes fift\'-six 

 seconds gain per day striven after. The subsequent 

 rate may usually be relied upon to within ver\- few 

 seconds a day. 



The adjustment of the clasps to the stand and 

 to the telescope ma\' then be seen to. The holes in 

 the Declination plate should just fit the lugs of the 

 clasp, and if there is any play wedges of thin zinc or 

 tin (not wood), must be used. Two large washers 

 for the screws to work against should also be pro- 

 vided, and everything should be screwed up well and 

 firmh'. 



If the clasps are not quite right in size, two pieces 

 of sheet lead of suitable thickness should be wrapped 

 round the telescope, and then screw on the clasps. 

 No \\edges of irregular size and thickness must on 

 any account be used. The telescope should, of 

 course, be clasped tightly, and no plav of an\- kind 

 should be even suspected. 



The adjustment of the instrument ma\" then be 

 taken in hand, and for this the method given in 

 Chambers and Loomis leaves nothing to be desired, 



if faithfullv and thoroughly carried out. Few, how- 

 ever, amongst amateurs do this, and hence a final 

 testing by means of circumpolar stars is desirable 

 and often essential. 



For this purpose, either the upper or lower cul- 

 mination of an}- circumpolar will suffice, but u and X 

 Ursae minoris are far the best, and the following 

 method may be adopted. 



A quarter of an hour or so before the actual 

 culmination, the Declination circle should be brought 

 to read the true Declination as given in the " Nautical 

 .\lmanac." minus the correction for refraction. The 

 R.A. circles should be set to the true R.A., and to 

 time respectively, and the screws at the bottom of 

 the Polar axis adjusted so as to bring the star 

 exactly in the centre of the field of an eyepiece of 

 not less than fifty to the inch of aperture, the teles- 

 cope being East. Every care should be taken to 

 perfect this adjustment and not to be content with 

 mere approximation only. Exactness in this point 

 is essential. 



The telescope should then be swung over to the 

 Western side and the circles re-fixed anew. If the 

 star is too high or too low it is probably the Declin- 

 ation vernier that is wrong, and the star should be 

 again centred by the Declination slow adjustment 

 and the new reading taken. Half of this will be the 

 error of the Declination vernier, and it must be 

 altered accordingly. If the star is right in this 

 respect, but appears to the East or West of the 

 centre, a packing of thin metal must be put under 

 one of the clasps until the error is halved. This is 

 the commonest fault, and though it ought to have 

 been detected before in the observation of six-hour 

 stars, yet, too often, it is passed over as "near enough." 

 .\ very small amount of packing will suffice, and 

 nothing is more convenient than a sheet of thin lead 

 from a tea chest, to be obtained at the grocer's 

 for the asking, .\gain, however, we must insist 

 on exactness, for nothing else will suffice, .\ 

 second, or even a third, testing will be amply 

 repaid. The time vernier may then be finally 

 adjusted in the meridian by transit of an equatorial 

 star, as given in the text books. 



The consequence will be that powers of one hundred 

 and fifty or two hundred may be used, with the 

 assurance that the object will be in the very centre 

 of the field of view with ordinary care in setting the 

 circles. There will be no "fishing" or waiting, and 

 above all there will be no mistaking one object for 

 another, or concluding that it is beyond the power 



423 



