Stellar Photography at Harvard

1883 Nature  
J'uly 12, 1883] NATURE STELLAR PHOTOGRAPHY AT HARVARD the plate is exposed. If the plate be exposed during ten seconds a distinct dot is obtained, whilst an exposure of A T the meeting of the Astronomical Society which was thirty seconds causes a short line to be formed. The held on June 8 last, Prof. Pickering of Harvard plates used at Harvard Observatory are six by eight College Observatory, so well known for his stellar obser-inches. . They are divided into six equal parts, each part
more » ... , each part vations, and who is a Foreign Associate of the Society, being in turn exposed. By this means six regions of the attended and gave an interesting account of the work heavens, each about I 5° square, may be photographed on which has been done during the past few years at his one plate; and by a variation in the dot and line system observatory. employed, sometimes taking the dot and sometimes the Some few years ago Prof. Pickering took up the work line first, three pictures may be taken on a single division of determining the intensity of the light of the principal of one of the plates without giving rise to any confusion, stars b y eye observation, without taking the question of Instead of simply six, therefore, eighteen photographs are colour into consideration, work which bas been already taken on one of these plates, so that on a single plate dwelt upon in this journal. For this purpose he used a a portion of the heavens of more than three hours' photometer, completing his observations, which number right ascension, and extending from 30° S. to 6o 0 N., may some 9o,ooo, about a year ago, and a large part of his results be included. Since each portion of the plate covers a are already in print. The published results of the more region of about I So, the camera mounting has a series of important investigators of star n'agnitudes, from the time notches or stops, by which it may be instantly moved of Almagest and Ulf1, have abo been reduced. Sir W. through that amount either of right ascension or Herschel's observations, which appeared almost a century declination. ago in the Philosophical Transactirms, have likewise been When photographing the following is the exact method taken in hand at Harvard Observatory and completely employed. The first exposure takes the region between di scussed. Sir John Herschel's works, the " U ranometria J0° and I 5° south declination, and between one hour and a Nova," the "Durchmusterung," as well as many other half and half an hour west of the meridian. Firstthe plate works in the same field, have also been made use of in is exposed for ten seconds, and each star records itself by a preparing the Harvard Catalogue, which therefore shows dot. The plate is then covered for ten seconds; next it is those cases in which the photometric observations carried exposed for a period of thirty seconds, and each star makes out by Prof. Pickering differ from the results obtained by a line on the plate. By means of the clamping arrangeother observers, when their observations are reduced to ment to which we have referred the plate is then moved the same system. These eye observations of stars having through one hour in right ascension. This takes up therebeen completed, Prof. Pickering, in conjunction with his maining few seconds of the minute, so that the taking of brother, Mr. W. H. Pickering, has taken up stellar photo-the next photograph begins with the first second of graphy from the same point of view. By this means a com-another minute. The camera is then on the meridian. pari son is obtained between the brightness of the star as The same part of the plate is again exposed, and in order seen by the eye, and its brightness as determined by its to distinguish this series of stars from those first photogreater or le's action upon the photographic plate; and graphed, this time the plate is exposed first during thirty oy a comparison of photographs taken on different nights seconds, and then during ten, so that the result is a line any variation in brightness may be detected; whilst the followed by a dot. This gives the second series. Bitt exact positions of stars may of course be more accu-the same portion of the plate may be again used. The rately and permanently recorded than by eye observa-remaining ten seconds of the second minute, like those tions. Mr. A. A. Common recently, by taking of the first, are spent in moving the camera through graphs of the nebula in Orion on different nights and another hour of right ascension. Then a· fresh excomparing them, has thus tee n able to detect a proba.ble posure is made for thirty seconds, a line simply being in one of the stars in the nebula, and in 1858 obtained without a dot, and this completes the series. Professor George P. Bond, by measuring the diameters of The first class of images is in dots and lines, the second stars in photographs was able to determine the relative in lines and dots, the third is recognised by the presence brightness of the two stars which form the double ( Urs:e of lines alone. The thirty seconds which remain of the Majoris. third minute are employed in exposing a second portion of But the work at Harvard University was to do more the plate, and changing the position of the camera, which than this. The stars which Prof. Bond examined were now takes in the region from 15° S. to the equator. The close together. Prof. Pickering wished to compare stars same process is then gone through again, three exposures far removed from each other. For this purpose the ordi-as before being made in three different positions of right nary method of stellar photography, by which photographs ascension. By continuing this process, taking three are taken at the foci of large telescopes, would not suffice. , photographs on each of the six portions into which the These photr graphs only comprise a small region of but plate is divided, the whole region included between the one or two degrees in diameter. A different method was declinations of -J0° and + 6o 0 , and between three hours
doi:10.1038/028255a0 fatcat:dmgsqb7or5a65nhf6mvf335ll4