James Edward Keeler

“ In making the photographs of nebulæ for which the Crossley telescope is at present regularly employed, it was at first our practice to adjust the driving-clock as accurately as possible to a sidereal rate, and then, when the star had drifted too far from its original position, on account of changes of rate or of flexure, to bring it back by the right-ascension slow motion, the observer either closing the slide of the plate-holder or following the motion of the star as best he could with the right-ascension screw. ”

“ The irregularities to which I have referred consist in sudden and unexpected jumps of the image, which always occur some time after the telescope has passed the meridian. These jumps are sometimes quite large—as much as one-sixteenth of an inch or 1. They are due to two causes: flexure of the tube, and sliding of the mirror on its bed. When the jump is due to sudden changes of flexure, the image moves very quickly, and vibrates before it comes to rest in its new position, and at the same time there is often heard a slight ringing sound from the tension rods of the tube. ”

“ If, for example, the spiral is the form normally assumed by a contracting nebulous mass, the idea at once suggests itself that the solar system has been evolved from a spiral nebula, while the photographs show that the spiral nebula is not, as a rule, characterized by the simplicity attributed to the contracting mass in the nebular hypothesis. This is a question which has already been taken up by Professor Chamberlin and Mr. Moulton of the University of Chicago.
The Crossley reflector promises to be useful in a number of fields which are fairly well defined. ”

“ The winding apparatus, contained in the large casting of the original mounting, has no maintaining power, and can not easily be fitted with one. The clock could in no case be wound during a photographic exposure, on account of the tremors attending the operation, but it would be somewhat more convenient to have the stars remain on the plate during the winding. With a little practice, however, one can wind the clock without actually stopping it, though the object must afterwards be brought back to its place by means of the slow motion in right ascension. ”

“ The most important improvements which have suggested themselves have indeed already [Pg 12] been made by Dr. Common himself, in constructing his five-foot telescope. The three-foot reflector is, in spite of numerous idiosyncracies which make its management very different from the comparatively simple manipulation of a refractor, by far the most effective instrument in the Observatory for certain classes of astronomical work. Certainly no one has more reason than I to appreciate the great value of Mr. Crossley’s generous gift. ”

“ But this mercury flotation, as applied to the Crossley telescope, is a delusion, as I think Mr. Crossley had already found. The mercury, it is true, relieves the thrust to some extent, but it greatly increases the already enormous side pressure on the steel pin at the bottom, thus creating a much greater evil than the one it is intended to remedy. The workmen who set up the mounting inform me that the small bearing at the lower end of the polar axis is badly worn, as I should expect it to be. ”

“ The star then reverses its motion and begins to drift toward the right; and so on throughout the exposure. The advantages of this method over the one previously employed are, that the star never has to be moved by the slow motion of the telescope, and that its general drift is in a known direction, so that its movements can be anticipated by the observer. In this way photographs are obtained, with four hours’ exposure, on which the smallest star disks are almost perfectly round near the center of the plate, and from 2′′ to 3′′ in diameter. ”

“ A collimating lens, placed at its principal focal distance from the incandescent filament of the lamp, makes the illumination of the wires nearly independent of their position on the slide, and a piece of red glass, close to the lens, effectually removes all danger of fogging the plate. The light is varied to suit the requirements of observation by rotating the reflector which throws the light in the direction of the eyepiece. ”

“ In this case the vertical component depends upon the reading of the declination circle, and the horizontal component gives the error of collimation. This method failed, however, to give consistent results, mainly on account of instability of the mirror, and was abandoned.
The use of the large mirror for purposes of adjustment was finally given up, and the axis was adjusted by observations of Polaris with the long finder, in the usual manner. In order to reach the star at lower culmination the finder tube had to be thrown out of parallelism with the main telescope. ”

“ The base-plate having no definite center of rotation in azimuth, and the wedges and crowbars used for moving it being uncertain in their action, a watch telescope, provided with a micrometer eyepiece, was firmly secured to the mounting throughout these operations, in such manner that a mark on the southern horizon could be observed through one of the windows of the dome. The errors of the polar axis were finally reduced to within the limits of error of observation.
The movable hour circle and driving wheel of the Crossley reflector has two sets of graduations. ”

“ The greatest southern declination which can be observed is 25°. In England this would doubtless mark the limit set by atmospheric conditions, but at Mount Hamilton it would be easy to photograph objects 15° farther south, if the telescope could be pointed to them.
[Pg 17] The original driving-clock having proved to be inefficient, at least without an electric control, a new and powerful driving-clock was made by the Observatory instrument maker, from designs by Professor Hussey. In its general plan it is like that of the 36-inch refractor. ”

“ The inclination of the declination axis was determined with a hanging level which had been provided by Mr. Crossley, the hour circle and polar axis being very firmly clamped. The clock correction being known from the records kept at the Observatory, the collimation and azimuth constants were found by the usual formulæ. This method failed to give satisfactory results, and it was found later that the declination and polar axis were not exactly at right angles.
There is only one part of the sky on which the telescope can be reversed ”

“ It is in fact out of proportion to the degree of refinement attained in other adjustments of the reflector, for a slight pressure of the hand on the draw-tube, or movement of the telescope to a different altitude, instantly destroys the perfection of the adjustment. ”

“ The terminals are at such a distance inside the principal focus of the objective, that the light from the spark, after reflection from the flat, appears to proceed from the center of curvature of the large mirror. The rays are therefore reflected back normally, and form an image of the spark which, when the mirrors are in perfect adjustment, coincides with the spark itself. ”

“ The construction here described is not followed exactly in the Crossley apparatus. The guiding eyepiece slides freely when not held by a clamp. Pin-holes for preventing fogging are unnecessary when red light is used.
[9] It so happens that the tension of the vertical thread is such that it begins to slacken when the temperature falls to within about 2° of the dew point. The thread thus forms an excellent hygrometer, which is constantly under the eye of the observer. When the thread becomes slack, it is time to cover the mirrors. ”

“ It is certain that the focus does change considerably, whatever the cause may be, and that the best photographic star images are obtained by keeping the focus of the guiding star unchanged during the exposures. ”

“ It is only when the dome has to be moved through a considerable angle, as in turning to a fresh object, or in photographing some object which passes nearly through the zenith, that the need for a mechanical means of rotation is felt. ”

“ The significance of this somewhat destructive conclusion lies in the fact that these figures of Herschel have sometimes been regarded as furnishing analogies for the figures which Poincaré had deduced, from theoretical considerations, as being among the possible forms assumed by a rotating fluid mass; in other words, they have been regarded as illustrating an early stage in the development of double star systems. ”

“ No. Date. Exposure. Enlargement. Orientation
Top. Remarks h m 1 224 1899, September 7 3 0 2.0 W Great nebula in Andromeda. 2 253 1902, December 18-20 3 0 2.5 S H V, 1. 3 598 1899, September 12 3 30 2.1 W M 33. 4 628 1899, October 31 4 0 3.4 S M 74. 5 650 1899, September 11 3 0 3.4 S M 76. 6 891 1899, November 6 4 0 3.4 S H V, 19. 7 1068 1899, December 3 3 0 7.2 S M 77. 8 .. 1899, December 28 4 0 2.1 W Pleiades. 9 1952 1899, December 24 2 0 3.4 S Crab nebula. 10 .. 1898, November 16 0 40 2.2 S Great nebula in Orion. 11 .. 1899, February 9 0 5 2.5 S Great nebula in Orion. ”

“ E45° 215 10 9 48.3 3.6290 +42 0 20 17.776 16 E95° 33′′ long small spur follows E45° 216 10 9 50.2 3.1172 + 4 10 50 17.776 17 vS bM N R 217 10 9 58.9 3.6294 +42 4 6 17.783 17 vS R 218 10 10 3.0 3.6318 +42 12 15 17.786 17 vvS stell 219 10 10 15.5 3.6205 +41 39 52 17.793 15 S E60° 220 10 10 16.8 3.6317 +42 15 56 17.795 vS R stell 221 10 10 16.8 3.6311 +42 14 7 17.795 18 vvS sbN Spiral? 222 10 10 21.8 3.1184 + 3 51 52 17.797 17 vS bM N Spiral 223 10 10 23.0 3.6194 +41 38 24 17.798 16 vS bM Spiral N 224 10 10 23.9 3.6208 +41 42 47 17.799 18 vvS R Spiral? ”