260 CARNEGIE INSTITUTION OF WASHINGTON. 



details of the tests already applied, which leave no doubt that this 

 powerful addition to the equipment of the Observatory will permit 

 our observational program to be greatly extended. 



In the preliminary tests of the. Hooker telescope, mentioned in the 

 last annual report, the star images at the 134-foot focus were imperfect, 

 and the driving-clock was found to have a small periodic error of very 

 short period, visible when the guiding-star was observed on the cross- 

 wires with powers higher than 1,000. This error was traced to a slight 

 springing of one of the shafts in the clock-train, and was practically 

 eliminated by introducing an additional bearing. The source of the 

 defective star images was somewhat obscure, but persistent work on the 

 part of Mr. Pease has disentangled the several elements involved. These 

 were three in number: (1) imperfect adjustment of the edge-support 

 of the 100-inch mirror; (2) occasional effects of coma and flare due to 

 atmospheric disturbances near the telescope, especially the escape of 

 warm air from the space behind the large mirror when the cooling coils 

 were not in operation; and (3) occasional atmospheric disturbances, 

 visible also with the 60-inch telescope, which produce an effect on the 

 star images closely resembling such astigmatism as might result from 

 distortion of the large mirror. 



The edge-support was adjusted by relining the edge arcs and bring- 

 ing their pressure to bear exactly in the correct plane. Flare was 

 eliminated by circulating water of the right temperature through the 

 coils provided for this purpose behind the mirror and encircling its 

 edge. Water pumped from tanks within the telescope pier was 

 temporarily employed, and a complete automatic control system, to 

 maintain the mirror throughout the day and night at any desired 

 temperature, is now being installed. The astigmatic effect sometimes 

 resulting from the atmosphere will be felt only in direct photography 

 of star-fields at the 134-foot focus, but the excellent images photo- 

 graphed on many occasions even with this great focal length show 

 that no apprehension need be felt as to the practical performance of the 

 telescope, which greatly surpasses the 60-inch reflector in every class 

 of work. 



Up to the present time, most of the observations have been made 

 with the Cassegrain combination of mirrors giving an equivalent focal 

 length of 134 feet. The focal plane of this combination, which in- 

 volves three reflections of light, is at a point on one side of the tube 

 about 11 feet from its lower extremity. The attachments available 

 for use at this point include a large double-slide plate-holder; a stellar 

 spectrograph,^ for use with one, two, or three prisms, having a colli- 

 mator of 40 inches focal length and several cameras; and a small 

 spectrograph, usually employed without slit, provided with a colli- 

 mator of 18 inches focal length, a single 39° prism, and a camera of 



^ Equipped by Mr. Babcock with a new form of thermostat. 



