284 CARNEGIE INSTITUTION OF WASHINGTON. 



WORK OF THE OPTICAL SHOP. 



In the optical shop, which has remained under the charge of Mr. 

 Ritchey, the work of changing the spherical surface of the 100-inch 

 mirror to a paraboloid has been under way for nearly a year. 80 per 

 cent of the total change necessary has been accomplished in this time, 

 involving 90 days of actual figuring with the large machine. Optical 

 tests have been made each morning after a day's figuring; frequently 

 on account of atmospheric disturbances beyond control, tests have been 

 necessary on more than one morning before figuring could be continued. 

 Two poUshing tools of 90° sector form, one of 850 square inches area, 

 the other of 415 square inches area, have been used in this figuring. 



Thus far, testing has been done entirely at the center of curvature 

 (about 84 feet from the glass). Now, however, the 60-inch plane mir- 

 ror has been finished, silvered, and placed in position on a massive iron 

 carriage with straight ways; this allows a motion of the plane mirror 

 for a distance of 66 inches in a straight line at right angles to the axis 

 of the paraboloid. This arrangement permits optical testing to be 

 done both at the center of curvature and at the focus of the paraboloid. 

 The former test is better for determining the figure of the mirror as a 

 whole, while the latter test is invaluable for detecting and correcting 

 zonal errors in the general curvature. 



In the work on the 60-inch plane mirror, improved methods of 

 figuring made it highly desirable that the very frequent optical tests 

 might be made without tipping the glass from the horizontal position 

 in which it lies and rotates while being figured. Accordingly, a sil- 

 vered spherical testing mirror of 14 inches aperture and 16 feet radius 

 of curvature was set up permanently, so that the 60-inch plane could 

 be tested at any time while lying horizontal. On account of the very 

 high foreshortening of the large plane, this test is extremely rigorous 

 for general curvature, i. e., for slight convexity or concavity. Figuring 

 was carried to a very advanced stage by means of this simple and 

 economical test. At a late stage, the 60-inch plane was placed on edge 

 in a vertical position and tested in combination with a 36-inch spheri- 

 cal testing mirror of 50 feet radius of curvature. This showed the 

 general curvature to be sensibly perfect, but revealed some fine narrow 

 zones which were not visible in the former tests on account of the very 

 high foreshortening. As it was not feasible to use the 36-inch spherical 

 mirror for the daily tests while eliminating these fine zones, a new 

 spherical testing mirror of 27 inches aperture and only 135 inches 

 radius of curvature was made, and was used in the manner in which 

 the 14-inch mirror had been used. With the aid of the tests which this 

 mirror afforded, the figuring of the 60-inch plane was readily finished. 



During three months in winter, when artificial heat is needed in the 

 optical shop, it is difficult to make the daily tests of the 100-inch mirror 

 satisfactorily. These months have been devoted to the work of 



