I. S. Bowen

The Aberrations of the Concave Grating at Large Angles of Incidence

Mack, Stehn and Edlen have treated the aberrations of a concave grating at large angles of incidence for the case in which both slit and plate are on the Rowland circle. The present paper shows that, by allowing minor departures from this position, it is possible to reduce the aberration to one-fourth of the value found by them. This increases the optimum grating size and maximum resolving power by a factor of 212. Certain conditions are discussed under which an even greater increase in the size of the grating is desirable.

THE ORIGIN OF THE CHIEF NEBULAR LINES

This paper is reproduced from Publ. Astron. Soc. Pac., Vol. 39, p. 295 - 297 (1927) to celebrate the centenary of the Publications. See also 004.069.

PROBLEMS IN FUTURE TELESCOPE DESIGN

The fifteen years since the war have seen drastic changes in astronomical equipment and observing techniques. Photomultiplier receivers have replaced the photographic plate for perhaps one-fourth to one-third of the observing time of most large telescopes, and this use is still increasing. Image-intensifier tubes are just beginning to become available. When these are perfected they will almost certainly replace the direct recording of star light on a photographic plate for much of the remaining observing time. Most large telescopes in the past have required from 5 to 20 years for their design and construction and have then had effective lives of a half-century or more. This means that telescopes now planned or in the design stage will be used chiefly with photoelectric devices rather than with the direct-photography instrumentation typical of the last few decades. Any prediction of the properties of a device such as an image-intensifier tube that is still in the early development stage is very dangerous. However, if these telescopes now planned are to have maximum effectiveness, it behooves us to make the best guesses that we can as to these properties and as to the telescope design that will take full advantage of them. The important influence of the properties of a receiver on telescope design may be illustrated by the changes brought about by the last such shift, namely, from visual to photographic observation. Thus the retina of the eye has a very narrow wavelength sensitivity range, about 80% of its sensitivity falling in a 1000 Â range. Furthermore the small field of view of high resolution of the eye makes it possible to vary the magnification and the intensity of illumination of the image on the retina to any value needed

FINAL ADJUSTMENTS AND TESTS OF THE HALE TELESCOPE

Since the inception of the 200-inch-telescope project it has been realized that the key to the success of the telescope lies in maintaining the exact optical shape of a mirror of this size in all orientations and often with rapidly changing temperatures. Thus, a simple dimensional analysis shows that the flexure of such a mirror under its own weight is proportional to the fourth power of the diameter of the mirror and inversely proportional to the square of its thickness. Obviously, if the standard astronomical practice of using a fixed ratio of mirror diameter to thickness (usually 8:1) is followed, the flexure under gravitational forces increases as the square of the aperture of the telescope. Because of the difficulties of carrying even the weight of such an 8-to-l mirror on a moving telescope, it is impractical to appreciably decrease this ratio of diameter to thickness. In order to obtain

Optical Problems at the Palomar Observatory

The great flexibility of the 200-inch mirror required the development of new and elaborate support systems to maintain the figure in all orientations. New methods had, therefore, to be devised for testing the mirror in the telescope while on these supports. The final figuring was done on the mountain on the basis of these tests.To increase the effectiveness of the instrument, corrector lenses were designed by Ross greatly to enlarge the field of the main mirror. Likewise the method of evaporating a reflecting coat of aluminum was developed by Strong to replace the old silvering procedures. Several new optical designs were introduced in the spectrographs to make optimum use of the light collected. These included the use of (1) a composite of four “blazed” gratings to permit a collimator aperture of 12 inches; (2) Schmidt cameras with half-strength corrector plate placed practically in contact with the grating, the light passing through the plate twice (this eliminates many vignetting problems and permits higher speeds and larger fields); and (3) a combination of a quartz aplanatic sphere with a Schmidt camera for the shortest focal-length camera to obtain a much higher speed (F/D = 0.7) than would otherwise have been possible.

Explorations with the Hale Telescope

The 200-inch telescope has performed much as Hale had hoped and predicted. Most of the programs he listed have made large advances. A very few have run into unforeseen difficulties. On the other hand, important breakthroughs have occurred along lines of which Hale had no inkling. If Hale were with us today I believe he would be content with the results of the great adventure he started.

The Presence of Sulphur in the Gaseous Nebulæ

MANY of the strongest lines in the spectrum of the gaseous nebulæ have been explained (NATURE, 120, p. 473; 1927. Astrophys. J., 57, p. 1; 1928) as forbidden transitions from low metastable states in oxygen and nitrogen. The analysis of the S II spectrum by Ingram (Phys. Rev., 32, p. 172; 1928), combined with the intercombination lines recently classified by L. and E. Bloch (C. R., 188, p. 160; 1929), makes possible the prediction of the position of lines due to similar jumps in singly ionised sulphur as follows: