Horace W. Babcock

Adaptive Optics Revisited

From the earliest days and nights of telescopic astronomy, atmospheric turbulence has been a serious detriment to optical performance. The new technology of adaptive optics can overcome this problem by compensating for the wavefront distortion that results from turbulence. The result will be large gains in resolving power and limiting magnitude, closely approaching the theoretical limit. In other words, telescopic images will be very significantly sharpened. Rapid and accelerating progress is being made today by several groups. Adaptive optics, together with the closely related technology of active optics, seems certain to be utilized in large astronomical telescopes of the future. This may entail significant changes in telescope design.

The Ruling of Diffraction Gratings at the Mount Wilson Observatory

The newer ruling engine of the Mount Wilson Observatory is now producing sizable diffraction gratings of high quality in all respects. Several recent gratings are 714 inches wide with grooves 534 inches long, and one is 8 by 514 inches. Curved-edge ruling diamonds, developed here, have been used in blazing these gratings to specifications for astronomical use. High luminous efficiency is combined with practically complete absence of scattered light, either general or local, in the spectrum. Resolving power of 500,000 is achieved. Rowland ghost intensity is held to about 0.00004 in the first order of 15,000 line per inch rulings. Most ruling to date has been at 300, 400, 600, or 900 grooves per millimeter, but other spacings are available. The rather considerable modifications of the Rowland-type engine are described, with particular reference to the monorail diamond carriage, the coupling of the nut to the grating carriage, the end-thrust bearing of the screw, the use of Nitralloy steel ways, and the spacing mechanism.That the principles of this entirely mechanical, single-screw machine are thoroughly sound is attested by the quality of its products. Blazed plane gratings have almost entirely supplanted prisms in fast stellar spectrographs of both short and long focus.Our methods of testing gratings are outlined and a formula is proposed for the evaluation of gratings.

Control of a Ruling Engine by a Modulated Interferometer

This paper describes a system of interferometric control as applied to the smaller ruling engine of the Mount Wilson Observatory. The usual mechanism intermittently advances the grating carriage with a spacing closely approximating an integral number of fringes of green Hg198 light. A Michelson interferometer monitors this motion. The interferometer is modulated by deflecting the compensating plate electromagnetically, thus correcting for barometric pressure changes and also causing the fringe pattern to oscillate with a small amplitude at 60 cps. The oscillating fringe pattern is scanned by a phototube and is reproduced on a monitoring cathode-ray tube; any decentering of the nth fringe is detected by synchronous demodulation and is converted to a stored electrical charge. During each spacing operation, a differential correction proportional to the stored error signal is introduced into the mechanism. Corrections average about 1 centifringe (2.5 × 10−7 cm). The system is relatively simple and ensures a very high order of precision in spacing, with extremely straight grooves.

MAPPING THE MAGNETIC FIELDS OF THE SUN

New apparatus for observing magnetic fields on the surface of the sun has recently been put into use at the Hale Solar Laboratory in Pasadena. It utilizes the longitudinal Zeeman effect, wherein a spectral line is split by the field into two overlapping oppositely polarized components. The new equipment has three noteworthy features : (1) a large plane grating superior to those that have been available previously; (2) a photoelectric detector having two slits symmetrically placed on the wings of the chosen line, where the profile is steepest, with two photomultipliers connected to a difference amplifier; and (3) provision for scanning the suns disk and mapping with a cathode-ray tube and a camera the intensity and polarity of the magnetic fields on the surface. The new plane grating used in the 75-foot underground Littrow spectrograph has a ruled area 13 by 20 cm with 600 grooves per mm, and is blazed in the fifth order green, where the dispersion is 1 1 mm per angstrom and the resolving power, measured photographically, is 600,000. Scattered light and ghosts are entirely negligible. The resolving power of this grating is ample to render line profiles with high accuracy, so that the interferometer, with its critical adjustments and low light transmission, can be eliminated.

REMARKS ON STELLAR MAGNETISM

Observations carried out within the past year strongly support the theory that rapidly rotating stars of early type possess general magnetic fields much more intense than that attributed to the sun by G. E. Hale. In particular, the metallic-line A-type star 78 Virginis exhibits an integrated Zeeman effect characteristic of a general magnetic field having a strength of at least 1500 gauss at the pole.1 Similar fields of the same order of magnitude have been found for γ Equulei, β Coronae Borealis, HD 125248, and a few other stars ; these results will be reported in detail elsewhere. It is significant that the observed magnetic polarities of the three stars just named are opposite in sign to the polarity of 78 Vir. The intensity and extent of the stellar magnetic fields, comparable in strength to the fields of sunspots, leads us to infer that magnetic and electric phenomena occur on a far grander scale in and near some of the early-type stars than in the sun.

INSTRUMENTAL RECORDING OF ASTRONOMICAL SEEING

The quality of astronomical seeing can be measured objectively and continuously by placing a reticle in the form of a fine grid or lattice in the focal plane of a telescope and analyzing the spectrum of frequencies in the transmitted light of a star with the aid of a phototube, a.c. amplifier, wave filters, and recorder. The lattice has openings somewhat smaller than the diameter of the stellar image under the best conditions, so that the image is always partially but never completely obscured. Thus, when the image is perfectly quiet and steady, the a.c. output is zero (except for negligible shot noise). With ordinary seeing, the image shifts rapidly and randomly with excursions comparable to the lattice spacing ; this results in a characteristic modulation of the transmitted light, so that a Fourier spectrum of frequencies is developed which depends on image diameter as well as on the amplitude and rate of image tremor. If, with very poor seeing, the image becomes large enough to cover several lattice openings, the frequency spectrum will be characteristically altered, and may be expected to reflect largely the effects of scintillation. In general, the Fourier spectrum will contain a great deal of information sensitively related to the quality of the seeing. This can be analyzed by using a multi-channel output or by scanning the spectrum in frequency with a tunable band-pass filter. But for site-survey work or general utility, a simple seeing monitor need record only the mean amplitude in a single, low-frequency band,

Diffraction Gratings at the Mount Wilson Observatory

The past century has seen major advances in diffraction gratings, motivated by pressure for new and better data in two rather distinct fields: laboratory spectroscopy—aimed at understanding atomic structure and testing quantum theory—and astrophysical spectroscopy—aimed at understanding objects ranging from the Sun and stars to faint sources at the limit of detection. In this historical account I trace some of the hard‐won gains in the technology of the ruling machines that produce the gratings, and I touch on the role that larger and higher‐quality gratings have played in scientific advances. I focus on the work at the Mount Wilson Observatory, where I directed the diffraction‐grating laboratory from 1948 to 1963, but I do so with full recognition of the important contributions made at numerous other laboratories, especially those at The Johns Hopkins University, the Massachusetts Institute of Technology and the Bausch & Lomb Optical Company.

Optical gyroscopes applied to telescopes

Optical gyroscopes mounted directly on a telescope can provide its precise angular coordinates referred to the fundamental inertial system. Tracking of a star can be accomplished by computing continually in real time its changing apparent place and by applying computer-generated corrections to the telescope to maintain coincidence between the output of the gyros and the apparent place of the star. This system circumvents mechanical imperfections of the mounting and drives of the telescope, thereby promising significant cost reduction as well as improved performance.

An Integrating Photometer for Low Light Levels

Proper estimation of exposure times in astronomical spectroscopy is often difficult, largely because the quality of the stellar image is so dependent upon “seeing.” The integrating photometer described here, while not new in principle, functions with satisfactory sensitivity and stability on only two percent of the light transmitted by the slit of the coude spectrograph of the 100-inch telescope. When the meter indicates a predetermined number of counts, the finished spectrogram will have the proper density regardless of seeing, slit width, passing clouds, and efficiency of guiding.

TEST FOR A MAGNETIC FIELD IN THE WHITE DWARF 40 ERIDANI B

This paper is reproduced from Publ. Astron. Soc. Pac., Vol. 60, p. 368 - 370 (1948) to celebrate the centenary of the Publications. See also 126.086.