R. Tousey

The extreme ultraviolet spectrum of the sun

The present state of knowledge of the spectrum of the sun from 3000 Å to 170 Å is critically reviewed. The characteristics of the different photographic and photoelectric spectroscopic instruments and the results are summarized and compared. Topics covered are line identifications, intensities, the continuum and its origin, the suns surface in Lyman alpha, the profiles of H and K of Mg II, Lyman alpha and beta, and monitoring work conducted from rockets and satellites, including OSO-I.

The Spherical Aberration of the Eye

The spherical aberration of the eye was measured by placing a series of centered annular apertures over the eye pupil, and determining the optimum spectacle correction for each aperture. A “double star” was used as a test object. Accommodation was controlled by reflecting a second test object into the field of view. The three eyes examined had positive (undercorrected) spherical aberration when unaccommodated; in one case 2 diopters at the pupil margin. The aberration was reduced with increasing accommodation and in one case became negative at high accommodation. Homatropine reduced the spherical aberration of two of the three eyes examined. A historical review of previous work is given.

Fluorescent Sensitized Photomultipliers for Heterochromatic Photometry in the Ultraviolet

Photomultipliers sensitized to the extreme ultraviolet by coating with a layer of a fluorescent material were found satisfactory for the measurement of radiation intensities at least down to 850 angstroms. The relative spectral quantum efficiencies of sodium salicylate, pump oil, anthracene, calcium and cadmium tungstates, and Eastman ultraviolet fluorescing lacquer were measured by comparison against a thermocouple, using a vacuum monochromator. Sodium salicylate was found to be the best suited for sensitizing a photomultiplier, since it had high sensitivity to the extreme ultraviolet and low sensitivity to long wavelength stray light. Heterochromatic photometry can be carried out with this combination, since the quantum efficiency of sodium salicylate was approximately independent of wavelength over the range 900–2300A. The properties of stilbene, phenanthane, and several miscellaneous phosphors were also measured.

Reflecting Coatings for the Extreme Ultraviolet

The conditions for preparing evaporated Al films of highest reflectance for mirrors and gratings to be used in the vacuum ultraviolet are summarized, and an account is given of aging studies. The preparation of bloom-free, one-micron thick coatings for grating blanks is described, and the effect of thin overcoatings on the efficiency of ruled gratings is discussed. Data are presented on the reflectance and aging of other film materials, such as Pt, ZnS, and Al2O3, which are more suitable for use at wavelengths below 1200 A. A discussion is given of the preparation and the reflecting properties of various multilayer film combinations suitable for reducing the reflectance of mirrors and gratings in the visible and the near ultraviolet while at the same time preserving a high reflectance in the extreme ultraviolet. A 250-A thick layer of MgF2 evaporated onto an Al coating immediately after its deposition was found both to raise the reflectance to about 80% at wavelengths down to 1200 A, and to retard aging.

Coronal changes associated with a disappearing filament

This paper describes Skylab/ATM observations of the events associated with a disappearing filament near the center of the solar disk on January 18, 1974. As the filament disappeared, the nearby coronal plasma was heated to a temperature in excess of 6 × 106K. A change in the pattern of coronal emission occurred during the 11/3 hr period that the soft X-ray flux was increasing. This change seemed to consist of the formation and apparent expansion of a loop-like coronal structure which remained visible until its passage around the west limb several days later. The time history of the X-ray and microwave radio flux displayed the well-known gradual-rise-and-fall (GRF) signature, suggesting that this January 18 event may have properties characteristic of a wide class of X-ray and radio events.In pursuit of this idea, we examined other spatially-resolved Skylab/ATM observations of long-duration X-ray events to see what characteristics they may have in common. Nineteen similar long-lived SOLRAD X-ray events having either the GRF or ‘post-burst’ radio classification occurred during the nine-month Skylab mission. Sixteen of these occurred during HAO/ATM coronagraph observations, and 7 of these 16 events occurred during observations with both the NRL/ATM slitless spectrograph and the MSFC-A/ATM X-ray telescope. The tabulation of these events suggests that all long-lived SOLRAD X-ray bursts involve transients in the outer corona and that at least two-thirds of the bursts involve either the eruption or major activation of a prominence. Also, these observations indicate that long-lived SOLARD events are characterized by the appearance of new loops of emission in the lower corona during the declining phase of the X-ray emission. However, sometimes these loops disappear after the X-ray event (like the post-flare loops associated with a ‘sporadic coronal condensation’), and sometimes the loops remain indefinitely (like the emission from a ‘permanent coronal condensation’).

Extreme ultraviolet spectroheliograph ATM experiment S082A.

The XUV spectroheliograph, Apollo Telescope Mount experiment S082A, is described. The instrument was a slitless Wadsworth grating spectrograph that employed photographic recording. The grating was of 4-m radius, ruled with 3600 grooves/mm. By rotating the grating to positions where the normal was at 255 A or 400 A, the spectral ranges 175-335 A and 320-480 A, respectively, were covered with 2-sec of arc spatial resolution. Close to the normal the resolution reached 2 sec of arc, but at the extreme limit, 630 A, it was 10 sec of arc or worse. The aberrations of the instrument are discussed in detail as are the provisions necessary to maintain optimum imagery and reliability in a space environment. During the mission about 1020 exposures were made covering 171-335 A or 320-630 A.

Forbidden lines of the solar corona and transition zone - 975-3000 A

New forbidden lines characteristic of 5 x 10/sup 4/--3 x 10/sup 6/ K plasmas are observed in ATM UV spectra. New identifications, accurate wavelengths, ionization classes, intensities, and half-widths are presented. Coronal blends with He II lambda1640 are noted. Variations in nonthermal velocities with limb distance are observed. Doppler shifts in the coronal lines observed on the disk may be related to the solar wind. The coincidence of two lines with F IV(/sup 3/P--/sup 5/S) is first evidence for atomic fluorine in the Sun.

A Study of Night Myopia

The phenomenon of night myopia, wherein the eye becomes relatively nearsighted in dim light, was investigated in detail using high contrast grating test objects. Night myopia first appeared at the brightness level where rod vision began to take place and grew larger as the brightness was further reduced. At the lowest brightness investigated, the myopia attained a value of 1.5 to 2.0 diopters, depending upon the observer. Night myopia appeared when accommodation was prevented by an optical method and also when accommodation was paralyzed with homatropine. It was therefore concluded that accommodation was not a significant cause of night myopia in the observers examined.The spherical aberration of the observers’ eyes was measured, and its effect upon the effective focal length of the eye was investigated with the aid of artificial pupils. Also studied were the properties of a simple glass lens having spherical aberration approximating that of the eye. All tests showed that night myopia, and its dependence upon the brightness level, is primarily a result of undercorrected spherical aberration of the eye. For some eyes, homatropine reduced night myopia slightly, but only to the extent that it reduced the spherical aberration. A review of the literature is included.

Reflectance of Evaporated Aluminum in the Vacuum Ultraviolet

The effect of speed of evaporation, pressure, thickness, and age on the reflectance of evaporated Al films was investigated in the extreme ultraviolet from 900 A to 2200 A. High speed of evaporation was found to be the most important factor in producing Al films of highest ultraviolet reflectance. By changing the evaporation time required to produce an 800 A thick coating from 2 sec to 130 sec at a pressure of 1×10−5 mm Hg, the reflectance is reduced from 90% to 67% at 2000 A, from 74% to 50% at 1600 A, and from 38% to 30% at 1200 A. For films produced at higher pressures such as 1×10−4 mm Hg the effect of evaporation speed on the reflectance is even more pronounced. Film thickness has very little effect on the reflectance as long as films are not thinner than 400–500 A and not thicker than 2000 A. The effect of aging on the reflectance increases strongly with decreasing wavelength. Between 2 hr and 1 month exposure to air the reflectance of good Al films decreases from 91.5% to 90.0% at 2200 A, from 75% to 69% at 1600 A, and from 44% to 27% at 1200 A.

White Light Coronagraph in OSO-7.

A small, externally occulted Lyot-type coronagraph, designed for use in the seventh unmanned Orbiting Solar Observatory (OSO-7), is described. Optical configuration, suppression of stray light, SEC vidicon detector, and data system are discussed, as well as integration of the instrument into the spacecraft and operation in orbit. Orbital operation produced daily images of the white light corona, from 2.8 to 10 solar radii, at least once per day for 2(3/4) yr. The first records of white light coronal transient events were obtained, and the corona was shown to be constantly changing.