The Large Angle Spectroscopic Coronagraph (LASCO) is a triple coronagraph being jointly developed for the Solar and Heliospheric Observatory (SOHO) mission LASCO comprises three nested coronagraphs (C1, C2, and C3) that image the solar corona for 11 to 30 solar radii (C1: 11 to 3 solar radii, C2: 15 to 6 solar radii, and C3: 3 to 300 solar radii) The inner coronagraph (C1) is a newly developed mirror version of the classic Lyot coronagraph without an external occultor, while the middle coronagraph (C2) and the outer coronagraph (C3) are externally occulted instruments High resolution coronal spectroscopy from 11 to 3 R solar radii can be performed by using a Fabry-Perot interferometer, which is part of C1 High volume memories and a high speed microprocessor enable extensive onboard image processing Image compression by factors of 10 to 20 will result in the transmission of 10 to 20 full images per hour

The Large Angle Spectroscopic Coronagraph (LASCO): Visible light coronal imaging and spectroscopy

The Large Angle Spectroscopic Coronagraph (LASCO) is a three coronagraph package which has been jointly developed for the Solar and Heliospheric Observatory (SOHO) mission by the Naval Research Laboratory (USA), the Laboratoire d’Astronomie Spatiale (France), the Max-Planck-Institut fur Aeronomie (Germany), and the University of Birmingham (UK) LASCO comprises three coronagraphs, C1, C2, and C3, that together image the solar corona from 11 to 30 R⊙ (C1: 11–3 R⊙, C2: 15–6 R⊙, and C3: 37 – 30 R⊙) The C1 coronagraph is a newly developed mirror version of the classic internally-occulted Lyot coronagraph, while the C2 and C3 coronagraphs are externally occulted instruments High-resolution imaging spectroscopy of the corona from 11 to 3 R⊙ can be performed with the Fabry-Perot interferometer in C1 High-volume memories and a high-speed microprocessor enable extensive on-board image processing Image compression by a factor of about 10 will result in the transmission of 10 full images per hour

The Large Angle Spectroscopic Coronagraph (LASCO)

Physics of the aurora and airglow

A Shack-Hartmann aberrometer was used to measure the monochromatic aberration structure along the primary line of sight of 200 cyclopleged, normal, healthy eyes from 100 individuals. Sphero-cylindrical refractive errors were corrected with ophthalmic spectacle lenses based on the results of a subjective refraction performed immediately prior to experimentation. Zernike expansions of the experimental wave-front aberration functions were used to determine aberration coefficients for a series of pupil diameters. The residual Zernike coefficients for defocus were not zero but varied systematically with pupil diameter and with the Zernike coefficient for spherical aberration in a way that maximizes visual acuity. We infer from these results that subjective best focus occurs when the area of the central, aberration-free region of the pupil is maximized. We found that the population averages of Zernike coefficients were nearly zero for all of the higher-order modes except spherical aberration. This result indicates that a hypothetical average eye representing the central tendency of the population is nearly free of aberrations, suggesting the possible influence of an emmetropization process or evolutionary pressure. However, for any individual eye the aberration coefficients were rarely zero for any Zernike mode. To first approximation, wave-front error fell exponentially with Zernike order and increased linearly with pupil area. On average, the total wave-front variance produced by higher-order aberrations was less than the wave-front variance of residual defocus and astigmatism. For example, the average amount of higher-order aberrations present for a 7.5-mm pupil was equivalent to the wave-front error produced by less than 1/4 diopter (D) of defocus. The largest pupil for which an eye may be considered diffraction-limited was 1.22 mm on average. Correlation of aberrations from the left and right eyes indicated the presence of significant bilateral symmetry. No evidence was found of a universal anatomical feature responsible for third-order optical aberrations. Using the Marechal criterion, we conclude that correction of the 12 largest principal components, or 14 largest Zernike modes, would be required to achieve diffraction-limited performance on average for a 6-mm pupil. Different methods of computing population averages provided upper and lower limits to the mean optical transfer function and mean point-spread function for our population of eyes.

Statistical variation of aberration structure and image quality in a normal population of healthy eyes.

Presbyopia and the optical changes in the human crystalline lens with age

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.

The spherical aberration of the eye.

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.

A Study of Night Myopia

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 2¾ yr. The first records of white light coronal transient events were obtained, and the corona was shown to be constantly changing.

White Light Coronagraph in OSO-7.

With a recording photometer of photopic sensitivity, measurements were made of many points in the sky during twilight for solar altitudes H=+5° to −15° for clear air and no clouds at two stations, one in Maryland, altitude 30 meters, and one on Sacramento Peak, New Mexico, altitude 2800 meters. The sky polarization on the meridian through the sun, and the illumination on a plane at various orientations exposed to the sky, were also recorded. For H from about −3° to −11° the entire sky changed in brightness at about the same rate of a factor of 10 for each 2° change in H. Except at the horizon the Sacramento Peak sky was about 23 to 12 as bright as the Maryland sky because of clearer air; at the horizon the two were about the same. At Sacramento Peak the ratio of the polarized components reached a minimum of about 0.06 at the zenith for H=−3°.

Measurements of the Brightness of the Twilight Sky

Photometers to measure the distribution in altitude of the λ 5577 line of OI, the sodium D-lines, and the continuum near λ 5300 were flown in an Aerobee rocket at the White Sands Proving Ground, New Mexico, on December 12,1955, at 10:00 p.m. MST. Though the photometers were extremely simple and lacked in-flight calibration and other refinements, definitive results were obtained.

The photometers and installation in the rocket were basically the same those described in the accompanying communication by Berg, et al. The differences were as follows: (1) Narrow band-pass multilayer filters were used, having bandwidths of 20A at half maximum, thus reducing the magnitude of the correction for the included “continuum.” (2) A third photometer was flown to measure the sodium D-lines. (3) The bakground “continuum” was measured at λ 5300, rather than λ 5200.

M. J. Koomen

Papers

The spherical aberration of the eye.

A Study of Night Myopia

White Light Coronagraph in OSO-7.

Measurements of the Brightness of the Twilight Sky

Distribution of the night airglow (OI) 5577A and Na D layer measured from a rocket