T. Stewart McKechnie

Atmospheric turbulence and the resolution limits of large ground-based telescopes

Conventional theory of imaging through the atmosphere is based on two main assumptions: (1) atmospheric turbulence is assumed to follow a Kolmogorov spectrum and (2) the outer scale, Lo, is assumed to be much larger than any telescope. There are numerous reports in the literature, however, of image properties that are not consistent with this theory—for example, cores in star images and lack of expected image motion. In almost every case these reports are consistent with a smaller value of Lo. There is also evidence of smaller Lo from other, more direct sources such as balloonborne temperature probes and long-baseline interferometry. If Lo is smaller than previously thought, as is suggested here, many long-held ideas about imaging with ground-based telescopes will have to be modified. A much more favorable picture emerges, especially at near-infrared wavelengths. At these wavelengths, resolution in the range 0.03–0.1 arcsec should be routinely attainable with 4–10-m telescopes, even though seeing at visible wavelengths is only 1 arcsec. To attain such high levels of resolution, telescopes must be built to diffraction-limited standards rather than to the currently accepted standards, which fall well short of this limit. Recent images obtained at 2.2 μm with the 4-m Kitt Peak telescope show that very high resolution (0.1 arcsec) is attainable. The images also show that telescope aberrations prevent even higher resolution (0.05 arcsec). A further benefit of a smaller Lo is that the isoplanatic angle of the atmosphere at near-infrared wavelengths is likely to be much larger than previously thought. Thus much wider angular regions are available from which to select suitably bright stars for guiding and tracking. A small Lo also means that ground-based infrared laser beams may be focused to diffraction-limited accuracy on targets in space without necessarily having to use wave-front compensation.

Light propagation through the atmosphere and the properties of images formed by large ground-based telescopes

A fresh look is taken at how light propagates through the atmosphere and how atmospheric turbulence affects images formed by large ground-based telescopes. Telescopes with fixed and adaptive optics are considered. The approach is based on a layered model of the atmosphere. It is shown that the atmosphere can be represented by an equivalent phase screen for the two quantities that determine most of the important image properties—the atmospheric modulation transfer function and the spectral correlation function. Techniques are described for measuring the parameters that define the equivalent phase screen. Expressions are given in terms of screen parameters for a number of image properties. Many of these properties are different from those in the conventional literature. Diffraction-limited cores in star images are discussed. An optimum wavelength at which resolution is maximized is also discussed. Resolution of the order of 0.05 arcsec is possible at this wavelength, but only if the telescope is near diffraction limited. The optimum wavelength can be used to produce maximum energy density at the focus of a ground-based laser beam directed at a target in space.

Diffraction analysis of bulk diffusers for projection-screen applications

A method of analyzing bulk diffusers based on diffraction theory is developed. Bulk diffusers are often used to diffuse the incident light in projection screens. They are modeled as glass spheres randomlyh suspended throughout a plastic substrate. The theory accounts for a distribution of sphere diameters. The method of analysis and results are presented. With this method of analysis, bulk diffusers can be designed with optimal optical performance.

Ultra-Wide Viewing Angle Rear Projection Television Screen

Consumer based rear projection television (PTV) systems are composed of three CRTs which form the red, green and blue portions of the TV picture. Three projection lenses magnify these images and converge them into a single plane. The viewing screen is placed in this plane.