Viewing Problems in Telescopes
The theoretical resolving power of any optical telescope is never fully realized. The lower layers in the earth's atmosphere are unsteady and turbulent; this turbulence blurs and distorts the star's image and makes it twinkle, or scintillate. The rapid scintillations break the starlight into many dancing specks of light, which in long exposu res merge to form the fuzzy stellar images we see in photographs. When the atmospheric turbulence is low, the stars twinkle, or scintillate, less, and the so-called seeing is improved. A planet, on the other hand, shines with a steady light because each point on the tiny disk twinkles out of step with neighboring points; we see an average of all the twinkling points.
A technique called speckle photography, which can be used with large telescopes, can get around the smearing and wiggling of the image that comes from atmospheric turbulence. In the exposure of the photographic plate for an extremely short time (less than 0.01 second), each star image appears as a cluster of sharp specks of different brightness. Then the photograph is run through a high-speed light-sensing device which measures the variations in brightness across each speck. The information from the assemblage of specks in each of many photographed images is fed into a computer that is programmed to analyze and reassemble the information into the unsmeared image of the star.
Other nuisances hamper our observation of the heavens. The night sky's transparency varies as smog, dust, and atmospheric haze cloud it. The upper atmosphere is also suffused with a faint light called airglow. Atmospheric atoms and molecules absorb the ultraviolet photons in sunlight and reradiate the energy in a few wavelengths of the green, red, and infrared spectral regions. On long exposures airglow fogs a photograph and reduces the contrast between the faintest images and the sky background.
Another problem is that starlight entering the atmosphere is bent increasi ngly toward the vertical so that we see a star slightly closer to the zenith (the point directly above the observer) than it really is. This atmospheric-refraction effect is greatest near the horizon (about OS), for there the light's path through the air is the longest. When we observe the rising or setting sun, it is really below our horizon,
but refraction raises the sun's image above the horizon by an amount equal to its apparent diameter, which is OS.
Other viewing problems are related to the geographical location of the observatory. An ideal site for an optical observatory is a mountaintop where the air is dry, transparent, and steady, and the sky is dark. An observatory also needs a minimum amount of wind and relatively easy access. The southwestern part of the United States satisfies most of these conditions and has many clear days and nights. Kitt Peak National Observatory is located there, about 65 miles southwest of Tucson, Arizona.
The theoretical resolving power of any optical telescope is never fully realized. The lower layers in the earth's atmosphere are unsteady and turbulent; this turbulence blurs and distorts the star's image and makes it twinkle, or scintillate. The rapid scintillations break the starlight into many dancing specks of light, which in long exposu res merge to form the fuzzy stellar images we see in photographs. When the atmospheric turbulence is low, the stars twinkle, or scintillate, less, and the so-called seeing is improved. A planet, on the other hand, shines with a steady light because each point on the tiny disk twinkles out of step with neighboring points; we see an average of all the twinkling points.
A technique called speckle photography, which can be used with large telescopes, can get around the smearing and wiggling of the image that comes from atmospheric turbulence. In the exposure of the photographic plate for an extremely short time (less than 0.01 second), each star image appears as a cluster of sharp specks of different brightness. Then the photograph is run through a high-speed light-sensing device which measures the variations in brightness across each speck. The information from the assemblage of specks in each of many photographed images is fed into a computer that is programmed to analyze and reassemble the information into the unsmeared image of the star.
Other nuisances hamper our observation of the heavens. The night sky's transparency varies as smog, dust, and atmospheric haze cloud it. The upper atmosphere is also suffused with a faint light called airglow. Atmospheric atoms and molecules absorb the ultraviolet photons in sunlight and reradiate the energy in a few wavelengths of the green, red, and infrared spectral regions. On long exposures airglow fogs a photograph and reduces the contrast between the faintest images and the sky background.
Another problem is that starlight entering the atmosphere is bent increasi ngly toward the vertical so that we see a star slightly closer to the zenith (the point directly above the observer) than it really is. This atmospheric-refraction effect is greatest near the horizon (about OS), for there the light's path through the air is the longest. When we observe the rising or setting sun, it is really below our horizon,
but refraction raises the sun's image above the horizon by an amount equal to its apparent diameter, which is OS.
Other viewing problems are related to the geographical location of the observatory. An ideal site for an optical observatory is a mountaintop where the air is dry, transparent, and steady, and the sky is dark. An observatory also needs a minimum amount of wind and relatively easy access. The southwestern part of the United States satisfies most of these conditions and has many clear days and nights. Kitt Peak National Observatory is located there, about 65 miles southwest of Tucson, Arizona.
